How the Battery Storage Calculator Works

The calculator takes your annual electricity use, whether you have solar, your tariff type and when you are usually at home. From these it recommends a battery capacity in kilowatt hours and models the savings from two sources, storing surplus solar generation for evening use and charging from grid at cheap overnight rates to avoid buying at peak prices.

The assumptions behind numbers reflect current UK conditions, grid electricity around 26 to 27p per kWh under Ofgem cap, Smart Export Guarantee rates between roughly 4p and 16p depending on supplier and off peak windows around 7 to 8p on smart tariffs.

What Size Battery Do You Need

Capacity should match your evening and overnight consumption, not the biggest unit your budget allows:

• 5 kWh: couples and small homes, covers a typical evening of cooking, lighting and TV
• 9 to 10 kWh: the most popular size for families with a 4kWp solar array, bridges evening through to morning
• 13 kWh and above: homes with a heat pump, an EV or a large solar array, where evening loads drain smaller batteries fast

To size from your own figures, work out your average daily use from a year of bills, then estimate share used between late afternoon and morning. That evening and overnight figure is what the battery needs to hold. Remember usable capacity is slightly less than headline figure because batteries reserve a margin, known as depth of discharge, to protect their lifespan.

How Much Can a Battery Save

With solar saving per stored kWh is gap between your import rate and your export rate. Importing at 27p and exporting at 15p makes each stored kWh worth about 12p. A 5 kWh battery cycling daily on those numbers saves roughly £200 a year, and more where export rate is lower. That leads to a counterintuitive point: a poor SEG rate makes a battery pay back faster, because surplus you would have exported cheaply is worth more kept at home.

Without solar a battery still earns through tariff arbitrage. Charging at around 8p overnight on a smart tariff and using that power during the day at 27p can save £250 to £550 a year depending on how much of your usage battery can shift.

Typical installed prices run from around £2,500 to £3,500 for 5 kWh up to £7,500 or more for 13.5 kWh, which puts realistic payback at 8 to 14 years against a working life of 10 to 15 years. Batteries also lose a little capacity each year, so model with some fade rather than assuming year one performance forever.

The 0 Percent VAT Window

Battery storage currently qualifies for 0 percent VAT in the UK, whether installed alongside solar panels or retrofitted on its own. The zero rate runs until 31 March 2027, after which the standard rate is due to return. On a £5,000 installation that is a £1,000 difference, which makes timing worth factoring into any decision. Use an MCS certified installer so system qualifies for export payments and keeps your paperwork clean for DNO notification.

Battery Storage With or Without Solar

With solar a battery lifts self consumption from around 35 to 40 percent to 70 percent or more, which is difference between exporting your surplus for pennies and using it yourself at full value. Without solar the case rests entirely on your tariff: on a flat rate tariff a battery has no gap to exploit, while on a time of use tariff with a big peak to off peak spread the sums can work on their own. If you have neither solar nor a smart tariff, sort the tariff first because it costs nothing.