Air-conditioning is the appliance that can match solar panels best and the one that most easily makes them look silly. Turn it on at 14:00 and you are eating kWh that this August were worth ~€0.045 on the grid and ~€0 as surplus. Leave it for 21:00 and you are buying at ~€0.31. Same cool air. Different bill. Summer is ending: this is the moment to write the rule down, not to sell you four extra modules “for August”.
This month’s prices are in record solar and the 21:00 bill. Here is the machine that moves that curve most in an east-coast house.
Three homes, the same August
Home office with air-con from 12:00 to 17:00. Panels at full tilt, air-con too. High self-consumption. Surplus shrinks. This is the textbook case: the sun eats the cold. You do not need to invent anything. Maybe not even a battery.
House empty in August, air-con off, alarm and fridge. The panels export. This month, exporting at 15:00 paid almost nothing. A large roof for a house that spends August at the beach is not paid back by surplus. It is paid back the rest of the year — or it is not. Sizing “for the holidays” is the classic mistake.
A family that gets in at 19:30, sets 24 °C and the hob. The sun is already falling. The grid is dear. This is the house August’s headline hurts. Here the debate is habits, pre-cooling, or a battery — not “more panel power” blind.
What does work, with no new hardware
- Pre-cool at noon. Drop to 25–26 °C between 13:00 and 16:00, blinds down, and the house stays inertially cool. At 21:00 the unit works less, or not at all.
- Do not set 21 °C at 22:00 “because I’m home now”. Each extra degree is extra power in the expensive hour.
- Schedule, don’t improvise. A split with wifi or a thermostat that starts at 13:00 on weekdays does more than an extra module on the north pitch.
- Hot water in daylight. If the tank is electric, charge it with sun, not at 07:00.
None of this is trade-show home automation. It is using the machine when the roof produces. In KNX is not always the answer we already said a well-set schedule beats an expensive bus.
What not to do
- Do not oversize the array for August 2026. One hot month does not justify three extra modules if in October the house is empty at noon. The extra becomes surplus at €0.
- Do not believe “with panels the air-con is free” at 23:00. At night the air-con is the grid, unless you have a battery or you pre-cooled.
- Do not switch the panels off in your head because the bill rose. If the air-con was on at 15:00, you already saved those hours. What hurts is the rest. Different problem, different fix.
And a battery, for the 21:00 air-con?
It makes sense if, besides night air-con, there is a hob, a tank or a car, and by day you systematically have energy left over. Then you are not “buying a battery for the split”: you are shifting a block of kWh every hot day. The spreadsheet for that shift, with this month’s prices, is in virtual versus physical.
If the only fat load is a 1 kW split for three hours at night, do the pre-cooling sum first. Sometimes the habit is cheaper than the cabinet.
September, when the year is decided
Summer is ending and the air-con stays. September on the east coast still wants cold in the mid-afternoon and is no longer August sun. It is the honest month to look at the curve: which hours you bought, what you self-consumed, whether a string sagged in July without you seeing it. If the plant produced as if panels were missing, that is not the air-con: it is a fault. We look at that in monitoring.
The short rule: air-con at 14:00 is an ally; air-con at 22:00 is a bill. Panels do not change the physics of the split. They change the hour at which it pays to switch it on.
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