Here's the question behind the question: your solar panels do their best work at 1pm, but you want your bedroom cool at 11pm. A battery is the bridge between those two moments. The good news is that air conditioning is a surprisingly light overnight load — lighter than most people assume — so the battery that covers it is smaller and cheaper than you might fear. Let's do the actual maths.
The 1pm to 11pm problem
Solar generation peaks around midday and tails off through the evening. British heat does the opposite: bedrooms in this country are often at their most stifling at bedtime, because our homes are built to trap heat and they do the same job in a heatwave. So the hours you most want cooling are precisely the hours your panels have clocked off.
Without a battery, a big slice of what your panels generate gets exported to the grid for a modest payment, and you buy expensive evening electricity back to run the air conditioning. A battery stores the 1pm surplus and releases it at 11pm, so far more of your own generation gets used at home rather than sold cheap and bought back dear. We've covered the wider picture in will solar run my air conditioning?; this guide is about getting the battery size right.
What air conditioning actually draws overnight
The headline capacity on a split unit — say 2.5kW or 3.5kW — is its cooling output, not what it pulls from your battery. Modern inverter-driven units are efficient, and crucially they throttle down once the room reaches temperature. Overnight, a bedroom unit spends most of its time ticking over, not working flat out.
As a deliberately cautious planning figure, budget for a bedroom split drawing around 0.5–0.8kW averaged across a warm UK night — many nights it will use less. Run it from 11pm to 7am — eight hours — and the sums look like this:
- Light night: 0.5kW × 8 hours ≈ 4kWh
- Warm, sticky night: 0.8kW × 8 hours ≈ 6kWh
So a full night of bedroom cooling costs you roughly 4–6kWh of stored energy. Two bedrooms running together won't simply double that — units cycle down once rooms are cool — but budgeting 6–9kWh for two rooms on a genuinely hot night is a sensible planning figure. For what those kilowatt-hours cost in money terms, see our guide to air conditioning running costs in the UK.
Rules of thumb for sizing
A few things installers know that spec sheets don't shout about:
- Usable capacity is what matters. Batteries are advertised on nominal capacity, but a slice is held back to protect the cells. A "10kWh" battery typically gives you somewhere around 9kWh usable. Always size on the usable figure.
- The battery isn't only feeding the aircon. Your fridge, router, standby loads and the odd kettle all draw from it overnight too — call it another 1–2kWh across a typical night.
- Power output rarely bites here. Batteries also have a maximum continuous output rating, but a bedroom split drawing under 1kW is well within what any mainstream home battery delivers. It only becomes a question if you're running several rooms plus other heavy loads at once.
- Leave headroom for degradation. Batteries slowly lose capacity over their life. A system sized with 20% spare on day one is still doing its job comfortably a decade in.
So what size should you buy?
| Usable battery size | What it realistically covers overnight |
|---|---|
| ~5kWh | One bedroom split all night on most warm nights, with little left for the rest of the house |
| ~8–10kWh | One to two rooms of cooling plus normal household baseload — the sweet spot for most homes |
| ~13kWh+ | Multi-room cooling on hot nights with comfortable margin, or homes with high evening usage |
For most households pairing solar with a bedroom or two of air conditioning, something in the 8–10kWh usable range is the sensible answer. It covers the worked example above with margin, absorbs the household baseload, and still performs after years of gentle degradation.
Why slightly too big beats slightly too small
An undersized battery fails on exactly the nights you bought it for. It runs flat at 3am during a heatwave, the aircon switches to expensive grid electricity, and every one of those nights chips away at the savings case. An oversized battery, by contrast, is never wasted: the spare capacity soaks up more of your solar surplus in summer and, on time-of-use tariffs, can charge on cheap overnight rates in winter to run your splits as heating — remember a modern split is a reversible heat pump. The price difference between a marginal size and a comfortable one is usually a small fraction of the total project, so when in doubt, go one size up.
What it costs, roughly
As a rough guide, home batteries tend to run somewhere around £3,000–8,000 installed depending on capacity and brand, and a typical 4kW solar array a few thousand more — and under current rules, solar and battery installations are zero-rated for VAT until spring 2027. Sized and installed together, solar and battery as one system is usually cheaper and neater than bolting a battery on later, because the inverter and wiring get planned once.
The bottom line
A cool bedroom costs roughly 4–6kWh a night. A well-sized 8–10kWh battery covers that, plus the rest of the house, plus a margin for hot spells and the passage of time. If you'd like real numbers for your home rather than rules of thumb, get a free quote — we'll match you with one vetted local installer per trade who'll size the system properly, not a list of firms racing to call you first.