What would solar, a battery and a smarter tariff really save you?

Move the sliders. The bill on the right redoes itself using half-hourly-style tariff maths – your daytime use, the cheap overnight window, what you send back to the grid – not a single average number.

00:0006:0012:0018:0024:00

Cheap overnight window (7p/kWh) – battery and car fill upSolar hours – panels power the house firstPeak rate – the battery covers this

Your home

Electricity a year (without a car)kWh On your bill or the supplier app. A typical UK home uses 2,700–3,500 kWh.

Where you live Sets how much a kW of panels makes in a year (790–980 kWh).

Electric car

Miles a yearmi 0 if you don't have an EV. Assumes about 3.5 miles per kWh, charged at home.

Solar & battery

Solar panelskWp About 10 panels = 4 kWp. 0 to leave solar out.

Home batterykWh A Powerwall 3 is 13.5 kWh. 0 for no battery.

Tariff

Import

Smart EV tariffs (like Intelligent Octopus Go) give a cheap window overnight. You need an EV or a battery to make the most of it.

Export

What you're paid for solar you don't use. Most suppliers' basic rate is far lower than Octopus's flat export rate.

Change the rates and prices

Price cap unit rate (p) Standing charge (p/day) Smart tariff peak (p) Smart tariff off-peak (p) Basic SEG export (p) Outgoing Fixed export (p) Solar cost (£) Battery cost (£)

Costs are filled in from typical 2026 UK quotes for the sizes above (0% VAT). Type your own quote to override; set to blank to go back to the estimate.

How it's worked out

Solar is split into what the house uses straight away, what fills the battery and what's exported. On a smart tariff the battery tops up overnight in the cheap window and covers the evening peak; the car charges overnight. Battery round-trip efficiency is taken as 90%.

What it leaves out

Gas, heat pumps and hot-water diverters
Selling battery power back at peak times (Agile / Flux) – usually extra
Panel ageing (about 0.5% a year) and future price changes

Rates used

Price cap Oct–Dec 2026: 26.32p/kWh and 54.83p/day (Ofgem, direct debit average). Smart EV tariffs and export rates vary by region and month – open “Change the rates” and use your own.