A battery without solar: does buying cheap and using it later pay?

The one advantage over a solar battery

A battery attached to solar has a problem the arithmetic on our sizing page makes plain: for a good part of the British winter there is not enough surplus generation to fill it. Capacity sits idle exactly when electricity is dearest.

A standalone battery does not have that problem. The cheap window comes round every night of the year, in December as much as in June. So per kilowatt-hour of capacity purchased, it does considerably more work.

That is the case for it, and it is a real one. What it gives up is the free half of the transaction: solar generation costs nothing per unit once installed, whereas every unit a standalone battery discharges was bought.

The arithmetic

Two things go into it and one comes out.

What you pay to fill it. Your off-peak unit rate, times the units you have to buy — which is more than the units you get back, because of round-trip losses.

What you avoid paying. Your peak or standard rate, times the units the battery delivers.

Written out, for a battery delivering D kilowatt-hours a year at round-trip efficiency η:

annual value = D × peak rate − (D ÷ η) × off-peak rate

Take a 5 kWh usable battery cycled most nights — call it 350 cycles, 1,750 kWh delivered a year — at 90% round-trip efficiency, avoiding electricity at the Ofgem price cap rate of 26.32p. The off-peak rate is yours to supply, so here is the shape of it:

The peak rate here is the price cap’s flat unit rate, which is a sourced number but not necessarily your number: on a time-of-use tariff the daytime rate is set by the supplier and may sit either side of it. Put your own two rates into the formula. The point of the table is the slope, not the rows.

The charge window, and why the power rating bites

A solar battery is limited by how much surplus there is. A standalone battery is limited by something more mundane: how much it can take in before the cheap rate ends.

Maximum overnight charge = charge power × window length.

At 3 kW over a seven-hour off-peak period, that is about 21 kWh — comfortably more than a domestic battery holds, so capacity is usually the binding limit. But charge ratings vary, and a battery that charges slowly against a short cheap window can greet the morning part-full, which quietly removes a share of the return you calculated.

Ask for the charge power rating, not just the discharge one. They are not always the same number.

Sizing is a different question without solar

With solar, useful size is capped by the daily surplus. Without solar, that ceiling disappears and only the demand-side one remains: you cannot save money on electricity you were not going to buy.

So the sizing question becomes: how much electricity do you use outside the cheap window? A household using 2,900 kWh a year, of which a substantial part falls in expensive hours, has a ceiling set by that, not by anything to do with sunshine.

In practice this makes standalone batteries sensibly larger than solar ones for the same household — and it makes the warranty arithmetic more pressing, because a battery cycling most nights of the year reaches a cycle or throughput limit far sooner than one cycling on sunny afternoons.

The SEG has nothing for you

Worth stating plainly, because it comes up. Ofgem’s Smart Export Guarantee covers solar PV, wind, micro-CHP, hydro and anaerobic digestion. Electricity storage on its own is not an eligible technology.

A standalone battery therefore has no export income. Everything it earns, it earns by reducing what you buy. If a sales pitch implies otherwise, that is the moment to stop.

If you do have solar and are considering grid charging, the rules are different again and they can cost you export payments you already receive — see will a battery affect my FIT or SEG payments?

VAT

A standalone battery counts. HMRC’s notice lists “the installation of a standalone battery for storing electricity from the grid” among the qualifying installations, alongside retrofitting one to existing microgeneration and installing one that does both.

Before you decide

• Get your actual peak and off-peak rates, and check whether the tariff requires a smart meter and half-hourly settlement — see half-hourly settlement.
• Check how much of your consumption genuinely falls outside the cheap window. If most of your usage is already at night, there is less to arbitrage than you think.
• Put the round-trip efficiency into the formula rather than ignoring it.
• Check which warranty limit your intended cycling reaches first.
• Remember the tariff can change and the battery cannot. A twenty-year asset is being bought on the strength of a tariff nobody has promised will exist next year.