There is a certain kind of British stubbornness about air conditioning. It is what other countries have — hot countries, unserious countries. Here we open a window, put a fan on, and complain. But sit with the actual temperature record for a moment and the joke starts to wear thin. As I write this on 8 July 2026, our own Daikin air-to-air system is quietly running in cooling mode, keeping the house at a civilised temperature. It is 30°C outside, the roof is producing more power than we're allowed to export, and the cooling is soaking up the difference — electricity that would otherwise simply be clipped and lost. In the truest sense, it is running on sunshine we couldn't have used any other way.
So we asked a simple question with a data answer. Over the last five years, how many days would you actually have wanted air conditioning in southern England? We pulled the daily maximum temperature for the Heathrow area — a good proxy for lowland south-east England — from the ERA5 climate reanalysis, every day from January 2021 to now, and counted.
What counts as an air-con day?
We used a threshold of 25°C daytime maximum. That is not an arbitrary number. Below about 25°C, most reasonably insulated homes stay comfortable with shading and a bit of night ventilation. Push past 25°C — especially two or three days running, when the fabric of the house has soaked up heat and stops shedding it overnight — and comfort drops away fast. Bedrooms in particular become a problem: overnight minima matter more than the daytime peak for sleep, and a run of hot days drags the nights up with it. So "days at or above 25°C" is a fair, conservative stand-in for "days you'd reach for cooling."
On that measure, here is the last five years, plus 2026 to date.
Figure 1 — Hot days per year, Heathrow area (2021–2026). Green bars count days with a maximum at or above 25°C; the amber overlay counts the genuinely hot days at or above 30°C. 2026 is only to 7 July and has already cleared 26 days above 25°C and 12 above 30°C. Source: Open-Meteo / ERA5 reanalysis, daily maximum 2 m air temperature.
| Year | Days ≥25°C | Days ≥28°C | Days ≥30°C | Peak | Character |
|---|---|---|---|---|---|
| 2021 | 13 | 4 | 0 | 29.5°C | Mild |
| 2022 | 31 | 12 | 6 | 37.0°C | Record heat |
| 2023 | 21 | 9 | 2 | 31.2°C | Warm |
| 2024 | 15 | 5 | 1 | 31.8°C | Average |
| 2025 | 35 | 18 | 8 | 33.0°C | Hot & long |
| 2026* | 26 | 17 | 12 | 34.3°C | Running hot |
*2026 to 7 July only — the main summer is still ahead. Note how the "≥30°C" column, effectively zero a decade ago in an average year, is now routinely mid-single-digits and already 12 this year.
The headline is not a smooth, tidy upward line — weather never is. 2021 was mild (13 days), and 2024 was fairly average (15). But the hot years are getting hotter and longer: 2022 gave us the country's first-ever 40°C reading nationally and 31 days above 25°C locally; 2025 topped even that for sheer duration, with 35 days above 25°C and 18 above 28°C. And 2026, before the peak of summer has even arrived, has already banked 12 days above 30°C — more than 2022, 2023 and 2024 combined. The five full years 2021–2025 average 23 hot days a year. That is a working month of weather where cooling earns its keep.
The season is stretching, not just the peaks
Averages hide the more interesting shift, which is when the heat now lands. If you tally every hot day across all six years by calendar month, the old idea of a short August peak simply isn't there any more. The hot days are spread from May clean through to September, and the single biggest month is June.
Figure 2 — When the hot days fall (all days ≥25°C, 2021–2026 pooled). The heat is no longer an August story. June is the busiest month, closely followed by July, with a long shoulder through May and September. A cooling system now has a genuine four-to-five-month duty season, not a fortnight.
This matters for the economics. A cooling appliance that only pays off during two weeks in August is a hard sell. One that does useful work from May to September — and, crucially, runs the same hardware backwards to heat your home for the other seven months — is a completely different proposition. Which brings us to the machine.
Why air-to-air, specifically
Most of the heat pump conversation in Britain is about air-to-water systems: they take heat from outside air and put it into your radiators and hot-water cylinder, replacing a gas boiler. They are the right answer for whole-home heating and they are what the Boiler Upgrade Scheme grant is aimed at. But they don't cool. Water-based radiator systems aren't designed to run cold (you'd get condensation and clammy rooms), so an air-to-water heat pump gives you winter warmth and nothing for summer.
An air-to-air heat pump is the other branch of the family — and it is, mechanically, exactly what people mean by "air conditioning." It moves heat between outside air and indoor air via wall-mounted units. The single, decisive difference from the aircon of cliché is that it is reversible: the same refrigerant cycle that dumps heat outdoors in July pulls heat indoors in January. One box on the wall, two jobs, and a seasonal efficiency (SCOP/SEER) that means you get several units of heat or cooling for every unit of electricity.
Our Daikin, soaking up power we'd otherwise lose
Here is the part that makes it genuinely painless — and it's better than "free." Our grid connection has a G100 export limiter set to 6 kW: the maximum we're allowed to push back to the network. On a bright day like today, the array can produce far more than that — we peaked at around 10 kW. If we did nothing with the difference, the inverter would simply throttle the panels down to 6 kW and that extra ~4 kW would never be generated at all. It isn't exported for pennies; it's clipped and lost.
So the trick is to give that power somewhere to go behind the meter. Today the 6 kW went to the grid and the other ~4 kW went into the EV charging and the Daikin cooling — load we chose to run precisely so the array could keep producing at full tilt. The cooling isn't just running on surplus sunshine; it's running on sunshine that would otherwise have been curtailed to nothing. Air-to-air cooling demand and peak solar land at exactly the same time of day, which makes it an almost perfect sink for clipped generation.
It is the tidiest bit of synergy in the whole low-carbon home: the hotter and brighter it gets, the more cooling you want, and the more otherwise-wasted power you have to do it with.
So — do you actually need it?
Honestly, "need" is a strong word for a country that managed without for centuries. But the case has shifted from luxury to reasonable, and the numbers above are why. If a system does real work for three or four weeks a year (rising), extends your comfortable use of the house, protects sleep during heatwaves, and — this is the clincher — also heats your home the rest of the year off the same kit, then it stops looking like an indulgence and starts looking like sensible infrastructure. Add solar, and the summer running cost rounds to zero.
A few honest caveats before you rush out. First, this is a single location (lowland south-east England); Scotland, the north and exposed coasts see far fewer hot days, and the case is correspondingly weaker there. Second, ERA5 is a gridded reanalysis, not your back garden — treat the day-counts as robust trends, not to-the-decimal truth. Third, our 25°C threshold is a proxy for comfort, not a law of nature; a well-shaded, heavyweight house copes higher, a west-facing flat with big windows suffers lower. And fourth, air-to-air currently sits outside the Boiler Upgrade Scheme grant, so unlike air-to-water you're paying the full cost up front — though the dual heating-and-cooling use changes that maths.
The bottom line
The British summer has quietly crossed a line. Two dozen days above 25°C a year — and rising — is no longer a heatwave, it's a season. You can meet it by suffering with a fan, or with a single reversible box on the wall that cools you in July, heats you in January, and — if you have panels — does the summer half on sunshine you'd otherwise have thrown away. Ours is running as I finish this sentence. The window, for once, is shut.