Fill a battery on cheap overnight electricity and run your home from it during the day, with or without solar panels. Six quick steps, no sign-up needed.
Battery only, charged overnight
£0 / yr
Battery plus panels
£0 / yr
Your household
We'll estimate your electricity use from the number of people at home. Override it if you know your annual figure from a bill.
Estimated annual electricity use
0 kWh
kWh
%
Most homes use about 80% of their electricity during the day and evening. Lower this if you already run the washing machine or dishwasher overnight. Estimates follow Ofgem's typical consumption values.
Your current tariff
Pick your supplier and tariff and we'll fill in typical rates. Adjust them to match your bill, rates vary by region.
p/kWh
p/kWh
p/day
The off-peak tariff you would move to
Choose an EV or smart tariff with a cheap overnight window. The battery scenarios all use this tariff. Staying put? Pick the same tariff as the previous step.
p/kWh
p/kWh
p/day
hours
hour (24h clock)
Battery
Pick a battery or enter your own figures. The recommendation on the results page will tell you if a different size suits your usage better.
kWh
£
£
W
W
%
%
%
%
Usable capacity per cycle
0 kWh
Since 27 August 2026 you can plug an 800W solar kit into a normal socket in Great Britain, but batteries are not covered yet. The 5 kWh units (EcoFlow STREAM 5000, Anker Solarbank 4 Pro, Marstek) must be hardwired by a Part P electrician, hence the installation cost. Manufacturers recommend charging to about 95% and stopping at 20% to protect the cells, so a battery gives you around three quarters of its rated capacity each cycle. Kettles, ovens and showers draw more than a plug-in inverter can supply, so they still come from the grid. Around 70% cover is realistic for an 800W unit, 90% for a 2,500W unit.
Solar panels
£
kWh/kWp
%
p/kWh
Changing the panel count sets the capacity at 440W a panel; drag the slider if your panels differ. Set the cost to 0 for panels you already own. A south-facing roof gives around 850 to 950 kWh per kWp a year, a balcony or fence rail nearer 500 to 700. Plug-in kits usually have no export meter, so leave export at 0 unless you have one.
Your results are ready
Pop your email in and we'll send you the full breakdown to keep, along with your recommended battery size.
Recommended battery size
0 kWh
Annual savingPayback (years)Recommended
Bigger is not always better. Once the battery covers your daytime usage, or exceeds what the socket can put in overnight, extra capacity adds cost but not saving.
Advertisement
Annual cost by scenario
Scenario
Cost / yr
Saving
Advertisement
Keep these results
Want a copy? Add your email and we'll send the full breakdown.
Ready to buy?
Links go to the retailer. Prices shown are the figures used in this calculation, installation not included.
How the maths works and what it assumes
Usage is spread evenly across the year. The share you enter as outside the window is bought at peak; the rest is bought off-peak at whichever tariff applies.
Each night the battery takes the smaller of its usable capacity and charge power multiplied by the window length. Round-trip losses come off before it is used.
Energy from the battery replaces peak-rate purchases, up to the coverable share of your daytime use. The charging energy is bought at the off-peak rate.
Solar is split across months using a typical UK profile (June makes about five times December). Solar used straight away comes first, surplus fills the battery for a second cycle, then the overnight grid charge covers what is left. Leftover solar is exported at the rate given, or wasted at 0.
The recommended size is the smallest that delivers 95% of the best achievable saving with your settings.
Payback uses hardware cost only and ignores battery degradation, inflation and tariff changes. Treat it as a guide, not a quote.
Plug-in solar kits up to 800W have been legal to connect to a normal socket in Great Britain since 27 August 2026. Plug-in batteries are not covered by that change, so larger units need a Part P electrician to hardwire them. Product prices are launch and pre-sale figures from September 2026 and will move.