Overnight Grid vs Solar Daytime Charging: Which Costs Less?

Two distinct strategies dominate home EV charging decisions: charging overnight at off-peak grid rates using a smart tariff, or using solar energy generated during the day. The question most EV and solar owners ask is: which is actually cheaper? The answer depends on your specific tariff, solar system size, driving patterns, and battery setup. This guide runs through the real numbers for common UK scenarios.

Overnight Grid Charging: The Numbers

The cheapest widely available overnight EV charging tariff in 2026 is Octopus Go, which offers a fixed rate of 7p per kWh between midnight and 6am. For a typical 60kWh EV battery, a full charge from 20% to 100% requires approximately 48kWh, costing £3.36 on Octopus Go. For a daily commuter adding 30 miles per day (approximately 10kWh), the daily charging cost on Octopus Go is around 70p. Over a year of daily commuting, this amounts to approximately £255 in EV charging costs — compared with the petrol equivalent of over £1,500 for the same mileage in an average family car.

Octopus Intelligent Go pushes the rate down further — sometimes to 6p or below — by automatically managing your car's charging via direct vehicle integration. Compatible vehicles include the Tesla range, Volkswagen Group EVs, and various others. The saving versus Octopus Go is modest but meaningful for high-mileage drivers.

Solar Daytime Charging: The Numbers

Solar charging your EV replaces grid imports at around 24p per kWh (the standard unit rate). A 4kW solar system in Yorkshire generates around 3,500-4,000 kWh per year. If a solar-diverting charger like the Zappi is installed, surplus generation during working hours can charge the car automatically. In summer, with good solar generation and the car at home during the day, this can supply 15-25 kWh per day — potentially covering a significant proportion of all EV charging.

At 24p per kWh saved, 2,000 kWh of annual EV charging from solar represents a £480 saving versus standard grid rate charging. At 7p per kWh (Octopus Go), the same 2,000 kWh would cost £140. This means solar EV charging saves more than Go overnight charging only when the alternative comparison is standard-rate grid power, not cheap overnight power.

The Optimal Strategy: Combine Both

The lowest total EV charging cost is achieved by combining solar daytime diversion with cheap overnight grid charging on a smart tariff. During the day, the Zappi diverts surplus solar to the car at effectively zero cost (avoided grid import or export). At night, the car tops up on Octopus Go at 7p per kWh. This layered approach can reduce EV charging costs to less than 3p per effective mile for a typical mixed solar-and-overnight scenario.

For households without solar, Octopus Go or Intelligent Go is the clear recommendation — cheapest overnight rates, automatic scheduling, and no upfront solar investment required. For solar owners who drive during daytime or work from home, a Zappi adds immediate value by converting surplus solar into EV miles that would otherwise export at 8-15p SEG rate. The Zappi's cost is recovered in saved charging costs within 2-3 years for a household that drives 10,000 miles per year.

What If I Have Battery Storage Too?

Adding battery storage to the solar and EV charger combination creates a third option: charging the EV from the house battery in the evening, which was itself charged from solar during the day. This is possible but needs careful configuration to prevent the EV charger from depleting the home battery before evening household loads. We configure all three systems — solar, battery, and EV charger — as an integrated system and set priority rules so house loads take precedence over EV charging when the battery is at lower states of charge.