Commercial Solar Self-Consumption: Optimize Sizing

Last updated: November 2026

SurgePV publishes this post and sells solar design software.

A commercial roof can produce a large annual energy total and still export much of it when the building has little demand. The design question is how generation, operating hours, and the electricity tariff overlap. Optimizing that overlap requires a load profile, not just the annual electricity bill.

This guide explains self-consumption, array sizing, load shifting, storage, demand charges, and proposal checks for commercial and industrial (C&I) projects. Worked numbers are labeled as illustrative. They show the calculation method and should be replaced with the site’s own data before an investment decision.

What Self-Consumption Ratio Means for C&I Solar

Self-consumption ratio describes how much generated solar energy is used on site. It is useful alongside total savings and load coverage; maximizing the percentage alone is not necessarily the best investment.

The Self-Consumption Formula Every EPC Needs

Self-consumption ratio (SCR) equals solar energy used on site divided by total solar energy generated, multiplied by 100. In an illustrative example, 560,000 kWh used on site from 700,000 kWh generated gives 80% SCR. Keep the measurement boundary and time period identical in the numerator and denominator.

For a system without storage, calculate direct use in each interval as the smaller of solar generation and site demand. Sum those values over the reporting period. Annual generation and annual consumption totals cannot reveal how much energy coincides in time.

Storage adds an accounting choice. Separate direct use, solar charging of storage, storage losses, and solar-derived discharge to loads. Counting all energy entering a battery as useful consumption can make the result look better without showing what the building actually receives. State the convention before comparing scenarios or vendor outputs.

Use monthly results to explain seasonal variation, then calculate annual SCR from annual energy totals. Do not take a simple average of monthly percentages unless generation is identical in every month. A low-generation month and a high-generation month carry different weight.

Self-Consumption vs. Self-Sufficiency

Self-sufficiency, also called solar load coverage here, measures the share of facility consumption served by solar. Its denominator is total load, not solar generation. A site may consume almost all its solar output and still buy most of its electricity from the grid.

For example, the same 560,000 kWh of on-site solar use against a 1,400,000 kWh annual load gives 40% solar load coverage. The illustrative project therefore has 80% SCR and 40% coverage. Neither number alone gives a bill saving because energy rates, demand charges, and export credits still matter.

Why SCR Belongs in C&I Proposals

SCR helps explain where the generated energy goes. A proposal should also show total generation, grid purchases, exports, curtailment, and the resulting bill change. A small array can produce a high SCR while leaving a worthwhile larger investment unexplored.

Avoid claiming a universal lender requirement or preferred financing rate for a particular SCR. Ask the lender what evidence it needs for this project. It may review load stability, tariff assumptions, equipment, contracts, and downside cash flow, with different priorities depending on the financing structure.

A useful proposal connects the metric to a decision. Explain why one layout was selected over another, which operating changes were assumed, and what happens if the customer closes a shift or adds a new load. A percentage without those assumptions cannot support a sizing decision.

How Export Tariffs and Net Billing Change the Economics

The value of exported electricity depends on the actual tariff or contract. Where exported energy earns less than avoided imports, using a solar kWh on site can be more valuable. That does not make every export uneconomic or every larger array a poor choice.

The California Public Utilities Commission’s net-billing overview states that net-billing export credits are usually below retail rates but can exceed them on late-summer evenings. Its page applies to the named large investor-owned utility territories. Check the actual customer tariff rather than applying a single California average to every commercial project.

This is also why an old net-metering assumption should not be copied into a new proposal. Confirm the customer’s enrollment and connection status, contract dates, applicable rate schedule, and treatment of storage. Keep the tariff source and effective date with the financial model.

Why Net Metering Alone Cannot Determine C&I Solar ROI

Annual offset is an energy comparison, not a complete financial model. Return on investment depends on when energy is used or exported, which bill components change, and the full project cost.

California Net Billing: Use the Applicable Rate Schedule

Use the CPUC overview as policy context and the utility tariff as the calculation input. Do not apply an assumed fixed export discount to every interval. The value of moving energy from midday to an evening period depends on the actual rates, storage losses, and operating constraints.

Separate legacy customers from new applicants and distinguish residential provisions from commercial provisions. A tariff option mentioned on a general policy page may not apply to the business being modeled. Ask the utility to resolve unclear eligibility before presenting savings as an expected contractual outcome.

Run the no-project bill and the proposed-project bill through the same tariff logic. Compare the results by month and by charge category. This makes it easier to find a model that has accidentally removed a standing charge, ignored a minimum bill, or credited exports at the import rate.

European and Australian Feed-in Tariffs

There is no single European or Australian import-export spread suitable for all C&I buildings. Tariffs depend on jurisdiction, supplier, contract, size, connection date, and sometimes the time of export. Keep values in the local currency unless a conversion date and rate are explicitly stated.

For Italy, ARERA’s 2025 Scambio sul Posto decision confirms the final application date under that mechanism. A new November 2026 design must identify an available export arrangement instead of treating the former route as the default. For other markets, obtain the applicable contract directly from the supplier or administrator.

Request the full tariff schedule, not a screenshot showing one energy rate. Include export conditions, demand periods, minimum charges, taxes where relevant, and any capacity or network charges. Assign an effective date and record which future changes are assumptions rather than contractual terms.

The Math: Avoided Import Value vs. Export Value

The spread between avoided import value and export value is useful only for the energy being shifted. It is not a universal penalty on the whole project. Use marginal avoided charges rather than assuming every part of a billed average rate disappears when imports fall.

Consider an illustrative fixed-rate case with avoided imports worth $0.25/kWh and exports worth $0.05/kWh. Using one solar kWh directly is worth $0.20 more than exporting that same kWh, before considering any extra cost needed to change its timing. These are teaching assumptions, not regional prices.

If storage delivers only 900 kWh from those 1,000 kWh, the avoided-import value at the assumed rate is $225. Foregone export income is $50, leaving $175 before storage ownership and operating costs. The 90% delivery ratio is an illustrative assumption, not a battery specification.

This example shows why the battery cannot capture the full retail-export spread without losses. It also shows why a positive energy spread is not enough to prove a positive investment return. Add capital cost, maintenance, replacement, and any other affected bill components.

When Sizing for 100% Annual Offset Misleads

Suppose a warehouse produces an assumed 420,000 kWh annually against a 400,000 kWh annual load. Its generation-to-load ratio is 105%. If only 280,000 kWh is used directly, the project still exports 140,000 kWh and imports 120,000 kWh, assuming no storage or curtailment.

Those flows produce different bill outcomes under different tariffs. Compare feasible array sizes using net present value, cash flow, or another agreed financial objective. A high SCR can be a constraint or a client preference, but it should not silently replace the investment objective.

Keep utility-scale generation-cost benchmarks separate from customer-sited savings. A published levelized energy cost for a large plant cannot determine the installed cost or retail-bill value of a particular rooftop system. Use local quotations and the site’s tariff for the commercial decision.

The Four Self-Consumption Optimization Levers

Load scheduling, array sizing, storage dispatch, and export controls affect different parts of the energy balance. Evaluate them together without adding generic percentage gains as though their effects were independent.

Lever 1: Load Shifting and Smart Scheduling

Load shifting moves flexible consumption into useful solar periods. Candidate loads can include water heating, vehicle charging, refrigeration scheduling, and batch processes. Whether they can move depends on production, comfort, safety, equipment, and staffing requirements.

Ask the facility operator to identify permissible time windows and minimum service levels. Record which loads are genuinely discretionary and which are essential. A design that assumes a production schedule the operator cannot accept is not an achievable self-consumption plan.

Pilot a proposed operating change where practical and compare the resulting load profile with the baseline. Include any extra energy consumption caused by preheating or precooling. Moving a load can improve overlap while changing total demand, so both effects belong in the model.

Lever 2: Right-Sizing the PV Array for the Load

Compare several feasible capacities against the same load and weather series. Include roof constraints, electrical connection limits, equipment choices, and installation costs. There is no universal conversion from daytime baseload in kW to the best PV capacity in kWp.

For each size, show annual generation, direct use, exports, and financial results. A larger system may have a lower SCR and a higher total saving. A smaller system may fit capital constraints or avoid an upgrade. Explain the deciding tradeoff rather than assuming the highest ratio wins.

Consider changes to future load only when there is evidence behind them. A planned new production line may justify a scenario, but its timing and energy demand should be sourced from the client. Keep the existing-load case available if the expansion is delayed or cancelled.

Lever 3: Battery Dispatch and Peak Shaving

Peak shaving uses battery discharge or other controls to reduce demand during the intervals that determine billed demand. It needs the tariff’s measurement rules and a load series capable of representing those intervals. A monthly energy total cannot show the required battery power or duration.

The National Laboratory of the Rockies’ System Advisor Model battery documentation describes dispatch options for peak shaving and time-varying prices, along with models for degradation. It provides useful modeling context; a particular battery’s performance and this building’s savings still need project-specific analysis.

A successful reduction on one day does not guarantee the billed peak remains lower. Another interval can set a new maximum, and ratchets or time-window rules may limit the saving. Simulate the full billing period and document the controller’s available information and reserve policy.

Lever 4: Export Limitation and Zero-Export Design

Export limitation controls power at the grid connection to meet an agreed limit. If generation exceeds loads and available charging capacity, the system may curtail solar output. The control design must follow the utility agreement and equipment requirements.

Curtailing export can raise reported SCR by reducing the generation denominator. It does not automatically increase useful solar energy or improve economics. Report curtailed energy separately so a high ratio cannot hide lost output.

A zero-export target requires an accepted control architecture, commissioning, and a defined response to sensor or communication faults. Do not promise absolute operational behavior from a spreadsheet scenario. The responsible engineer and utility must review the actual installation.

How the Levers Interact

A load shifted into midday may reduce the surplus available to charge a battery. A smaller array may reduce battery utilization. A battery reserved for backup may have less capacity available for bill management. These interactions are why gains should be calculated in one consistent model.

The table is a design comparison, informed by the cited tariff and battery-model documentation. It does not assign universal SCR gains or rank equipment vendors. Model each option using the same baseline, then test combined options with their actual constraints.

How to Size a C&I Solar System for Self-Consumption

Start with accepted interval data and feasible layouts. Run the energy balance before deciding whether the project needs storage or an operating change.

Collecting and Cleaning 12-Month Interval Data

Request a full seasonal cycle of consumption data, preferably including the intervals relevant to the customer’s tariff. Twelve months is a useful planning scope, not proof that the future will match the past. Record shutdowns, tenant changes, unusual weather, and major equipment additions.

Check timestamps, time zone, daylight-saving treatment, units, meter identifiers, and whether each value is energy or average power. Reconcile interval totals with the corresponding bills. Investigate differences instead of accepting a universal error tolerance.

Keep missing and estimated intervals visible. Document any imputation method and run sensitivity checks where gaps affect important periods. Do not silently delete a poor month or replace it with another season just to produce a complete-looking dataset.

Mapping Daytime Baseload vs. Peak Demand

The minimum daytime load helps identify periods when exports may occur, while peak demand informs electrical and billing questions. Neither can replace the whole time series. Define the window being examined and retain the building’s overnight demand when calculating annual coverage.

Segment weekdays, weekends, holidays, and operating shifts without losing their actual calendar weighting. Average profiles help explain behavior, but the final bill calculation should preserve the variation that determines exports and demand charges. A mean weekday can hide both low-load days and exceptional peaks.

Ask the client which unusual events will recur. An annual maintenance shutdown is part of normal operations; a one-time closure may belong in a sensitivity case. Document the decision so reviewers understand why the modeled year differs from raw history.

DC/AC Ratio and Self-Consumption

The DC/AC ratio compares module nameplate capacity with inverter AC capacity. A ratio above one does not by itself prove that clipping occurs in every project or define the amount. Location, orientation, temperature, losses, equipment, and weather affect the result.

Compare feasible inverter and module combinations using manufacturer limits and a generation model. Show clipping separately from export curtailment. Inverter clipping is an equipment-output constraint; export curtailment is an operating constraint at the site connection. Storage may interact with them differently depending on its architecture.

The 2024 Annual Technology Baseline commercial-PV page publishes modeled assumptions and scenarios with a stated base year. It is useful context for understanding benchmark boundaries, not a substitute for a 2026 contractor quote or the site’s operating cost. Its exact nlr.gov URL was checked in November 2026.

Seasonal Variation: Why Annual Averages Mislead

Calculate monthly solar use, exports, and imports to show when the system performs differently. Cooling load, production schedules, holidays, and weather can all change overlap. Do not assign a fixed summer or winter SCR to a building category.

A project may rationally accept seasonal exports if the added generation produces sufficient value. Alternatively, a customer may prioritize low exports because of a connection constraint. State the objective and compare options against it rather than imposing an unsupported monthly minimum SCR.

Storage usually shifts energy across its designed operating window, not from summer into winter. A school with a long summer closure cannot assume a normal daily-cycling battery stores the entire seasonal surplus for the next term. Include the real calendar and available loads.

Modeling Granularity: Hourly vs. Subhourly Simulation

Use a time step appropriate to the decision and available data. Hourly data can support an initial energy assessment, but it may smooth shorter import peaks or export events. Demand-charge modeling should match the relevant billing interval and preserve the peaks that determine charges.

Finer resolution is not automatically more accurate if the source data or control assumptions are poor. Record how solar and load series are aligned, whether one series was interpolated, and which uncertainty remains. A detailed-looking graph should not hide an estimated load profile.

Shading affects the generation available to meet demand. A reduction in generation may raise SCR while reducing useful energy and savings. Keep those results separate when comparing roof areas or deciding whether a shaded group is worth installing.

Battery Storage Sizing for Commercial Self-Consumption

Size storage from the service it must deliver, its power and energy limits, and a dispatch model. A fixed kWh-per-kWp rule cannot establish the right battery for every commercial building.

Why a kWh-per-kWp Rule Is Only a Screening Assumption

The PV array size says little about when surplus occurs or when the building needs energy later. Two sites with identical arrays can have very different storage opportunities. Use surplus duration, discharge demand, and operating constraints to develop candidate capacities.

Keep battery nameplate energy separate from usable energy at the relevant operating conditions. Check state-of-charge limits, reserve, efficiency, temperature, and warranty terms in the selected manufacturer’s documentation. Do not assume one cycle-life or depth-of-discharge specification for an entire chemistry.

Start with several feasible candidates and compare incremental savings. A larger battery may capture more energy but cycle less or earn insufficient extra value. Show the marginal benefit of additional capacity rather than treating the largest SCR improvement as the purchase recommendation.

Sizing for Demand Reduction vs. Energy Shifting

Power rating determines how much demand the battery can offset at a moment. Usable energy determines how long it can sustain that output. A short high peak and a long moderate peak can need different combinations of kW and kWh even if the peak height is similar.

For an illustrative constant 100 kW reduction lasting two hours, delivered energy would be 200 kWh before additional allowances. The required installed capacity depends on losses, reserve, equipment limits, and degradation. This arithmetic is not a complete battery specification or a guaranteed monthly demand saving.

Energy shifting requires available charging energy and a later valuable discharge window. Use the actual tariff and account for foregone export credits. If grid charging is allowed, distinguish it from solar charging so the reported solar-use metric remains meaningful.

Daily Cycling vs. Backup

Backup reserve may conflict with daily bill optimization. Establish which service has priority and how much capacity must remain available. A battery cannot allocate the same stored energy simultaneously to an expected outage and a scheduled discharge unless the operating policy allows that tradeoff.

List critical loads, their starting requirements, acceptable interruptions, and the desired service duration. Check the transfer and islanding architecture with the electrical designer. Multiplying load by hours is a useful energy estimate but does not demonstrate that the system can start or sustain every critical load.

Compare the warranty with the proposed duty cycle, temperature, and throughput. Include replacement or augmentation assumptions where justified. Calendar aging and operating conditions matter as well as cycle count, so a lightly cycled battery is not automatically a lifetime asset.

AC-Coupled vs. DC-Coupled Efficiency

AC-coupled and DC-coupled systems place conversion equipment at different points in the architecture. Compare complete energy paths and the manufacturer’s efficiency definitions. A component peak efficiency is not interchangeable with delivered round-trip efficiency.

A retrofit may favor retaining a suitable existing PV inverter, while a new system may offer more architecture choices. Compare compatible equipment, controls, installation scope, service access, and operating limits. Neither coupling label is a universal recommendation.

Check whether a claimed clipping-recovery benefit is physically available in the proposed configuration. An AC-side battery cannot receive DC energy that never passes through the PV inverter merely because the model includes storage. Use the actual power-flow boundaries in the simulation.

Battery Costs and the Investment Case

Obtain an installed quote that separates battery equipment, conversion equipment, controls, civil works, electrical works, commissioning, maintenance, and connection costs. State currency, tax treatment, usable capacity, power, and quote date. A utility-scale benchmark should not be presented as the price of a small commercial installation.

Calculate incremental storage returns against the same solar-only baseline. Include the change in imports, exports, demand charges, operating costs, and replacement assumptions. If storage is selected for resilience, identify that objective rather than making the energy-savings model carry the entire justification.

The EPC Workflow: From Load Profile to Client Proposal

A useful workflow produces a traceable energy balance and a tariff-specific financial comparison. Engineering, procurement, and customer commitments should all use the same design revision.

Step 1: Extract Interval Data and Build the Load Profile

Start with the raw utility or facility-meter records and preserve an unchanged copy. Record the coverage period and any gaps or corrections. Verify that the meter boundary includes the loads the proposed solar system will actually serve.

Tag working days, shutdowns, and known changes in operations. Keep EV charging or other flexible loads identifiable if a scheduling scenario depends on them. Ask the operations team to approve the assumed flexibility before using it to justify a smaller connection or a larger array.

Reconcile the historical bill before estimating savings. If the model cannot explain the baseline charges, its proposed savings need further review. Resolve mismatched periods, units, meter aggregation, and tariff logic rather than compensating with an unexplained adjustment factor.

Step 2: Size the Array Against the Load Shape

Develop feasible layouts using verified roof or land constraints and equipment. Model generation with documented weather, orientation, shading, and loss assumptions. Keep the electrical connection and export limit visible throughout the comparison.

Compare multiple capacities rather than selecting one from annual consumption alone. For each candidate, calculate direct solar use, export, curtailment, and residual imports. Examine low-load days and seasonal closures as well as the annual result.

Use the financial objective agreed with the client to select a preferred option. A capital budget, a connection limit, an investment hurdle, and an emissions objective can lead to different choices. Record which constraint actually determined the recommendation.

Step 3: Model Self-Consumption and Export Scenarios

Show the baseline solar-only result before adding storage or load shifting. Then change one assumption at a time and document what it affects. This makes the source of each improvement visible and helps the client understand the operating commitments involved.

Use monthly tables for direct use, exports, imports, and solar coverage. Calculate annual ratios from the corresponding annual energy totals. If storage is included, identify solar-derived discharge and losses so charging energy is not counted twice.

Value exports at the correct time and rate. Do not assume export income is negligible without calculating it. A project with valuable export periods may need a different dispatch strategy from one designed primarily to avoid imports.

Step 4: Size Storage and Simulate Dispatch

Choose candidate power and energy ratings from the required service, then apply equipment operating limits. Simulate the actual dispatch priorities, including backup reserve where relevant. Review periods with low solar, high demand, and competing uses of storage.

A battery reaching its minimum state of charge is not automatically undersized, and a battery retaining reserve is not automatically oversized. Interpret the result against the control objective, warranty limits, and reliability requirement. The correct question is whether the extra capacity earns sufficient benefit or meets a necessary service requirement.

SurgePV’s documented scope includes energy-yield and financial modeling. A specific battery-dispatch optimizer, interval-data integration, or live control function should be verified separately before being promised. Use a suitable dedicated model where the project requires capabilities outside the documented scope.

Step 5: Generate the Client Proposal

Lead with the decision: recommended system, project cost, expected bill effect, and key operating assumptions. Include SCR and load coverage as explanations of the energy flows. Neither should replace the cash-flow comparison or the connection conditions.

Show demand savings separately from energy savings and export income. State the baseline tariff, price date, modeled load year, storage assumptions, and ongoing costs. Label sensitivities as scenarios, with their changed inputs clearly identified.

Attach enough detail for a reviewer to reproduce the result. Keep the load summary, generation assumptions, equipment schedule, tariff source, and dispatch outputs linked to the same revision. Explain which approvals and commercial terms remain open before the client commits.

Self-Consumption by Building Type: What to Measure

Building type is a starting point for questions, not a reliable SCR benchmark. Actual operating hours, equipment, occupancy, and array size determine the result.

Offices and Mixed-Use Commercial

Check weekday attendance, cooling and heating systems, server loads, and weekend operation. Hybrid working can change the load pattern without changing the building’s floor area. Separate landlord and tenant meters where the solar connection cannot serve them all.

An office that closes in the evening may have little later load for a battery to serve, or it may retain substantial equipment demand. Measure it. Compare storage with a smaller array, a different tariff where available, or flexible daytime loads before assuming a battery is the next step.

Retail and Shopping Centers

Retail demand may extend into the evening and vary by tenant. Refrigeration, lighting, cooling, and delivery schedules can create different profiles within the same site. Establish which loads sit behind the proposed connection and who benefits from the generation.

Parking-canopy solar may be useful where roof space is constrained, but its structure and civil works affect cost. If EV charging is included, model its schedule and billing separately. Do not assume that visitor charging demand coincides perfectly with solar output.

Manufacturing and Industrial Facilities

Use actual shift and process schedules. Some factories have steady demand while others run short, high-power batches or stop for long maintenance periods. A generic manufacturing SCR range hides those differences.

Ask the production team to approve any shifted loads and document quality or safety limits. The energy model should not assume compressors, ovens, or process cooling can move freely. Include the consequences of a production expansion or shutdown as separate scenarios where relevant.

Warehouses and Cold Storage

A lightly occupied warehouse and a refrigerated logistics facility can have very different electricity use. Large roof area does not imply a large daytime load. Inspect refrigeration cycles, forklift charging, automation, lighting, and tenant arrangements.

For cold storage, evaluate any proposed precooling with the operations team. Product temperature and process requirements take priority over a higher SCR. Include changed total consumption if the operating strategy uses more energy while shifting its timing.

Healthcare and Hospitals

Separate ordinary bill management from critical-power requirements. A hospital’s emergency supply, transfer arrangements, and clinical loads require specialist review. A solar-plus-battery model cannot replace the applicable resilience and safety design.

Use the measured load profile and approved operating boundaries. A round-the-clock facility may use substantial solar directly, but that still depends on array size and demand. Do not assume a fixed backup duration or that zero-export control is incompatible with every emergency supply architecture.

Schools and Universities

Include holidays, term dates, evening use, and research or residential loads. A school closed during sunny periods may export more than a campus with continuous laboratory demand. Annual consumption alone will not capture that difference.

A battery can shift energy within its operating window but cannot solve a seasonal surplus without an appropriate scale and economic case. Compare summer exports and connection limits directly. Keep educational or sustainability objectives separate from claims of bill savings.

This is a measurement checklist, not a set of observed performance benchmarks. Use it to request evidence and identify which scenario the client should review before accepting a design.

Conclusion and Three Action Items

Optimize the financial and operating result, using SCR to explain the energy balance. A high ratio is useful only when the design also meets connection requirements, equipment limits, and the client’s objectives.

• Audit the load and tariff inputs before comparing capacities. Keep missing evidence and assumptions visible.
• Compare solar-only, operating-change, and storage scenarios against the same baseline. Report exports, curtailment, losses, and demand savings separately.
• Review the proposal against the final design revision and obtain the required engineering and utility decisions before committing to the scope.

Use SurgePV’s generation and financial workflow to review the solar assumptions, then bring a commercial project to a demo to check the outputs your team needs.

Frequently Asked Questions

What is a good self-consumption rate for commercial solar?

There is no universal target that proves a commercial project is worthwhile. The appropriate ratio depends on array size, load timing, tariffs, and cost. Compare total savings and load coverage alongside self-consumption.

How do you calculate solar self-consumption ratio?

Divide solar energy used on site by total solar generation and multiply by 100. Calculate time-matched use before adding up the annual values. State how storage losses and solar-derived battery discharge are counted.

How does battery storage improve solar self-consumption?

Storage can move solar surplus into a later period when the facility needs energy. The useful gain depends on losses, capacity, power, and dispatch limits. Compare incremental savings with the full storage cost.

Can one battery discharge remove a monthly demand charge?

One discharge cannot guarantee a lower monthly demand charge. Another interval may set the billed peak, and tariff ratchets or time windows may also apply. Model the full billing period and the actual controller limits.

Does zero export guarantee the best solar return?

Zero export does not guarantee the best return. Curtailment can raise the self-consumption ratio while reducing useful generation. Apply the connection requirement and compare the remaining energy and financial results.

How should a commercial battery be sized?

Size battery power and usable energy from the required service and a dispatch model. Include tariff intervals, surplus solar, later loads, reserve, losses, and equipment limits. A fixed kWh-per-kWp rule is only a screening assumption.