A home battery makes its money at night, not in the sun

The first question people ask about a home battery is a solar question: is my roof any good, is my postcode sunny enough, how much will I actually generate?

It is the wrong question, and you can see why from the arithmetic. A battery does not generate anything. It buys electricity at one price and supplies it back to your house later, when the alternative would have cost more. What it earns is the difference between those two prices, multiplied by how many units it can move in a day. Sunshine is one way of filling it cheaply. A cheap overnight tariff is another, and unlike sunshine it is identical in Inverness and in Brighton, in December and in June.

The gap, from published rates

Checked 29 August 2026, on one supplier's published figures.

Octopus's Intelligent Octopus Go gives six hours of overnight electricity at 8p/kWh, between 23:30 and 05:30, for households with an eligible EV or chargepoint (octopus.energy/smart/intelligent-octopus-go, read 29 August 2026). On the same page, Octopus states that the average unit rate of a standard variable tariff under the July 2026 Ofgem price cap, paying by direct debit, is 26.11p/kWh.

That is a spread of roughly 18p a unit. A battery that moves 8 kWh a night from the cheap window into the expensive part of the day is displacing about £1.45 of electricity a day, against about 64p of electricity bought — a gross margin of a little over 80p a day, or somewhere near £290 over a year if it does it every night.

Every one of those numbers is an assumption and should be treated as one:

8 kWh a night moved. A real battery is limited by its usable capacity, its charge and discharge rates, and by whether your house actually consumes 8 kWh during the expensive hours. A battery that fills up and has nowhere to send the energy earns nothing that day.
Round-trip efficiency ignored. You do not get out what you put in. Losses in the region of 10% are ordinary, and they come straight off the margin.
Every night of the year. Nothing is out of service, nothing is derated in cold weather, the tariff does not change.
26.11p/kWh as the displaced price. That is a national average quoted by a supplier. Unit rates under the price cap are regional — the actual figure for your region is on Ofgem's price cap page, and that is the number to use, not this one.
Nothing here is a payback calculation. We have not touched the installed cost of a battery system, because that is a quote you get and not a figure we hold. Divide your quote by your own version of the annual margin and you have a payback period; do not take ours.

[NEEDS VERIFICATION: the 26.11p average unit rate read directly off Ofgem's own price cap page. Ofgem publishes it behind a region selector that we could not read automatically on 29 August 2026, so the figure above is Octopus's statement of it, cited to their page.]

Why this makes postcode the wrong question

Solar yield varies meaningfully across the UK — that is why our savings calculator asks for a postcode and looks the location up in PVGIS rather than using a national average. For panels, geography is a real input.

For the arbitrage above, it is not an input at all. The overnight rate and the day rate are set by tariff, not by latitude. A battery in a north-facing flat in Aberdeen with no solar whatsoever can run exactly the same cycle as one in a south-coast bungalow, and earn the same spread.

That has a consequence people find counter-intuitive: a battery can make sense without any solar at all, and it is sold that way. It also means a battery's case is much more sensitive to tariff availability than to weather. If you cannot get onto a tariff with a genuinely cheap window, the spread collapses and most of the case goes with it.

And the tariff that gives you the cheap window may have conditions. The 8p rate above is an EV tariff and requires an eligible vehicle or chargepoint. Others require a smart meter that can be read at half-hourly resolution. Check what you must have before you assume you can get the rate.

Where solar comes back in

Adding solar changes the maths in two directions and it is worth being clear about both.

It gives you a second, cheaper filling source. Electricity from your own roof costs nothing at the margin, so a unit stored from solar has a bigger spread than a unit bought at 8p.

And it reduces what you get paid for export. Every unit you put into the battery is a unit you do not sell. Whether that is a gain depends entirely on whether your export rate is higher or lower than the rate you would otherwise pay to import — and export rates in this market range from 4p to 20p a unit depending on who supplies you, which we have written about separately. If you are on a 20p export tariff and a cheap overnight import rate, storing solar rather than exporting it can genuinely lose you money.

What actually governs a home battery

Since this site publishes a register, it is worth being precise about which rules apply here, because the plug-in solar ones do not.

A home battery has to be fitted by an electrician. You cannot plug one in. The company that owns the cables in your street has to be told it is there — a notification for a small system, an application for a bigger one — and whoever fits it needs to be certified, because that certification is what decides whether you can get paid for exporting and whether you qualify for grants. The names for those things, in order: G98 and G99 for the network paperwork, and MCS for the installer's certification.

The 800W cap, the no-battery test and the plug-and-socket test that you may have read about all come from SI 2026/848, which created the plug-in solar route in August 2026. That instrument governs plug-in solar and nothing else. Applying its numbers to a home battery would be simply false: 800W means nothing at all in this category.

Checking the equipment on the tested list

We hold the storage side of the ENA type test register and you can search it. One thing to know before you do: the register lists the grid-interfacing device — the hybrid inverter, or the all-in-one unit that has one inside it — and never the battery module itself. One major storage manufacturer holds over a hundred rows on the register and not one of them is a rack-mounted module.

So a battery module that does not appear is unremarkable, and asking a seller for "the battery's register entry" is asking for something that does not exist. Ask about the inverter. We have set out what the register does and does not cover here.

The register also says nothing about your installation: not who fitted it, not whether it was notified, not whether the installer holds MCS certification.

Rates and price cap figures on this page checked 29 August 2026. Tariffs change; check them before you rely on them. Nothing here is a quotation, a guarantee, or financial advice.