The short answer
Solar panels can supply some or all of an air conditioner's electricity while they're generating, which suits cooling on sunny days far better than heating in winter. Solar electricity you use yourself costs you any export payment you'd otherwise have earned.
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Rooms tend to overheat on the same sunny days when solar panels generate the most, so cooling and generation often line up. How much of an air conditioner's use your panels actually cover comes down to timing, the size of both systems and the weather.
How your home uses solar electricity
Solar panels feed your home first. Whatever is switched on at that moment runs on solar power, and the balance shifts moment by moment:
- If the house needs more than the panels are making, the rest comes from the grid.
- If the panels make more than the house needs, the surplus goes to a battery if you have one, or is exported to the grid.
A passing cloud can cut the panels' output quickly, and the grid makes up the difference without you noticing.
An air conditioner needs no special connection to use solar electricity. Wired into your home's electrics like any other appliance, it draws on the panels' output in the same way as a kettle or a fridge, and the outdoor unit doesn't need to be anywhere near the panels.
What solar electricity is worth
Using your own solar electricity isn't quite free. If you're paid for what you export, every kWh the air conditioner takes from the panels is a kWh you aren't paid for.
| Situation | The unit runs on | What that electricity costs you |
|---|---|---|
| Panels making more than the home uses | Solar | The export payment you give up |
| Panels making less than the home needs | Solar, topped up from the grid | Your import rate, for the grid part |
| Evening or night, no battery | The grid | Your import rate |
| Battery charged earlier from solar | The battery | The export given up, plus storage losses |
Say the unit uses 3 kWh during an afternoon when your panels' surplus covers all of it. At an example export rate of 10p per kWh, you give up 30p of export payments. Buying the same 3 kWh from the grid at an example import rate of 25p per kWh would cost 75p. Both rates vary between suppliers, so check your own. If you aren't paid for exports, surplus you don't use earns nothing.
Matching use to generation
Panels generate most in the middle of the day and in summer. East- or west-facing panels shift some of that output towards the morning or the late afternoon.
- Rooms that heat up during the day are the best match, since the unit works hardest while the panels are producing.
- Rooms that catch the evening sun, and bedrooms at night, need most of their cooling after generation has tailed off.
- Pre-cooling means running the unit while generation is strong to bring a room down before the evening. It uses some extra energy, because a cooler room gains heat faster, and it works best in a room that holds its temperature with blinds and doors closed. A timer or schedule makes this easier.
- Other appliances compete for the same surplus. A dishwasher, washing machine, electric car charger or hot water diverter may already be using it.
If your solar system has monitoring that shows when you're exporting and when you're importing, it helps you judge when to run the unit.
Heating in winter
Panels produce much less in winter, when days are short, the sun is low and cloud is common. Heating is needed most in early mornings and evenings, outside generation hours, so much of the electricity an air conditioner uses for heating will come from the grid or a battery.
Mild, sunny days in spring and autumn give a better overlap, when a little heating during the day can run on solar.
Panel size and batteries
A solar system's size is given in kWp, its peak output under standard test conditions. Real output is usually lower and changes through the day, so compare the unit's draw with what your monitoring shows the panels actually producing rather than with the kWp rating. How much does air conditioning cost to run? explains how to estimate the unit's draw.
A home battery can store daytime surplus for cooling in the evening or at night. Some energy is lost in charging and discharging, though, and the battery's capacity is shared with everything else you use after dark. Some households also charge a battery from the grid during the cheaper hours of a time-of-use tariff.
Related guides
- How much does air conditioning cost to run?Multiply the electricity it uses, in kWh, by your unit rate. How much a unit uses depends on the room, the weather, the temperature you set and its efficiency, so estimate it from its average power draw and the hours it runs.
- Time-of-use tariffsTime-of-use tariffs charge different unit rates at different times of day. Whether one suits you depends on when your air conditioning and the rest of your home use electricity, and how those rates compare with what you pay now.
- Timers and schedulesMost systems have on and off timers that count down a set number of hours, and many controllers and apps add clock times and weekly schedules. Used well, they have a room comfortable when you need it and the system off when you don't.
- Air conditioning and carbon emissionsAn air conditioner's effect on the climate comes from two places: the electricity it uses, and its refrigerant if it leaks. Using less electricity, running it when the grid is cleaner and keeping the system leak-free all reduce it.