What will your new build's electricity cost — and would a battery pay?

Assumptions

• Standard rate 26.32p per kWh: the Ofgem price cap for 1 October to 31 December 2026, with no VAT on electricity until 31 March 2027. Ofgem
• With an electric car: off-peak rate 8p per kWh on Intelligent Octopus Go, 11:30pm to 5:30am, checked September 2026. It needs a compatible electric car and charger — the cheapest overnight rates require an electric car. Its daytime rate is assumed to equal the standard rate. Octopus
• Without an electric car: Economy 7, the overnight tariff generally available without one, at 14.19p per kWh overnight and 32.65p in the day. Rates for Flexible Octopus Economy 7, average of the 14 GB regions, paying by Direct Debit, from 1 October 2026, checked September 2026. Octopus Energy tariff data
• None of your household use (other than car charging) is assumed to fall in the off-peak window. That makes the Economy 7 figures conservative: households on Economy 7 usually run some appliances overnight, which lowers their bills with or without a battery. On these rates, Economy 7 beats the standard rate once about 34% of your use is overnight. We haven't found a reliable figure for how much households actually shift, so we don't assume any.
• The calculator uses whichever tariff is cheaper, separately for the home without a battery and with one.
• Household electricity uses the appliance, cooking and lighting formulas in the government's Home Energy Model for Future Homes Standard assessments, and its standard occupancy by bedrooms. HEMFHS-TP-05, HEMFHS-TP-02
• Space heating and hot water electricity is not included.
• Car charging at 3.5 miles per kWh; default 7,100 miles a year, the 2025 average for all cars in England. DfT
• Solar generation: the European Commission's PVGIS model gives about 3,600kWh a year for a south-facing 3.7kWp array near Derby (SARAH-3 dataset, 40° pitch, 14% system losses, queried September 2026), with about 437kWh in May and 131kWh in December. The calculator scales this in proportion to your array size. PVGIS
• Output depends on orientation, roof pitch and shading, which the calculator doesn't know. Panels facing east or west produce less, and a north-facing roof will generate materially less than the figure shown. Homes further north generate less than Derby, and homes further south more.
• The default 3.7kWp is a typical system — what the Approved Document L 2026 benchmark gives for a 42m² ground floor — not an average. We haven't found a published average array size for new builds.
• Without a battery, a home uses 30% to 50% of its solar generation; with a battery, around 70%. These are industry estimates, not measurements. Solar self-use can't exceed your household use, and solar is assumed not to charge the car.
• Export is paid at 12p per kWh under the Smart Export Guarantee (Outgoing Octopus, checked September 2026). Many export tariffs pay less. Octopus Energy
• The battery is 5kWh usable unless you change it, loses about 10% of what it stores, and does at most one full cycle a day, shared between stored solar and off-peak charging. Whichever is worth more per kWh is assumed to go first. It doesn't allow for limits on how fast a battery can charge or discharge, or for months when there's no solar surplus to store.
• Payback divides the typical £3,500–£5,500 installed price of a 5kWh retrofit battery (Heatable and Home Energy Quotes cost guides, checked September 2026) by the yearly saving, rounded up. It assumes today's prices and no loss of battery capacity. Battery warranties are typically around 10 years. Heatable, Tesla warranty
• Standing charges are the same on either tariff and aren't included.