The Short Answer
Whatever's on at the time. Solar feeds your consumer unit and your appliances use it before drawing from the grid; you don't choose what runs on it. A 4kW system makes about 3,800 kWh a year on our figures, but about 446 kWh in June and only 146 in December.
In a power cut, an ordinary system switches off: you need a battery set up for backup to keep anything running.
How Solar Flows Through Your House
How it flows
From sun to socket
Sunlight
Solar panels
Generate DC electricity
Inverter
DC to AC (mains-compatible)
Your consumer unit
The fuseboard your house already has
1st priority
Appliances
Drawn first, automatically
2nd priority
Battery
If you have one
3rd priority
Grid export
Whatever's left
Your appliances always pull from the nearest source. When the panels are producing, that's solar. When they're not making enough, your battery (if you have one) and then the grid make up the rest. No switch, no decision.
The panels make DC electricity; the inverter turns it into AC and feeds it into your consumer unit alongside the grid supply. Whatever your home is using at that moment takes the solar first. Any surplus charges a battery if you have one, then goes out through the meter to the grid. If the panels aren't making enough, the grid tops you up without you noticing.
What Your System Makes, Month by Month
What Can Your Solar System Actually Power?
Pick a system size and a month to see what it makes, with appliance equivalents. Based on a south-facing, unshaded roof in the Midlands.
This month
446kWh
Daily average
15kWh
Whole year
3800kWh
In June, that's enough for:
4014
boils
Kettle boils
558
cycles
Washing machine cycles
401
days
Fridge-freezer days
1472
miles
EV miles charged
4460
hrs
Hours of TV
299
hrs
Hours of heat pump running (at about 1.5kW)
12-month picture (your 4 kW system)
Click a bar to switch months
Monthly shape from PVGIS for the Midlands, scaled to our yearly figure for a south-facing, unshaded roof. PVGIS puts Inverness about 17% lower and Plymouth about 13% higher. Appliance figures are our estimates. On the government's SAP 10.2 self-use formula, a 4kW system in a home using Ofgem's typical 2,500 kWh a year uses about 26% of its solar as it's made, and about 56% with a 10 kWh battery (8 kWh usable, our assumption).
Summer and Winter
On the PVGIS profile we use (the Midlands, south-facing), 67% of a year's output comes from April to September and 18% from November to February. June makes about 3 times as much as December. So in summer a 4kW system covers a lot of daytime use and exports the rest. In December it makes a little on a bright day, and without a battery you'll buy from the grid most winter evenings whatever size you fit. Our guide to solar panels in winter has winter output for eight UK places.
| Month | 4kW system | A day |
|---|---|---|
| January | 146 kWh | 4.7 kWh |
| February | 208 kWh | 7.4 kWh |
| March | 340 kWh | 11.0 kWh |
| April | 423 kWh | 14.1 kWh |
| May | 449 kWh | 14.5 kWh |
| June | 446 kWh | 14.9 kWh |
| July | 461 kWh | 14.9 kWh |
| August | 412 kWh | 13.3 kWh |
| September | 352 kWh | 11.7 kWh |
| October | 243 kWh | 7.8 kWh |
| November | 176 kWh | 5.9 kWh |
| December | 146 kWh | 4.7 kWh |
Appliance by Appliance
What matters is how much power an appliance draws while it runs against what the panels are making at that moment. A 4kW system only reaches its full rating in bright, cool sun, so short bursts and small loads suit solar; long, heavy loads mostly come from the grid. Typical ratings (check the plate on yours):
| Appliance | Typical rating | On solar |
|---|---|---|
| LED bulb, router, laptop | Under 100W | Easily, in daylight |
| Fridge-freezer | Around 100 to 150W, cycling | In daylight; a battery covers the night |
| Washing machine, dishwasher | Up to about 2kW while heating | Run them around midday |
| Kettle | About 3kW for a couple of minutes | Partly, on a bright day |
| Electric shower | About 8.5 to 10.5kW | Mostly grid |
| Electric car charger | About 7kW | Only partly, unless it can follow the solar |
Power Cuts
When the grid goes down, a grid-connected inverter switches itself off. This is a safety rule in the network connection standards (G98 up to 3.68kW on a normal single-phase supply, G99 above): engineers working on the lines need them to be dead. So with an ordinary system your home goes dark with everyone else's, even with the sun on the panels. The exception is a battery system with a backup or "emergency power" function, which separates your home from the grid and keeps chosen circuits running. If that matters to you, ask for it by name and ask which circuits it will cover.
What a Battery Changes
On the government's SAP 10.2 self-use formula, a 4kW system in a home using Ofgem's typical 2,500 kWh a year uses about 26% of its solar as it's made, and about 56% with a battery (8 kWh usable, our assumption).
Without a battery, the electricity you don't use is exported. It earns 12p a kWh on Octopus's Outgoing tariff (or around 5p on a typical standard export deal, our estimate), and you buy it back in the evening at 26.32p. A battery stores the surplus for later, so each stored kWh is worth 14.32p more than one exported on Outgoing (about 21p on a 5p deal), less what the battery loses in storing it. Whether that pays back is a separate question: our battery guide runs the numbers.
Electric Cars
A typical home charger draws about 7kW, more than a 4kW system can make even at noon in June, so the grid supplies the rest. There are two fixes, and we think the second is simpler: a charger that follows your solar surplus and charges more slowly from it, or an overnight tariff such as Intelligent Octopus Go, 8p a kWh from 11:30pm to 5:30am. Our solar and EV guide compares them.
Heat Pumps
A heat pump uses most in winter, when panels make least, so solar covers part of its spring, summer and autumn use (much of it hot water) and little of its winter heating. Our solar and heat pump guide puts numbers on it.
What Solar Can't Do Well
- An electric shower. It draws more than a typical home system makes.
- Winter heating. The sun isn't there when you need it.
- Overnight loads without a battery.
- A standard car charger without solar-following control.
- Anything in a power cut without a backup-capable battery.
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