Best solar battery storage UK 2026: how many kWh you need, and what it really saves against your export rate

A storage battery keeps what the roof makes in the day for what you spend at night. Everyone agrees that far. The argument starts at what each stored kilowatt hour is worth, and that is where almost every sum in circulation goes wrong.

If you are on a Smart Export Guarantee tariff, the kilowatt hour your roof exports right now is already taking money off your bill. Storing it does not save you the price you buy electricity at: it saves you the difference between that price and what you are paid to export. Ofgem's price cap for 1 October to 31 December 2026 puts the electricity unit rate at 26.32p per kWh. Which? checked the SEG tariffs in April 2026 and found the range runs from 1p to 25p. So the value of storing a kilowatt hour in a British home is somewhere between 25p and about 1p, depending on nothing but which tariff you are on.

And here is the turn that changes the whole calculation, and that we have not seen written anywhere: the best export rates require you to have a battery. In that same Which? table, Octopus Intelligent Octopus Flux pays 23p per kWh to customers with solar and a battery, and Good Energy's Solar Savings Exclusive pays 25p to customers whose panels and battery were installed by Good Energy Solar, while Octopus's standard SEG rate, open to anyone, is 4.1p. Read those two facts together and the battery stops being a machine that stores kilowatt hours and becomes the thing that moves you from a 4p tariff to a 23p one. Which is worth far more than anything it saves by storing.

This is the part of the arithmetic that decides whether buying the battery yourself is clever or expensive, and it is a tax rule rather than a spec. Since 1 February 2024, "batteries for storing energy converted from electricity" are on the list of energy saving materials in HMRC's VAT Notice 708/6, and installing one in a home is zero rated. The relief runs from 1 May 2023 to 31 March 2027, and the notice is explicit about what happens next: "From 1 April 2027 onwards these will revert to the reduced rate of VAT of 5%."

The catch is in the same notice, and it is the sentence nobody quotes. The zero rate is on the installation, not on the box. HMRC writes it plainly: "if you supply energy-saving materials without installing them your supply will be standard-rated. For example, the sale of energy-saving materials by a retailer is always standard-rated." Buy a battery module from a marketplace and you pay the standard rate on it. Have an installer supply and fit the same battery and the whole invoice is zero rated.

There is a middle path, and the notice covers it too: the zero rate applies to "the services of installing energy-saving materials in residential accommodation", including the case "where a customer employs a business to install energy-saving materials that the customer purchased directly from a retailer". So the labour can be zero rated even on a battery you bought yourself. The tax on the hardware cannot.

The capacity that fits you does not come from the square metres of roof or the kilowatts peak installed: it comes from what you use once the sun has gone. That is the energy the battery has to hold, and everything beyond it is capacity you pay for and never touch.

The calculator does that sum with four figures that are on your bill and in your contract, and then carries it to the end: the hours of household running that capacity covers, what it saves a year with the export rate already taken off, and how long it takes to pay back. Under the result is the arithmetic, step by step, so you can redo it on the back of an envelope. It opens on a 250 kWh month with 40% of it after dark, the October 2026 capped unit rate and a 15p export tariff, which is roughly where a mid range British household sits.

The battery is the cheap half. The other half is the hybrid inverter if the one you have is not one, the protection it has to sit behind, and the two pieces of paper that decide whether the thing is connected legally and whether you keep getting paid to export. A cabinet of cells bought online includes none of that, and it is the difference between the price of the machine and the price of having it working.

The connection rule runs on a single threshold. A system that can export up to 3.68 kW per phase falls under G98: you can fit it and then notify your distribution network operator, within 28 days. Above that it is G99, which means an application to the DNO and approval before installation, with the usual wait measured in weeks rather than days. The classification follows the inverter rating, not the battery capacity: a 20 kWh battery behind a 3.68 kW inverter is still a G98 job, and a modest battery behind a big hybrid inverter is not.

The second piece of paper is the one with money attached. Smart Export Guarantee eligibility runs on certification: the installation and the installer have to be MCS certified or equivalent, and suppliers ask to see the certificate when you sign up. If your panels are certified and you then add a battery yourself, the sensible order of operations is to ask your supplier what that does to your export tariff before you buy the battery, not after. On the numbers in the first section of this guide, an export tariff is worth more a year than the battery saves by storing.

Not everyone looking for a battery has a hybrid inverter waiting for it. Anyone with a balcony array and a microinverter has another option: storage that sits between the panels and the microinverter, or works in alternating current, and never enters the consumer unit. What you gain is obvious, because it installs like an appliance. What you give up is real too: storing and returning in alternating current adds another conversion, it does not back up the house in a power cut, and it costs more per kilowatt hour than a cabinet module.

And in Great Britain there is now a third thing to check, which is three weeks old at the time of writing. The Plugs and Sockets etc. (Safety) Regulations 1994 and Electricity Safety, Quality and Continuity Regulations 2002 (Amendment) Regulations 2026, SI 2026/848, were made on 16 July 2026 and came into force on 27 August 2026. They are what opened the door to plug-in solar in England, Wales and Scotland. Read the definition carefully, though, because a plug-in battery is not obviously on the other side of that door.

A battery does not age with the calendar, it ages with the number of times it is emptied and refilled. That is why manufacturers publish cycles and not years: the same machine lasts twice as long in a house that uses it half the week as in one that empties it every night.

The problem with this catalogue is what is missing next to that figure. Of the 33 listings we counted on 8 September 2026, eleven publish cycles, and the figures run from 3,000 to 15,000. At one cycle a day that is between eight and forty one years. Forty one years is longer than the inverter lasts, longer than the panel warranty and longer than anyone can be held to a promise, which is the first clue that the number is a laboratory maximum rather than a service life.

For a grid connected home with a hybrid inverter, the short answer is a 48 volt battery (51.2 real) of lithium iron phosphate, with its management system inside and able to talk to the inverter. The 12 and 24 volt ones store energy just as well, but that is the voltage of a van, a shed or an irrigation pump: a domestic hybrid inverter does not connect to them.

And there is a third figure almost nobody looks at that decides as much as the other two: communication. If the battery and the inverter do not understand each other, the inverter does not know how much charge is left inside and works blind, which in practice means it either stops short of full or pushes the cells further than it should. Three of the 33 listings we counted name it.

The question almost always arrives through the price: a lead battery, AGM or gel, costs considerably less per nominal kilowatt hour than a lithium one. The sum changes when you look at usable capacity, which is the part you can take out without shortening its life.

The technical guidance published by retailers in the sector puts the recommended depth of discharge for lead at around 50% and for lithium at 80 to 90%. Applied to the lead battery in this guide: 1.28 nominal kilowatt hours become about 0.64 usable. With that correction the cost per usable kilowatt hour stops being half of lithium and lands somewhere close to it, and you need twice as many boxes on the wall to get there.