Why heating demand and solar generation sit almost opposite each other

A heat pump works hardest when the temperature difference between outside and inside is greatest. In a UK winter that means long run hours, colder source air and a lower coefficient of performance — so the unit draws more electricity per unit of heat delivered, at the same time as it is being asked for more heat overall.

Solar generation is governed by an entirely different variable: how much light reaches the panels. In midwinter the sun tracks low and shallow across the sky, daylight hours are short, and cloud cover is at its most persistent. A rooftop array that comfortably produces several hours of strong output on a June afternoon may deliver only a thin trickle across a December day, and much of that arrives in the middle of the day rather than the cold, dark early morning and evening when the heating load is heaviest.

So the two curves are not slightly out of step — they are close to inverted, both across the year and across the day. That is the single most useful thing to understand before you plan either technology, because it changes what you are actually buying solar for.

The shoulder seasons: where the overlap genuinely happens

There is a real overlap, and it is worth having. In the shoulder seasons — roughly March to May, and September into October — daylight is long enough and bright enough to produce meaningful solar output, while the house still has a modest heating requirement on cooler mornings and evenings.

This is the window where a heat pump can genuinely run on self-generated electricity for part of the day. Crucially, these are also the conditions in which a heat pump is at its most efficient: mild outdoor air means a smaller temperature lift, so each kilowatt-hour of electricity delivers more heat. Solar is contributing electricity at the exact moment the heat pump needs the least of it per unit of warmth. That is a favourable combination, and it is why shoulder-season overlap punches slightly above its weight.

What it is not, however, is a winter solution. Treat the spring and autumn overlap as a welcome bonus rather than the core reason to install solar.

How much of the heating can solar realistically cover?

Honestly: a modest share, weighted heavily towards spring and autumn. Anyone promising that a domestic rooftop array will carry a UK heating season is describing a different country's sunshine.

Midwinter heat comes from one of two places. The first is straightforward grid import — your heat pump runs on mains electricity, as it is designed to. The second, and the more interesting one, is stored cheap import: a home battery charged during a low-cost overnight tariff window and discharged during the expensive part of the day. In winter, that battery is mostly acting as a price-shifting device rather than a solar store, because there is very little solar to store. In summer it flips role and soaks up genuine daytime surplus.

That dual behaviour is the key planning insight. A battery bought purely as a "solar store" looks weak in January. A battery understood as a time-of-use tool that also stores solar looks sensible all twelve months. Same hardware, very different expectations.

The cold-weather paradox worth knowing

Here is a true fact that surprises most people, and it makes the mismatch even more poignant: solar panels are actually more efficient in cold weather than in hot weather. Photovoltaic cells lose a measurable amount of efficiency as their temperature rises, which is why a bright, crisp, cold day can produce a strikingly good instantaneous output figure while a sweltering July afternoon delivers slightly less per unit of light than you might expect.

The limiting factor in a British winter is therefore not temperature — it is light. Short days, a shallow sun angle and heavy cloud simply mean there is not much resource to convert, however willing the panels are. Related and equally useful: UK homes do still generate genuinely useful power on overcast days. Diffuse light is not zero light. It is not summer output, but it is not nothing either, and it is one of the reasons UK solar performs better over a full year than pessimists assume.

Both facts point the same way. Solar is a strong annual-energy proposition in Britain. It is a weak midwinter-heating proposition. Those are different claims and should be assessed separately.

Follow-up questions people usually ask next

**Does this mean solar and heat pumps are a bad pairing?** No. It means the marketing shorthand is wrong. They pair well across a full year because the heat pump raises your total electricity demand and solar reduces your total electricity bill. They just do not shake hands in January.

**Would more panels fix the winter gap?** Only marginally. Scaling up an array multiplies a very small winter number and a very large summer one, so you mostly create more summer export rather than more winter heating. Roof space and budget are usually better weighed against other measures.

**Would a bigger battery fix it?** A battery cannot store energy that was never generated. In winter it is charging from cheap grid import, not from your roof. Size it around your daily consumption and tariff window, not around a hoped-for winter yield. **What about east–west arrays?** Splitting an array across two aspects tends to widen the generation day and flatten the peak, which can help self-consumption across the shoulder seasons. It does not change the fundamental midwinter light shortage. **Is it better to install both at once or in stages?** Either can work. Installing together allows a single coherent electrical design; staging lets you observe real consumption data before sizing the array. What matters is that whoever specifies the second element sees honest data from the first.

Video transcript

Mo: So my solar panels will run the heat pump? RoboMo: Rarely. Heating peaks as solar collapses; they overlap only in shoulder seasons.

Mo: So in summer the solar just goes to waste? RoboMo: No. Surplus serves hot water, appliances and export — heating simply isn't asking for it.

Mo: So how much of the heating can solar actually cover? RoboMo: A modest share, mostly spring and autumn. Midwinter heat comes from the grid or stored cheap import. Mo: So my solar panels will run the heat pump? RoboMo: Rarely. Heating peaks as solar collapses; they overlap only in shoulder seasons. / Mo: So in summer the solar just goes to waste? RoboMo: No. Surplus serves hot water, appliances and export — heating simply isn't asking for it. / Mo: So how much of the heating can solar actually cover? RoboMo: A modest share, mostly spring and autumn. Midwinter heat comes from the grid or stored cheap import.

Wrapping up

The curves do not overlap, and pretending otherwise is how expectations get broken. A UK heat pump's demand peaks in the exact months a UK solar array's output collapses, and the genuine overlap is confined to the mild shoulder weeks of spring and autumn — where, helpfully, the heat pump is also at its most efficient. The rest of the winter is met from the grid or from stored cheap import, which is entirely normal and entirely fine. Meanwhile, the summer surplus that the heating never asks for is not wasted: it goes into hot water, household appliances, EV charging and export. Understood properly, solar and a heat pump are a sound annual pairing built on two different jobs rather than one powering the other. Understood badly, they produce a disappointed homeowner in February. The difference is nothing more than reading the monthly numbers before you sign anything.