Your battery stores the electricity. Your inverter decides how fast it moves.
The battery is the tank and the inverter is the pipe. It sets how quickly cheap overnight electricity can get in, and how much of it can come back out while the house is busy.
Size the pipe wrong and the tank never fills before the cheap window closes. This page explains the two numbers that matter and how we choose between them.
The inverter on the wall, the battery below it. The inverter's rating caps both.
To refill a 20 kWh battery from 10% through a 3.68 kW inverter, before charging losses
18 kWh of charge divided by 3.68 kW of inverter power. Charging losses add about half an hour, so it fills inside a six-hour cheap window and falls slightly short in a five-hour one. Checked 18 September 2026. Your own timing depends on the length of your tariff's window and how empty the battery is when it opens.
The mechanism
Two numbers, and they measure different things
A battery is rated in kilowatt-hours, which is how much electricity it can hold. An inverter is rated in kilowatts, which is how much it can move in any one moment. The first is the size of the tank; the second is the width of the pipe.
The inverter does the converting too. Grid electricity arrives as AC, the battery stores DC, and everything passing between them goes through the inverter in both directions. Its kilowatt rating caps the charge going in overnight and the power coming back out during the day.
So a 3.68 kW inverter can put about 3.68 kWh into the battery each hour, and no more, however large the battery behind it. That single fact decides most of what follows.
Grid, inverter, battery, home. Everything passes through the middle box.
The objection
A bigger battery does not automatically need a bigger inverter
The inverter's size matters for two reasons only. The first is whether the battery can refill inside one cheap-rate window. The second is whether the house sometimes draws more power at once than the inverter can supply.
For most homes neither bites. A 20 kWh battery refills through a 3.68 kW inverter in just under five hours, and a 3.68 kW output covers the ordinary evening of lights, cooking and television. Anything the house draws above that comes from the grid as normal, so the lights stay on and only the saving shrinks.
The larger inverter earns its cost when a 30 kWh battery has to refill in the same window, or when an EV charger, a hot tub and an electric hob run together and you want the battery, not the grid, to carry them.
The cheap window is fixed. The inverter decides how much fits inside it.
The choice
Two routes, and the difference is paperwork
The 3.68 kW line is not arbitrary. It is 16 amps per phase, the threshold below which fully type-tested equipment can be installed first and the network operator notified afterwards. Above it, the operator has to agree before anything is fitted, and it may approve, impose an export limit, or ask for changes.
A larger inverter is also not the only route to a faster refill: two smaller inverters on one battery bank are sometimes the cheaper answer, and the recommendation says so when they are.
| Fit, then notify | Approval first | |
|---|---|---|
| Battery | 20 kWh | 30 kWh |
| Inverter | 3.68 kW | 6 kW |
| Fills in a six-hour window | Yes | Yes |
| Network operator | Told within 28 days of fitting | Must agree before fitting |
| Supplies at once | 3.68 kW; above that the grid tops up and nothing switches off | 6 kW |
| Suits | Up to about 20 kWh of daytime use, or a wish to have it in soon | More than that every day, a heat pump, or frequent power cuts |
A 3.68 kW inverter cannot put more than about 20 kWh into a battery in six hours, so a bigger battery on the smaller inverter is capacity that never charges. 40 and 50 kWh systems need a three-phase supply.
The installer pays us a percentage of the installed price when the work is finished. You pay the installer directly and no money comes to us. The percentage is the same whichever system you choose, but the bigger system costs more, so it earns us more. That is why your estimate shows both, with the sums.
How we would work it out
We size the inverter from your bill, not from the battery
Send a recent bill and a photo of your fuse board
The bill tells us how much you use and when. The fuse board (your consumer unit) tells us what is already connected, which counts toward the 16-amp threshold.
We model your cheap window
Your tariff's overnight hours, your consumption and the battery size that fits them set the inverter rating needed to refill in one night.
Your recommendation names the inverter and the route
Before anyone visits, you know the battery size, the inverter rating, whether it is G98 or G99, and what that does to the timescale.
The survey confirms it
The installer partner's visit checks the recommendation rather than upgrading it, and they submit the notification or the application. Before they leave, have them show you the charging schedule running.
Your bill, read properly rather than guessed at.
Questions people ask
The four things to know before you compare quotes
Can the battery charge and power the house at the same time?
Yes, but the inverter's rating is shared between the two. If it is putting 3.68 kW into the battery overnight and the tumble dryer comes on, the dryer draws from the grid at the overnight rate. That is fine, because the overnight rate is the cheap one.
Will a 3.68 kW inverter limit what I can run in the evening?
It limits what the battery can supply at once, not what the house can use. A kettle and an oven together can pass 5 kW; the inverter covers its 3.68 kW and the grid covers the rest at the daytime rate. Most evenings sit well under the limit.
Does G99 approval delay the installation?
It can, because the network operator has to agree before anything is fitted, and it may approve, set an export limit or ask for changes. Your recommendation tells you which route applies so the timescale is not a surprise. Nobody can guarantee the outcome.
Do I need solar panels for any of this to work?
No. A battery and inverter on a time-of-use tariff save money on their own by buying overnight and using through the day. Solar changes the sizing, so tell us if you have it or plan to.
What if my quote lists a bigger inverter than you recommend?
Ask what it is for. A higher rating costs more and can push you from G98 into G99 without adding any saving if the smaller one already refills your battery in the window. We will look at the quote with you.
How big an inverter would your house need?
Four questions. No contact details until there is an estimate to send you, and the workings beside it when there is.