Heat Pump COP vs SCOP and Solar Self-Consumption

Last updated: November 2026

Picture a homeowner in Cardiff with a 5 kWp rooftop solar array who adds a 9 kW air-source heat pump with a datasheet SCOP of 4.2. The installer says to expect about 60% solar coverage of the heat pump’s electricity. Then December arrives. The array makes about 170 kWh that month, and the heat pump needs more than 600 kWh. Solar can cover at most a quarter of it, even if every solar kWh is used.

This is a worked example built to show the arithmetic, not a real customer. Its assumptions are listed in each table. The equipment in the example performs as advertised. What goes wrong is the assumption that one SCOP figure can predict self-consumption in every month. Heat pump COP is lowest in winter, exactly when solar output is lowest, and the two weaknesses multiply.

This guide is for installers, EPC designers, and homeowners who want the real numbers behind the marketing claims. It covers the COP versus SCOP distinction, how COP varies with outdoor temperature, why that interacts with the solar generation curve, what field data says about self-consumption with and without storage, and how to model the combined system in solar PV design software.

In this guide:

• COP, SCOP, and SPF: which number to use and when
• How outdoor temperature changes COP through the year
• Why COP matters specifically for solar self-consumption math
• The seasonal mismatch between PV output and heat demand
• Realistic self-consumption rates, with and without storage
• How weather compensation lifts COP at peak solar hours
• Worked example: a Cardiff 5 kWp plus 9 kW air-source heat pump
• Load-shifting strategies that bias more demand to daytime
• What a combined PV and heat pump model needs

COP, SCOP, and SPF: Three Numbers, Three Meanings

Most installer datasheets mix these three terms and homeowners assume they are interchangeable. They are not. Picking the wrong one for a sizing calculation can throw the electricity estimate off by a quarter or more, as the worked example below shows.

COP (Coefficient of Performance) is the instantaneous ratio of heat output to electricity input at a single operating point. A datasheet line that reads “COP 4.5 at A7/W35” means the unit delivers 4.5 kW of heat for every 1 kW of electricity drawn, when the outdoor air is +7°C and the heating water flow temperature is 35°C. Change either condition and the COP changes.

SCOP (Seasonal Coefficient of Performance) is the weighted annual average COP across a defined climate. The European standard EN 14825 defines three reference climates: Average (Strasbourg), Warmer (Athens), and Colder (Helsinki). SCOP is normally reported for a low-temperature application (35°C flow) and a medium-temperature one (55°C flow), so the same unit has two different SCOP values. For Europe-specific compliance details, see Europe solar compliance.

SPF (Seasonal Performance Factor) is what your specific installation actually delivers, measured by metering over a year. SPF differs from SCOP because your climate, insulation, flow temperature, and control logic differ from the test conditions. The Fraunhofer ISE field monitoring of 77 heat pumps in existing one- to three-family homes found a mean SPF of 3.4 for air-source units (range 2.6 to 4.9) and 4.3 for ground-source units (range 3.6 to 5.4), according to its November 2025 project press release.

The relevance to solar sizing is direct. The PV array must cover the electrical load created by the seasonal COP across the year, not by the best-case COP printed in the brochure. A 5% optimistic COP assumption means roughly 5% less electricity demand than the house really has, so the PV and self-consumption forecasts are off by a similar margin.

Why Datasheets Quote Multiple SCOPs

Manufacturers publish SCOP at two flow temperatures because real installations use different emitters. Underfloor heating works at low flow temperatures, modern low-temperature radiators somewhat higher, and older radiators or fan coils need the highest. The datasheet for a given unit therefore carries a higher SCOP for the 35°C line and a lower one for the 55°C line.

As a purely hypothetical illustration, a unit published at SCOP 4.5 at 35°C and 3.2 at 55°C would show a 29% drop in efficiency between emitter systems with the same hardware. Heating the same building through radiators instead of underfloor pipes would raise its electricity demand by about 40% (4.5 divided by 3.2), and the PV array would need to grow by a similar proportion to cover the same share of that load.

How COP Varies with Outdoor Temperature

The most useful chart an installer can show a homeowner is the COP-versus-ambient curve. It explains why winter performance disappoints and why oversizing the heat pump rarely helps.

For an air-source heat pump (ASHP), COP falls as outdoor temperature falls. The driver is thermodynamics: the bigger the temperature gap the pump has to bridge, the more electrical work is needed per kWh of heat delivered. Defrost cycles also start to matter around 0 to +5°C, and they cost extra electricity without delivering useful heat.

The table below shows the typical shape of the curve. The values are assumed for explanation and are not taken from a specific datasheet. Replace them with the published COP points for the exact unit and flow temperature you are designing.

Source: illustrative values showing the typical curve shape, not measured data and not from any one manufacturer.

Two lessons stand out. First, an air-source COP drops steadily as the air gets colder. Second, ground-source units (GSHPs) barely move across the year because the ground loop holds a far steadier temperature than the air does. The ground-source winter penalty is small.

Defrost Cycles and the Hidden COP Penalty

Below about +5°C ambient, ASHPs can accumulate frost on the outdoor coil. The unit periodically reverses the cycle to melt the frost, which uses electricity without delivering useful heat. Frequency depends on humidity: dry cold weather triggers fewer defrosts than damp cold weather, so a wet maritime climate such as South Wales is likely to see more defrost activity than a dry continental one at the same temperature. Read the heat pump defrost cycle guide for solar winter design for the detail.

Defrost losses are rarely visible in headline SCOP but they matter for winter self-consumption math, because they add load in exactly the months when PV is weakest. Ask the manufacturer for defrost-inclusive performance data if you are designing for a damp, cold site.

Why COP Matters for Solar Self-Consumption

The self-consumption math has three inputs: PV generation, heat pump electricity demand, and the timing overlap between the two. COP affects the middle term directly and the overlap term indirectly.

A higher COP shrinks the electricity demand for the same heat output. That makes each kWh of solar generation cover a larger share of the total heat load. The relationship is simple arithmetic, but the impact grows when you add storage and time-of-use tariffs.

The table below turns that into numbers. It shows the PV size that would generate 75% of the heat pump’s annual electricity as annual energy, using a specific yield of 1,000 kWh/kWp (close to the 1,029 kWh/kWp the EU’s PVGIS tool gives for a south-facing 35° array in Cardiff). It ignores timing, so it is a sizing yardstick, not a self-consumption forecast. For a primer on the yield input, see specific yield.

Source: arithmetic worked from the assumptions stated above (electricity = heat demand divided by SCOP; PV = 75% of electricity divided by 1,000 kWh/kWp), checked November 2026.

Moving from SCOP 3.0 to 4.0 cuts the array needed to match a given share of the load by 25%. That is one of the cleaner cases in solar design where better equipment pays back through a smaller PV system, though you should compare the real price difference between heat pumps before drawing a conclusion for a given project.

The Indirect Effect on Overlap

There is a second effect that many guides miss. A heat pump with a higher COP draws less power during peak heating hours. That lower peak demand is more likely to sit under the PV inverter’s midday output, so a larger share of the heat pump load can be served directly from PV without going through battery storage round-trip losses.

As an illustration, a 9 kW air-source heat pump delivering its full rated heat at -2°C and a COP of 2.8 draws about 3.2 kW. The same output at a COP of 3.5 draws about 2.6 kW. If the array is producing 2.5 kW at noon in March, the higher-COP unit can be matched almost entirely from PV. The lower-COP unit pulls the gap from the grid.

The Seasonal Mismatch Problem

The hard part of solar plus heat pumps is the calendar. PV peaks in summer when heat demand is near zero. Heat demand peaks in midwinter when PV is at a fraction of its summer output. The two curves are close to mirror images.

The table below pairs real PV output with an assumed heat-demand shape. PV figures come from the European Commission’s PVGIS for Cardiff (51.48 N, 3.18 W), a south-facing 35° array, 14% system losses, PVGIS-SARAH2 data, in kWh per kWp. The heat-demand share is an assumed shape for a typical UK home, not measured data.

Source: PVGIS 5.2 monthly output for the stated location and settings (rounded), run November 2026; heat demand shape assumed for illustration.

In December, a 5 kWp array in this setup produces around 166 kWh in total. A typical UK home in the worked example below needs about 660 kWh of heat pump electricity that month. Even with perfect timing, PV can cover only about a quarter of it. With realistic timing, where solar peaks around 11:00 to 14:00 but heat demand peaks in the early morning and evening, the figure falls further.

This is why summing the year into a single number is misleading. In the example, the array makes about 5,140 kWh a year against about 3,780 kWh of heat pump electricity, which is 136% of the annual load and looks like full coverage. The monthly breakdown shows otherwise. Summer output exceeds the heat pump’s needs several times over, and in December and January it covers a quarter of the load at best.

Why Oversizing the Array Does Not Fix It

The intuitive fix is “just install a bigger PV array.” It helps less than it sounds. Doubling the example array from 5 kWp to 10 kWp doubles winter output too, but December only moves from about 166 kWh to about 330 kWh, against a heat pump demand of about 660 kWh. A shortfall of more than 300 kWh remains. The summer surplus, meanwhile, grows from already surplus to far more than the home can use, and most of it is exported at the export tariff.

At some point, each additional kWp adds less to self-consumption than the same money spent on a thermal store, a battery, or smarter controls. That is the central trade-off in heat pump plus solar design. Where the point sits depends on your tariffs, roof space, and load shape, so model it rather than applying a rule of thumb.

Realistic Self-Consumption Rates With and Without Storage

The numbers homeowners quote often come from manufacturer marketing and assume best-case behavior. Field monitoring tells a more sober story.

Fraunhofer ISE analysed six heat pump and PV combinations as part of its WP-QS im Bestand project. Two terms matter here. Autarky is the share of the building’s electricity demand met by on-site PV. Self-consumption is the share of PV output used on site. The press release reports these ranges:

Source: Fraunhofer ISE press release on the WP-QS im Bestand project, six heat pump and PV combinations, 3 November 2025. A small sample, so treat the ranges as indicative.

Two points follow. A battery moves self-consumption sharply upward, from at most about 37% to as high as 83% in the sample. And the ranges are wide, which tells you that array size, load shape, and controls matter as much as the hardware list. The press release also names the classic approach to raising self-consumption: lifting setpoints when there is surplus PV so the building stores heat.

Why Self-Consumption Stops Rising

The last part of demand is structurally difficult to cover. Late evening and early morning loads occur when PV is offline. Even a large battery struggles to bridge a long overnight gap on a December evening when both solar generation and stored capacity are depleted. The marginal hardware spend to push self-consumption from good to near total is rarely justified.

Better strategies for the final share of demand involve tariff arbitrage rather than storage. A time-of-use tariff, such as Octopus Cosy in the UK or aWATTar in Austria and Germany, lets you draw cheap overnight electricity to pre-charge the thermal store before the morning peak. That turns a self-consumption problem into an energy cost problem, which is what the homeowner cares about.

Weather Compensation: The COP-PV Sweet Spot

Weather compensation is a control feature on most modern heat pumps that varies the flow temperature based on outdoor temperature. Cold outdoor air triggers a higher flow temperature, and mild outdoor air triggers a lower one. The mechanism is simple, and the effect on COP can be significant.

A lower flow temperature raises COP for the same reason the temperature-versus-COP curve behaves as it does: a smaller temperature lift requires less electrical work. How large the gain is depends on the unit. Compare the manufacturer’s COP at your design flow temperature and at a lower one before assuming any figure.

What makes weather compensation useful for solar self-consumption is the timing. Mild outdoor temperatures most often occur on spring and autumn afternoons, which are the hours and seasons when PV output is strong relative to heating demand. The control logic naturally pushes COP up when solar electricity is available.

The table below is an assumed illustration of that pairing, not manufacturer data.

Source: assumed values for explanation only, not measured data.

Spring and autumn become the high-yield self-consumption period. PV is generating well, ambient is mild, weather compensation drops the flow temperature, and COP is at its highest. In those shoulder seasons, a solar plus heat pump system can cover most of the heat pump’s load on a sunny afternoon, even without a battery.

Setting Weather Compensation Correctly

Many installations leave weather compensation in factory-default mode, which is typically tuned for comfort under worst-case conditions. Lowering the heating curve gradient, the slope that relates outdoor temperature to flow temperature, often does not affect comfort if the radiators are generously sized. The COP gain at mild temperatures can be substantial.

In the UK, the MCS standard for heat pumps covers commissioning, but the weather compensation setting is often not tuned for solar pairing. Adjusting the curve to push flow temperatures lower during shoulder months is a low-cost improvement that costs only commissioning time. Check the settings against the manufacturer’s guidance and the building’s heat loss.

Worked Example: A Cardiff 5 kWp Plus 9 kW Heat Pump

This is the project we opened with. It is a hypothetical example, so every input is stated. Let us work through the numbers properly to show how the design should be done.

The building: 3-bed semi-detached, 1970s construction, 215 m² heated floor area. Annual heat demand assumed at 11,500 kWh per year.

The heat pump: 9 kW air-source unit with a datasheet SCOP of 4.2 at 35°C flow. The flow temperature is set to 45°C because the radiators were not upgraded.

The PV array: 5 kWp, south-facing at 35° tilt. Monthly output comes from PVGIS for Cardiff (table above), which gives about 5,140 kWh a year.

The Optimistic Original Sizing

The original installer used the headline SCOP 4.2 to compute electricity demand: 11,500 ÷ 4.2 = 2,738 kWh per year. Against a 5,140 kWh annual PV output, the simple annual coverage figure was 188%, which led to the verbal claim of roughly 60% solar coverage.

The Reality Check

The flow temperature is 45°C, not 35°C, so the 4.2 figure no longer applies. Assume monthly effective COPs fall from 4.2 in summer to 2.6 in December and January. These values are assumptions for the example, not datasheet figures. Applying them to the assumed heat demand shape gives a seasonal average COP of about 3.0 and an annual electricity demand of about 3,780 kWh, which is 38% higher than the original estimate.

The table below compares each month’s heat pump electricity with the PV output. The last column is a ceiling: it assumes every kWh of solar generated in the month can be used, which no real hourly profile achieves.

Source: worked example. PV from PVGIS (Cardiff, 5 kWp, south, 35°, 14% losses); heat demand shape and monthly COPs assumed for illustration; not measured data and not a SurgePV output.

Even the monthly ceiling is 58% of the heat pump’s electricity, and the real figure will be lower because the heat pump runs mostly in the early morning and evening. The verbal promise of 60% was a best case that real operation cannot reach. In December and January, the ceiling is 25%.

What Could Have Changed the Outcome

Three design interventions would shift the numbers:

1. Lower the flow temperature. Upgrading the radiators to allow 35°C flow would lift the seasonal COP. If the monthly COPs improved by 0.5 to 0.7 across the year (assumed), electricity demand would fall to about 3,060 kWh, a seasonal COP near 3.75, and the monthly ceiling would rise to about 65%.

2. Add a buffer tank. This lets afternoon PV pre-heat the buffer when generation exceeds heating demand, moving heat pump runtime into the solar window. The gain depends on tank size and controls, so model it for the real building.

3. Install a battery. A battery captures evening household load as well as some heat pump load. In the Fraunhofer sample, batteries raised self-consumption a long way, but the cost-benefit for heat pump shifting alone is usually weaker than a thermal store.

The lesson is that the equipment was fine. The flow temperature assumption and the use of annual totals were the problem.

Load-Shifting Strategies That Bias More Demand to Daytime

Self-consumption comes down to generation in each hour and how much of the heat pump load coincides with it. PV generation is fixed by weather and array geometry. The lever the designer can pull is overlap. Every strategy below shifts heat pump electricity from non-PV hours into PV-producing hours.

Anticipatory Heating

Smart controllers can raise setpoints when PV surplus is available. Fraunhofer ISE describes lifting setpoint temperatures during surplus PV as the classic approach to raising self-consumption of locally generated electricity. The building’s thermal mass and any storage then carry the extra heat into the evening.

The control logic usually reads PV output, and sometimes weather and PV forecasts, then raises flow or storage temperature for a few hours around midday. The result depends on the building’s thermal mass and how much temperature drift the occupants accept.

Hot Water Pre-Heat

Domestic hot water (DHW) is the easiest load to shift because the cylinder already stores thermal energy. Heating water takes about 1.163 Wh per litre per kelvin, so a 200 litre cylinder lifted by 20 K stores roughly 4.7 kWh of heat. At a COP of 3.5 that is about 1.3 kWh of electricity. If the heat pump heats the cylinder at 11:00 instead of 06:00, that electricity moves cleanly into the PV window.

Many systems already include a DHW schedule or priority mode. Setting it to coincide with the PV peak rather than the morning peak is a control-only change. Check the cylinder’s legionella and temperature requirements before changing setpoints.

Thermal Store Sizing

A dedicated thermal buffer between the heat pump and the heating circuit lets the heat pump run at higher output for short periods, charging the buffer, then idle for longer. For solar pairing, the heat pump runs hard during the PV peak, then space heating draws from the buffer for some time after PV falls away. See the thermal storage for solar heat pump systems guide for design detail.

A commonly quoted rule of thumb is 25 to 40 litres of buffer per kW of heat pump rating, so a 9 kW heat pump pairs with a 225 to 360 litre buffer. Follow the manufacturer’s sizing guidance. The usable energy follows from physics: litres × temperature difference × 0.001163 kWh per litre per kelvin. The table below uses a 15 K difference.

Source: calculated from the stated rule of thumb and a 15 K temperature difference, November 2026.

The last column is a reminder that the rule-of-thumb buffer holds well under an hour of full-output heat. On a mild day when the heat pump runs at a third of its rated output, the same store covers roughly an hour and a half to two hours. That is enough to move a block of demand into the PV window, but it does not carry a house through the evening.

SG Ready and HEMS Integration

SG Ready is a label for heat pumps that defines operating states a controller can request over a standard interface. According to the German heat pump association’s SG Ready page, state 3 is an increased-operation recommendation for space heating and hot water, and state 4 is a firm start command where the control settings allow it. The interface can be used for grid load management or to raise self-consumption with a PV system.

Pairing SG Ready hardware with a home energy management system (HEMS) gives the installer finer control of load shifting. The HEMS reads PV inverter output, forecasts the next hour or so, and decides whether to boost heat pump output, charge a battery, or export. Check that the specific heat pump, inverter, and HEMS support each other before promising any result.

What a Combined PV and Heat Pump Model Needs

Modelling solar plus heat pumps correctly needs hourly resolution and a working knowledge of the COP curve. The combination is more demanding than either system alone because the interactions matter more than the headline numbers.

A useful model combines four data layers:

1. PV generation at hourly or finer resolution, using PVGIS or local typical-year data, with shading from a 3D site model. Shadow analysis identifies shading problems before installation.
2. Building heat demand from a recognised heat loss calculation, bucketed by hour using a degree-day or temperature-based profile.
3. COP curve from the manufacturer datasheet at the chosen flow temperature, applied to each hourly ambient temperature.
4. Storage state (battery state of charge, buffer temperature) tracked through the year to enforce realistic constraints.

The output is an hourly profile of PV generation, heat pump electricity, household baseload, storage states, and grid import and export. Aggregating to monthly and annual self-consumption is then simple. Adjusting flow temperature, heat pump model, buffer size, or battery size lets the designer iterate quickly.

The generation and financial modeling tool runs the cost analysis on top of the energy model, so you can compare configurations once the load profile is defined. The right PV size for a given heat pump depends on the homeowner’s tariff structure, roof space, and storage choice, so test several options.

Common Mistakes Installers Make

The same errors tend to recur in solar plus heat pump projects. Most are control or sizing decisions that cost the homeowner self-consumption.

Mistake 1: Using nameplate COP instead of SCOP at the actual flow temperature. In the worked example, this understated electricity demand by 38%. Always design from the datasheet line that matches the emitter system.

Mistake 2: Quoting annual coverage as if it equals self-consumption. A 188% annual coverage figure can still produce a low self-consumption share because the surplus is in the wrong months. Quote monthly.

Mistake 3: Skipping the thermal buffer. For heat pump load specifically, a buffer is often cheaper per kWh of storage than a battery. Many installers add the battery first because the margin is bigger.

Mistake 4: Leaving weather compensation in factory default. Factory curves are usually tuned for comfort under worst-case conditions. A lower curve can lift shoulder-season COP without affecting winter operation, provided the radiators can still deliver the heat.

Mistake 5: Sizing the array to total annual kWh rather than the heat pump’s draw profile. Heat pump load is highly seasonal. PV oversizing helps only if it falls in shoulder months when the load is meaningful.

Mistake 6: Ignoring defrost. Damp, cold sites add electricity demand that the datasheet SCOP may not show. Ask the manufacturer for defrost-inclusive data.

Mistake 7: Programming DHW priority for early morning. This pushes daily demand into the worst PV hours. Move DHW heating to the late morning or midday window and let the cylinder coast through the morning peak.

EU Energy Label Efficiency (ηs) Versus SCOP

European Ecodesign labelling uses seasonal space heating energy efficiency, written ηs (eta s). The figure comes from Commission Delegated Regulation (EU) No 811/2013, published on EUR-Lex. ηs differs from SCOP in two ways. It accounts for primary energy using a conversion coefficient of 2.5 for electricity, and it is expressed as a percentage rather than a ratio.

In practice, ηs for a heat pump comes out near SCOP divided by 2.5, expressed as a percentage, less small corrections for controls and other components. On the label scale, A+++ starts at ηs of 150% and A++ covers 125% to below 150%. The European Commission has been reviewing these energy label rules, so check the regulation currently in force for your market.

For solar sizing purposes, always use SCOP, not ηs. SCOP gives you the electricity demand you need to cover. ηs is a label figure for comparing products and does not give you kWh of electricity.

Conclusion: Three Things to Do Before Quoting

If you take three actions from this guide, make them these:

• Calculate the heat pump’s effective SCOP at your actual flow temperature, not the headline figure. In the worked example, running at 45°C instead of 35°C pushed the seasonal COP from 4.2 on paper to about 3.0 in the assumed monthly profile. Resize your PV expectation from the corrected figure.

• Run the monthly self-consumption breakdown, not the annual one. December and January will dominate the homeowner’s perception of value. If those months show a low coverage ceiling, no amount of summer surplus fixes the lived experience.

• Model load-shifting interventions before committing to extra PV. A thermal buffer, weather compensation tuning, and DHW timing often deliver more self-consumption per pound than another kWp of array, and they do not depend on roof space.

For a project you are about to quote, book a demo and bring the roof, tariff, and heat pump datasheet so the PV model starts from real inputs.

Frequently Asked Questions

What COP do I need for my heat pump to pair well with solar?

Look at the seasonal figure (SCOP) at your own flow temperature, not the headline COP. There is no published minimum for solar pairing, but a higher SCOP means less electricity to cover. In Fraunhofer ISE field monitoring of 77 heat pumps, air-source units averaged a seasonal performance factor of 3.4 and ground-source units 4.3.

How does COP change with outdoor temperature?

Air-source heat pump COP falls as the outdoor air gets colder and as the required flow temperature rises. The unit has to lift heat across a bigger temperature gap, so it uses more electricity per kWh of heat. Read the manufacturer’s COP table at several outdoor temperatures for your flow temperature, because the curve differs by model.

Why does COP matter for solar self-consumption?

A higher COP means less electricity is needed for the same heat, so each kWh of solar covers more of the load. A building needing 12,000 kWh of heat uses 3,429 kWh of electricity at SCOP 3.5 and 2,857 kWh at SCOP 4.2. That is 572 kWh less load for PV to cover every year.

What self-consumption rate can I realistically expect with a heat pump and PV?

Expect roughly a fifth to a third of PV output to be used on site without a battery. Fraunhofer ISE analysed six heat pump and PV combinations and found 22 to 37% self-consumption without a battery and 40 to 83% with one. Your result depends on array size, load shape and controls.

Does weather compensation help solar self-consumption?

Yes, weather compensation lowers the flow temperature on mild days, which raises COP. Mild spring and autumn afternoons are also when PV output is strong, so the two effects line up. How much COP improves depends on the unit, so check its datasheet at the lower flow temperature.

Is a battery or a thermal store better for solar self-consumption?

For heat pump loads, a thermal store is often the cheaper way to hold energy per kWh, but a battery can serve every other load in the home. Compare installed quotes for both, and size the store from the heat pump’s output. Many homes use a modest battery for general loads and a thermal store for heat shifting.

Is the EU seasonal space heating efficiency (ηs) the same as SCOP?

No, ηs is a percentage on the EU energy label, while SCOP is a ratio. Under Regulation 811/2013 the label figure divides by a conversion coefficient of 2.5 for electricity. For PV sizing, always use SCOP to work out the electricity demand.

For a detailed treatment of pairing PV and heat pumps at the system level, see our solar plus heat pump system design guide. For the load-side analysis methodology, see residential solar load analysis for heat pump and EV homes. If you are designing a UK project with battery storage, the UK battery and solar system design guide covers tariff and DNO considerations. To run the self-consumption math yourself, try the solar self-consumption rate calculator.