Winter
Heating demand is highest because temperatures are low.
Solar days are shorter, the sun is lower and total PV generation is much lower than in summer.
Heat pumps and solar explained
Is solar worth it with a heat pump?
A heat pump can transform the electricity demand of a home. That can justify substantially more solar generation, more battery storage and sometimes a larger inverter.
But the maths is more complicated than simply saying “double the panels”. Heat demand peaks in winter, solar generation peaks in summer, and the system still has to work when December is dark, cold and expensive.
The short answer
Yes. A heat pump can make a larger solar system much easier to justify.
Solar panels offset electricity consumption.
A heat pump moves a large part of the home's heating demand onto electricity.
So once a heat pump is added, the property often has considerably more annual electricity demand available for solar generation to offset.
That can justify:
more solar panels;
a larger inverter where appropriate;
greater battery capacity;
and more sophisticated use of time-of-use electricity tariffs.
But “bigger” should never mean “because we can sell you more”.
The larger system should be justified by calculated household demand, expected heat-pump electricity consumption, roof yield, tariff strategy and the amount of energy the household can realistically use or export.
Why the electricity bill changes
Removing gas heating can almost transform one energy bill into another.
Comparing the percentage of the old gas bill with the percentage increase in electricity is misleading.
Gas and electricity have different unit prices.
More importantly, a gas boiler and a heat pump produce useful heat in completely different ways.
A boiler burns fuel to make heat.
A heat pump uses electricity to move heat from outside the building into it.
The right question is not “What percentage of my old bill was gas?”
It is “How many kilowatt-hours of useful heat does this house need, and how much electricity will the heat pump need to deliver it?”
The basic heat-pump maths
Useful heat demand ÷ seasonal efficiency = heat-pump electricity.
Annual useful heat demand ÷ expected SPF = estimated annual heat-pump electricity
SPF means Seasonal Performance Factor.
It describes the relationship between the heat delivered over a period and the electricity consumed to provide that heat.
For example, if a heat pump achieved an SPF of 3.0, then over the period considered:
1 kWh
Electricity consumed
≈3 kWh
Useful heat delivered at an SPF of 3.0
3.0
Illustrative seasonal performance factor
SPF 3.0 is used here for simple illustration only. Real performance varies with the heat pump, weather, building, emitter system, flow temperature, hot-water production, controls and installation quality.
Worked example
Why a heat pump can nearly double a home's annual electricity demand.
An illustrative home before and after electrifying the heating.
3,500 kWh
Existing annual household electricity
10,000 kWh
Calculated annual useful heating requirement
3.0 SPF
Illustrative seasonal heat-pump performance
≈6,833 kWh
New combined annual electricity demand
The heating calculation is:
10,000 kWh useful heat ÷ 3.0 SPF = approximately 3,333 kWh heat-pump electricity
Add that to the house's existing 3,500 kWh of electricity:
3,500 + 3,333 = approximately 6,833 kWh per year
The home's annual electricity demand has increased by about 95%.
This is why “roughly double the solar” can sometimes land surprisingly close.
But the correct conclusion came from the heating demand and expected heat-pump performance — not from an automatic rule that every heat-pump house should have twice as many panels.
One number homeowners often misunderstand
An 8kW heat pump does not normally consume 8kW of electricity.
The headline kW rating on a heat pump normally refers to its thermal heating output under specified conditions.
It does not mean an 8kW unit continuously draws 8kW from the grid.
Electrical consumption changes with:
outdoor temperature;
the water flow temperature required by the heating system;
the heat demand of the building;
hot-water production;
defrost cycles;
and how efficiently the system has been designed and commissioned.
That is why annual kWh matters far more than simply reading the model number on the outdoor unit.
Heat-pump sizing
Bigger is not automatically better — and smaller is not automatically more efficient.
The correct heat pump should be selected against the calculated heat loss of the building under the appropriate design conditions.
That normally means a detailed room-by-room assessment of:
walls;
windows and doors;
floors;
roof or ceiling;
ventilation and infiltration;
required room temperatures;
and the property's local design outdoor temperature.
Selecting a heat pump from floor area alone is not enough.
Two houses with the same floor area can have completely different heat losses because insulation, glazing, construction, exposure and air leakage are different.
Too small
What happens when the heat pump cannot meet the house's heat loss?
An undersized unit may operate at or close to maximum output for long periods during cold weather and still struggle to maintain the required indoor temperature.
Where direct electric backup heating is fitted, that backup may then be called upon more often.
Direct resistance heating effectively produces around one unit of heat for one unit of electricity consumed.
That can dramatically worsen running costs compared with obtaining the heat through an efficiently operating heat pump.
Long running is not automatically the problem.
Modern inverter-driven heat pumps are designed to run steadily for long periods. The concern is a machine that cannot deliver the required heat under design conditions — not simply one that remains switched on.
Too large
Oversizing the heat pump can create a different problem.
A much larger heat pump is not automatically safer.
Every inverter-driven unit has a range over which it can modulate its output.
If the building needs less heat than the machine can reduce itself to, the system may repeatedly switch on and off.
This is known as short cycling.
The target is correct sizing — not maximum size.
A good design matches the heat pump to the calculated building requirement while considering its actual output and modulation characteristics at the temperatures it will experience.
The solar problem
The heat pump wants electricity most when solar gives you the least.
Summer
Solar generation can be excellent.
But space-heating demand may be close to zero, so a large part of the array's output must find another useful destination.
You cannot generate a surplus in June and put those electricity units
in a battery until January.
That sounds obvious, but it is one of the most important limitations when somebody claims that solar will simply “run the heat pump”.
So why install more solar?
Because annual electricity demand still matters.
The seasonal mismatch does not make solar pointless.
A larger array can still produce valuable electricity across the year.
Spring and autumn solar can directly support space heating.
Solar can contribute to domestic hot-water production.
Ordinary daytime household loads consume generation.
EVs can absorb surplus generation.
Batteries can move daytime energy into the evening.
Surplus summer generation can be exported where a tariff is available.
The house simply has more opportunities to use electricity than it did before the heat pump was installed.
Should I double my panels?
Sometimes — but calculate first.
Suppose the existing electricity demand is 3,500 kWh and the proposed heat pump is expected to add another 3,300 kWh.
The house has gone from roughly 3,500 kWh to roughly 6,800 kWh.
Designing solar only around the original 3,500 kWh would ignore a huge new electrical load.
A near-doubling of electricity demand can justify something approaching a near-doubling of solar capacity.
But the final panel count still depends on roof yield, orientation, shading, export value, inverter size and how the household can use the additional generation.
A better sizing sequence
Work from consumption — not panel count.
| Step | Question | Why it matters |
|---|---|---|
| 1 | What does the home currently use? | Establish the non-heating annual electricity demand. |
| 2 | What is the building's useful heat requirement? | This should come from proper heat-loss and energy calculations. |
| 3 | What heat-pump efficiency is realistically expected? | Heat demand divided by seasonal performance estimates the additional electrical demand. |
| 4 | What other electrification is planned? | EVs, electric hot water and future loads can materially change the answer. |
| 5 | How much PV can the roof sensibly produce? | Orientation, shading, pitch and usable roof space determine the solar opportunity. |
| 6 | What happens to surplus generation? | Battery charging, EV charging, hot water and export all affect value. |
The winter strategy
A battery can do something solar cannot: move cheap electricity through the day.
A battery cannot carry summer solar into winter.
But it can shift electricity by several hours.
That becomes very useful on a time-of-use tariff.
A household may be able to:
charge the battery from the grid during a cheap overnight period;
run the heat pump directly during part of that cheap period;
then discharge stored electricity later when grid prices are higher.
In winter, the battery can become a tariff-management tool as much
as a solar-storage device.
Battery sizing
The battery needs enough energy to do the job you are asking it to do.
Consider a simple winter example.
Suppose the heat pump and home together are expected to consume around 12 kWh during the expensive part of the day.
A nominal 5 kWh battery cannot magically cover 12 kWh.
Even its full advertised capacity may not all be usable, and there are conversion losses.
Battery kWh is only half the question.
The inverter must also be able to discharge quickly enough. A 20 kWh battery connected to a low-power inverter may still need substantial help from the grid when the heat pump and household loads are high.
A battery example
Why two batteries are not necessarily overselling on a high-demand home.
Imagine the winter target is to avoid expensive-rate imports.
8 kWh
Heat-pump demand during expensive hours
6 kWh
Other household demand during the same period
14 kWh
Total electricity that may need covering
>14 kWh
Nominal storage may be needed after allowing for reserve and losses
In that scenario, recommending 15–20 kWh of nominal storage might be entirely rational.
In another house where only 4 kWh needs shifting, the same battery proposal could be excessive.
The question is not “one battery or two?”
It is “How many usable kilowatt-hours do we actually need to move through the expensive part of the day?”
Hot water
Domestic hot water creates another useful solar opportunity.
Many heat-pump homes use a hot-water cylinder.
That cylinder stores thermal energy.
Depending on the system design and controls, the household may be able to schedule hot-water production into favourable periods.
Solar electricity can therefore contribute to heating stored hot water even when there is little or no space-heating demand.
A hot-water cylinder is effectively a form of energy storage.
It does not replace an electrical battery, but shifting hot-water production into periods of cheap electricity or strong solar generation can reduce the amount of expensive electricity required later.
Inverter size
More panels may justify a different inverter too.
Increasing the panel count without revisiting the inverter can leave the design unnecessarily constrained.
The correct inverter depends on:
total PV capacity;
roof orientations;
string design;
MPPT requirements;
battery charging and discharge capability;
household power demand;
and G98/G99 network requirements.
A larger annual electricity demand can therefore change more than just the number of modules on the roof.
Solar, battery and inverter sizing should be considered together.
Designing each component in isolation is how apparently large systems can still end up with frustrating bottlenecks.
Bigger roofs become more interesting
High electricity demand changes the value of marginal solar panels.
On a low-consumption home, filling every usable piece of roof may produce large amounts of electricity that the household cannot use.
A heat-pump home has a much larger electrical appetite.
That can make:
east-facing panels;
west-facing panels;
additional secondary roof sections;
and in suitable cases even some north-facing capacity
more commercially interesting than they would be on a low-demand property.
The correct test is still expected generation versus installed cost and the value of the electricity produced.
Trust the calculation
A good installer should be able to show you why the system is that size.
Homeowners are right to be cautious when one installer proposes substantially more equipment than another.
If one quotation contains:
a larger heat pump;
more solar panels;
more battery storage;
and a larger inverter;
it is perfectly reasonable to wonder whether the installer simply wants a larger sale.
The answer should not be:
“Trust us.”
It should be:
Here is the room-by-room heat loss.
Here is the required heat-pump output.
Here is the expected seasonal performance.
Here is the additional annual electricity demand.
Here is the PV generation estimate.
Here is what the battery is expected to cover.
Here are the tariff assumptions.
MCS
Competent heat-pump design already requires evidence behind the proposal.
MCS-certified heat-pump work is not intended to be designed from a salesperson's instinct.
The customer should receive meaningful performance information, and the detailed design process should establish the building's heating requirement and the suitability of the proposed emitters and system.
The heat pump should therefore be linked to documented design work, not simply chosen because:
“This size normally works on a four-bedroom house.”
Floor area and bedroom count do not replace proper heat-loss calculations.
EPVS
Independent validation can reduce the fear that the figures were invented to make the sale.
EPVS — the Energy Performance Validation Scheme — provides an additional consumer-protection layer for participating installers.
EPVS members are expected to use approved and standardised calculation methodologies for performance benefits and savings.
For heat-pump membership evidence, EPVS currently expects material including:
heat-loss calculations;
a heat-pump performance estimate;
hot-water calculations;
an emitter schedule;
the customer quotation;
and the associated savings illustration.
This does not mean the most expensive proposal automatically becomes correct.
It means the installer should be able to demonstrate that the performance and financial claims are based on a recognised method rather than unsupported sales promises.
A better way to choose an installer
Stop asking only “Who do I trust?” Ask “Who can show me the strongest evidence?”
Show me the heat loss
The heating requirement should be connected to the actual construction and condition of the property.
Show me the heat-pump performance
Ask what seasonal efficiency has been assumed and why.
Show me the new electricity demand
The solar calculation should include the additional electricity created by moving heating onto the grid.
Show me the solar yield
Roof direction, pitch and shading should support the generation estimate.
Show me what the battery does
The proposal should explain why that storage capacity and discharge power are appropriate.
Show me the assumptions
Tariffs, export rates, electricity prices and performance assumptions should all be visible.
“Bigger” is not overselling when the calculations show the house needs
it. The same calculations should also protect the customer from paying
for capacity they cannot realistically use.
A useful design philosophy
Under-provision can be just as disappointing as overselling.
Solar and home electrification are long-term infrastructure decisions.
A system that is materially too small can leave the homeowner:
buying far more expensive electricity from the grid than expected;
unable to cover planned EV or heat-pump demand;
with too little battery storage to exploit cheap tariffs;
or wishing that extra roof capacity had been used during the original installation.
But that does not mean “fill everything regardless”.
The objective is sufficient capacity with evidence behind it.
Properly designed does not mean smallest possible. It does not mean biggest possible either. It means enough.
Frequently asked questions
Heat pumps, solar and batteries answered.
Is solar worth installing with a heat pump?
Yes. A heat pump can materially increase annual electricity demand, giving solar more electricity consumption to offset. The important limitation is that heat demand peaks in winter while solar generation peaks in summer.
Should I double my solar panels when adding a heat pump?
Sometimes that may be close to the correct answer, but it should not be automatic. Calculate the home's existing electricity use, expected heat-pump electricity demand and realistic solar yield first.
How do I estimate heat-pump electricity use?
A useful approach is annual useful heating demand divided by the expected seasonal performance factor. A 10,000kWh heat requirement at an SPF of 3.0 would require approximately 3,333kWh of electricity.
Does an 8kW heat pump consume 8kW?
No. The 8kW figure normally refers to thermal heating output under specified conditions. Electrical input is lower and varies according to temperature, flow temperature and operating conditions.
Why can solar not run my heat pump all winter?
Winter heat demand is high when solar generation is at its seasonal low. Solar can still reduce annual grid imports, but summer surplus generation cannot simply be stored for several months.
Does a bigger battery make sense with a heat pump?
It can, particularly on a time-of-use tariff. The battery can store cheap off-peak electricity for use during more expensive periods, but it should be sized against the actual energy that needs to be shifted.
Can I charge the battery overnight and run the heat pump from it later?
Yes, where the tariff, inverter and battery controls allow it. Usable capacity, discharge power and round-trip losses should all be included in the calculation.
Should a heat pump run for long periods?
It can. Modern inverter-driven heat pumps are designed to modulate and may operate steadily for long periods. Repeated short cycling is generally more concerning than long controlled operation.
What happens if the heat pump is too small?
It may struggle to meet the building's heat loss in cold weather and may require more assistance from direct electric backup heating where that is installed.
Can a heat pump be too large?
Yes. Unnecessary oversizing adds cost and can contribute to poor cycling behaviour where the machine cannot modulate low enough for the building's demand.
Should the installer show me the calculations?
Yes. Heat loss, expected performance, annual electrical demand, solar generation and battery assumptions should all be capable of being explained from evidence rather than sales opinion.
What does EPVS add?
EPVS provides an additional validation framework for participating installers, requiring approved calculation methods and supporting documentation for performance and savings claims. It helps reduce reliance on unsupported sales illustrations.
Official guidance checked
Heat-pump design should be evidence-led.
This guide was reviewed against current material available in August 2026 including:
- MCS — Heat Pump System Performance and installation guidance.
- Energy Saving Trust — Heat Pump Installer Toolkit and heat-pump sizing guidance.
- EPVS — membership requirements, approved calculation methodologies and heat-pump validation evidence.
- Ofgem — time-of-use and off-peak electricity tariff guidance.
Heat-pump performance is highly property-specific. Actual heat loss, emitter design, flow temperatures, product performance data and the final installer design take precedence over the illustrative examples used in this homeowner guide.
Related Tom Solar guides
Understand the rest of the system.
Tom Solar view
Electrifying the heating changes the whole energy design.
A heat pump should not simply be bolted onto the old electricity calculation.
Once the heating moves onto electricity, panel count, battery storage, inverter capability and tariff strategy deserve another look.
Sometimes that produces a substantially larger system.
That should not frighten the homeowner if the design can show exactly why the capacity is required.
Do not buy the smallest system, the biggest system or the cheapest system.
Buy the system whose design can be justified by the property and whose calculations you can understand.