Solar Heat Pump System Design: PV Sizing Method

Solar installers and EPC engineers face a specific design problem when a client wants to heat a building with both a heat pump and rooftop solar: the load profile and the generation profile are badly misaligned. PV generation peaks in summer around midday. Heat pump demand peaks in winter, mostly in the morning and evening. Matching annual kWh (“the array produces as much as the heat pump uses”) gives a number that looks right on paper but underdelivers in practice, because the surplus arrives at the wrong time of day and the wrong time of year.

This mismatch is not a niche concern. Many homeowners and businesses now want both technologies, and electrification policy is pushing heat pump adoption in several markets. The question for every designer is how to size and control the combination so self-consumption stays high, grid import stays low, and the financial case holds up.

This guide covers how to calculate heat pump electricity demand from first principles, how to turn demand into PV array size, how to choose between battery and thermal storage, and what a published field study says about real-world results. It also lists the sizing mistakes that appear repeatedly, and shows where a cloud solar design platform fits in the workflow.

Last updated: November 2026

In this guide:

• Why solar + heat pump is an engineering problem, not just an equipment pairing
• How to calculate heat pump annual electricity demand
• SCOP, COP, and SPF: which number to use and why it matters
• Step-by-step sizing from building heat demand to PV array kWp
• What a Fraunhofer ISE field study measured for a PV, battery, and heat pump system
• Battery versus thermal storage, and SG Ready control
• The most common sizing and integration mistakes

Why Solar + Heat Pump Pairing Is Engineering, Not Just Add-On

The simplest version of this design decision goes: “The heat pump uses X kWh per year, so the array should produce X kWh per year.” That works for annual billing estimates. It does not work for self-consumption, and it ignores the two kinds of load-generation mismatch that define these systems.

Daily mismatch. A heat pump in space-heating mode draws most of its energy before and after peak solar generation. On a clear summer day, the array may produce well above household demand through the middle of the day while the heat pump barely runs because the building is warm. That surplus either exports at a feed-in rate or is wasted if there is no export tariff. Without storage or load shifting, a large share of PV energy never meets the heat pump load.

Seasonal mismatch. This is the harder problem. At northern latitudes, winter PV yield is a small fraction of summer yield, while heat pump demand is highest in winter. For several winter months the array contributes little to heat pump supply, and for several summer months it produces far more than the heat pump needs. Check the monthly data for your site, for example from the European Commission’s PVGIS tool, before promising anything.

Two design philosophies respond differently. The self-consumption design accepts that it cannot solve the winter gap and focuses on capturing summer and shoulder-season generation through a buffer tank, a battery, or both. The export-friendly design oversizes the PV array to maximize export revenue under a favorable feed-in tariff, accepting lower self-consumption. Which one makes sense depends more on the local tariff than on any technical factor.

Our view: where export tariffs are modest, the self-consumption path usually gives better economics for the homeowner, but model both against the actual tariff. A designer who treats the project as an equipment compatibility question and skips load-profile analysis will size the array to the wrong target. Getting it right starts with the building’s actual heat demand.

Heat Pump Electricity Demand: How Many kWh Per Year

Before sizing any PV array, a designer needs a reliable figure for annual heat pump electricity consumption. That figure is not the heat pump’s rated power in kW. It is the annual kWh the heat pump will draw. The two most reliable inputs are the building’s Energy Performance Certificate (EPC) or a heat loss calculation, combined with the heat pump’s SCOP.

Published consumption figures vary widely by building type, insulation level, emitter type, and climate, so this guide does not give generic ranges. Use the building’s own heat demand. The table below shows how much the heat pump’s efficiency changes the electricity needed for the same heat demand, using an assumed 12,000 kWh per year of heat.

Illustrative worked example, not market data. Electricity = heat demand divided by SCOP.

A higher SCOP matters most where the existing emitters force a high flow temperature. Two figures are regularly confused in this context:

SCOP (Seasonal Coefficient of Performance) is a standardized metric calculated under EN 14825 test conditions. It represents average performance across a reference climate. Manufacturers publish SCOP on datasheets, and it is the right input for sizing calculations.

SPF (Seasonal Performance Factor) is the field-measured ratio of heat delivered to electricity consumed over an actual operating period. SPF is site-specific and is usually below SCOP, because it includes defrost cycles, pump loads, and control behavior. Use SCOP for initial sizing, then apply a derate you can defend to the savings projection, and state it as an assumption in the proposal.

SCOP, COP, and What They Mean for PV Sizing

COP (Coefficient of Performance) is the ratio of heat output to electricity input at a single operating point. A COP of 3.5 means 3.5 kWh of heat for every 1 kWh of electricity at that instant. It varies with outdoor temperature, flow temperature, and part-load ratio, so a single COP figure from a datasheet says little about annual energy use.

SCOP is the seasonal aggregate from EN 14825 testing. It is the right number for annual sizing, and it depends heavily on the flow temperature the heat pump must deliver. Manufacturer claims show the effect. pv magazine reported in April 2026 that Samsung claims a SCOP of up to 4.9 “under low-temperature floor heating conditions” and about 3.78 at 55 degrees C for its new residential air-to-water heat pump. Those are manufacturer claims, so check the datasheet for the exact model on your project.

Why SCOP drives PV array size. The formula is:

Electricity for heat pump (kWh/year) = Building heat demand (kWh/year) ÷ SCOP

At SCOP 3.5, a home needing 12,000 kWh per year of heat needs 3,429 kWh per year of electricity. At SCOP 4.5, the same home needs 2,667 kWh. That 762 kWh difference matters for array size. At an assumed specific yield of 900 kWh/kWp/year, the higher SCOP cuts the PV needed for the heat pump by about 0.85 kWp, before baseload.

Worked example (assumed inputs):

• Building heat demand: 12,000 kWh/year
• Heat pump SCOP: 3.5
• Heat pump electricity demand: 12,000 ÷ 3.5 = 3,429 kWh/year
• At an assumed specific yield of 900 kWh/kWp/year: 3,429 ÷ 900 = 3.8 kWp dedicated to the heat pump

Add baseload consumption and you get the full array size, covered in the next section.

For more on how PV, battery storage and heat pumps interact in measured operation, pv magazine’s January 2024 report on the Fraunhofer ISE study is worth reading. We use its numbers below.

Step-by-Step Sizing: From Heat Demand to PV Array Size

This is the core engineering workflow. Follow these five steps in order.

Step 1: Determine the building’s annual heat demand (kWh/year).

The most reliable source is the building’s EPC, which in many European countries states estimated space heating demand. If none is available, commission a heat loss calculation, for example under EN 12831 in Europe or ACCA Manual J in the US.

For this worked example, assume annual heat demand = 14,000 kWh/year for a detached house with an air-source heat pump.

Step 2: Divide by SCOP to get heat pump electricity demand.

Heat pump electricity demand = 14,000 ÷ 3.5 = 4,000 kWh/year

If the homeowner upgrades to a higher-efficiency unit (SCOP 4.5), the same heat demand needs 14,000 ÷ 4.5 = 3,111 kWh/year, a 22% reduction that flows into array size.

Step 3: Add baseload electricity consumption.

Baseload covers lighting, appliances, cooking, and other non-heat-pump loads. Take it from the customer’s bills. For this example, assume 3,500 kWh/year.

Total electricity demand = 4,000 (heat pump) + 3,500 (baseload) = 7,500 kWh/year

Step 4: Set the PV coverage target.

The coverage target is a design decision, not a fixed rule. Two common starting points:

• Grid-tied baseline: roughly 100% of total annual demand. Some months export, some months import.
• Self-consumption optimized: above 100% of annual demand with storage, so low-yield shoulder months still produce a surplus to store.

For this example, use a 105% target: 7,500 × 1.05 = 7,875 kWh/year from the PV array.

Step 5: Convert kWh/year to kWp using site-specific yield.

Specific yield (kWh/kWp/year) depends on irradiance, shading, and system losses. Take it from a tool such as PVGIS or from your design software for the exact roof. For more on shading, see the shading analysis guide.

Using an assumed 900 kWh/kWp/year:

PV array size = 7,875 ÷ 900 = 8.75 kWp, rounded up to about 9 kWp

For a roof with shading, use solar shadow analysis software to confirm the actual annual yield at the chosen layout meets the 7,875 kWh target. Shading losses vary by roof and would add to the array size you need.

To cross-check whether incentives change the financial case for your client’s location, see European solar incentives and solar subsidies in Germany.

What a Field Study Measured: PV, Battery, and Heat Pump

Most published numbers for these systems come from simulations. Fraunhofer ISE published measured data from a real building, reported by pv magazine in January 2024 from the paper “Analysis of the performance and operation of a photovoltaic-battery heat pump system based on field measurement data” in Solar Energy Advances.

The system: a 1960 house in Freiburg, Germany, with 256 m² of heated area, a 13.9 kW ground-source heat pump, 12.3 kW of south-facing PV at 30 degrees tilt, and an 11.7 kWh DC-coupled battery. Data are at one-minute resolution for 2022.

This is one building with one control setup, so it is not a universal benchmark. It does show why a design target of “cover the heat pump with solar” needs a measured basis.

Battery vs Thermal Storage: The Self-Consumption Trade-Off

Storage raises self-consumption, but the type matters for cost, performance, and installation.

Battery storage captures surplus PV electricity and discharges it when generation falls below demand. It can serve all electrical loads, including the heat pump, baseload, and EV charging. Cost and round-trip efficiency depend on the product, so use the manufacturer’s datasheet and current quotes. See our glossary entries on battery storage and round-trip efficiency.

Hot water buffer tank (thermal storage) captures surplus PV as heat by running the heat pump or an immersion heater during the midday surplus. It is usually much cheaper per unit of stored energy than a battery, but it can only serve the heat load. Heat losses from the tank reduce the useful return, so size it for the daily surplus, not for the largest tank that fits.

Many designs combine both: a battery for evening baseload and EV charging, and a thermal buffer for the large midday surplus that a battery cannot absorb cost-effectively. Peer-reviewed studies of hybrid battery and thermal storage exist, but results depend on building, tariff and control, so model the specific case.

For storage economics beyond heat pumps, see adding battery storage services and battery solar system design for the UK.

Control Strategy: Diversion, Schedules, and SG Ready

Hardware sizing sets the ceiling on self-consumption. Control strategy decides how close you get to it.

PV-prioritized hot water diversion. The most basic control senses when PV generation exceeds household demand by a set threshold and signals the heat pump or an immersion heater to raise the hot water tank temperature. This stores excess PV energy as heat for the evening.

Time-based scheduling. For heat pumps without SG Ready, programming the pre-heating cycle for the midday window approximates the same effect, especially in shoulder months.

SG Ready. SG Ready is a German label for heat pumps that defines four operating states sent by an energy management system (EMS). Velasolaris, a simulation software vendor, describes them as blocked, normal, encouraged and ordered operation:

• State 1, blocked: The heat pump does not operate.
• State 2, normal: The heat pump follows its own control logic.
• State 3, encouraged: The heat pump is encouraged to run at higher output when conditions allow.
• State 4, ordered: The heat pump runs at maximum output, typically charging the buffer using surplus PV.

The thresholds that trigger states 3 and 4 are set in the EMS, so confirm them per project. Velasolaris states that intelligent SG Ready control has an amortization period of about 10 years, against about 6 to 7 years for a fixed-schedule temperature increase, so a simple schedule can pay back faster.

SG Ready control also has a cost in efficiency. In the Fraunhofer ISE study, SG Ready operation raised the hot water supply temperature by 4.1 K and lowered SPF by 5.7% (from 3.5 to 3.3). In space heating, SPF fell 4% (from 5.0 to 4.8). More self-consumption does not always mean lower cost.

Time-of-use tariffs. Time-of-use tariffs add a third control dimension: the EMS can suppress heat pump operation during peak pricing and favor cheap windows or the solar window. Capture the tariff structure in the financial model to produce an accurate payback calculation.

EMS integration. Many hybrid inverters and third-party EMS platforms can send SG Ready signals alongside battery management. Treat the EMS as part of the system design scope, not an afterthought.

Common Sizing & Integration Mistakes

These mistakes appear often in field reviews and post-installation checks.

1. Sizing PV to summer peak instead of annual demand including the winter heat pump load. Size to total annual demand, then check whether the winter gap is acceptable under the chosen tariff.

2. Ignoring SCOP degradation at low ambient temperatures. SCOP comes from a reference climate that does not represent a severe cold snap. Real efficiency falls in cold weather, which raises consumption just when PV generation is lowest. A flat SCOP in the financial model can overstate savings in cold climates.

3. Oversizing the battery relative to winter heat pump demand. In deep winter a heat pump can use more energy in a single day than a typical home battery holds, so the battery is empty by the evening. Winter self-consumption through battery storage is structurally limited. Thermal pre-heating in any available PV window is often more effective.

4. Skipping the buffer tank. Operating a heat pump without adequate buffer volume can cause short-cycling, which wears the compressor and hurts efficiency. Follow the heat pump manufacturer’s buffer sizing guidance.

5. Using nameplate COP instead of SPF in financial models. A model built on a datasheet COP assumes steady operation at mild temperatures. Field SPF is lower after defrost, pumps, and control behavior. Use SCOP for sizing and derate the savings.

6. Mismatched flow temperature. A heat pump connected to legacy radiators designed for high flow temperatures will run at a much lower efficiency, as the SCOP table above suggests. Either upgrade the emitters or accept a lower SCOP and size the array to match.

7. Forgetting export limits when oversizing PV. Grid operators may require an application above certain array sizes and may impose export limits. Check the local network operator’s rules before oversizing, because curtailed export lowers effective yield.

How a Cloud Design Tool Models Solar + Heat Pump Together

These projects take more iterations than a PV-only job. Array size, battery capacity, and thermal storage interact with site irradiance, the building’s heat demand, and the tariff. A spreadsheet is slow and error-prone for this. A cloud-based PV design software platform handles the geometry, yield, and financial layers in one workflow.

Here is where SurgePV fits:

3D roof modeling and array layout. The designer builds the roof model and places modules. Testing several array sizes against the sizing target takes minutes.

Shading analysis. Shadow analysis checks whether the chosen layout delivers the annual kWh target, with near and far shading across months. For heat pump projects, check the shoulder-month yield, because that is when thermal pre-heating has the most PV to use.

Energy-yield and financial modeling. SurgePV’s generation and financial tool projects yield and payback under the site’s tariff. Ask during a demo how a heat pump load profile should be entered for your workflow. Results depend on source data, assumptions, equipment models and review.

Proposal output. Solar proposal software turns the design, yield and financial analysis into a client document. For a heat pump project, showing PV and heat pump savings against the current heating bill on one page is the comparison the customer needs.

Conclusion

Three actions decide whether a solar + heat pump system performs as designed.

1. Pull the heat demand from the EPC or a heat-loss calculation before sizing PV. A documented annual kWh figure is the foundation of every downstream calculation. An error in heat demand flows straight into array size and the savings projection.

2. Use SCOP for sizing, and derate for field performance. Apply a defensible derate to the savings projection and stress-test the model in cold climates. Fraunhofer ISE’s measured data, with a third of heat pump electricity covered, is a reminder that winter coverage is limited.

3. Decide the tariff strategy first. If the tariff rewards export, oversizing PV and minimizing storage spend may win. If it penalizes export, self-consumption with a thermal buffer, a battery and good control usually wins. Make this decision before the array size is fixed.

When you are ready to model a real project, book a SurgePV demo.

Frequently Asked Questions

What size solar PV system do I need for a heat pump?

Size the array from annual electricity demand, not the heat pump’s nameplate rating. Divide annual heat demand by the heat pump’s SCOP, add household baseload, then divide by the site’s specific yield. With 4,000 kWh for the heat pump and 3,500 kWh baseload at a 105% target and 900 kWh/kWp, the example here gives about 9 kWp.

How many kWh does a heat pump use per year?

It depends on the building’s heat demand and the heat pump’s efficiency. Divide annual heat demand by SCOP: 12,000 kWh of heat at SCOP 3.5 needs about 3,429 kWh of electricity. Use the home’s EPC or a heat loss calculation for the heat demand rather than a generic range.

What is SCOP and why does it matter for sizing?

SCOP, the Seasonal Coefficient of Performance, is the ratio of annual heat output to annual electricity input under standardized EN 14825 test conditions. It is more useful than a single-point COP because it reflects a whole season. It sets the electricity demand that determines your PV array size.

Should I add a battery when pairing solar with a heat pump?

Not automatically. A battery serves all electrical loads, but a hot water buffer tank is usually cheaper for storing heat. Model both against your tariff and roof size, and consider SG Ready or scheduled control first, since control can raise self-consumption without a battery.

Can solar fully cover heat pump electricity in winter?

In practice, no. Winter PV output is low just when heat pump demand is highest. In Fraunhofer ISE’s measured Freiburg system, PV and battery covered 36% of heat pump electricity over 2022 and 28% in space heating mode, so plan for grid import in winter.

What is SG Ready and how does it help self-consumption?

SG Ready is a German label for heat pumps with four control states: blocked, normal, encouraged, and ordered operation. An energy management system sends the signals, so the heat pump can pre-heat a buffer when PV surplus is available. It has an efficiency cost, as the Fraunhofer ISE study measured an SPF drop of 4% to 5.7%.

Air-source or ground-source heat pump for solar pairing?

Both work with PV. Ground-source units usually have higher seasonal efficiency, so the same heat demand needs less electricity and a smaller array, but installation costs more and needs suitable ground. Air-source units are simpler to retrofit. Compare datasheet SCOP at your design flow temperature and model the full cost.