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Solar + Battery + EV Combined Savings Calculator UK

Model the combined savings of solar panels, a home battery and a home-charged EV on a UK time-of-use tariff. Sees where the money actually comes from — self-consumption, SEG export, battery arbitrage, EV off-peak — and gives a 15-year net-benefit figure on the whole stack.

#solar #evs #tariffs

Interactive combined solar, battery and EV savings calculator

Worked example

A UK household with 4 kWp solar, a 5 kWh battery, and an EV doing ~8,000 miles/yr, on Octopus Agile with 85% of EV charging overnight:

Year-1 total saving
£1660
Stack cost
£12700
Solar year-1
£844
Battery year-1
£457
EV year-1
£359
Payback on the whole stack
6.8 yrs
15-yr net benefit
£20544

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Ready to model individual components in more depth?

The dedicated solar, battery-via-Agile and EV calculators each model their piece more thoroughly.

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Why the stack numbers matter

  • Battery on its own rarely pays back. Pure-arbitrage payback is typically 10–20 years at current Agile spreads.
  • Battery + solar is different. Lifts solar self-consumption from 35% to 65–70%, compressing solar payback by 2–3 years.
  • EV + time-of-use tariff is where the real money is. An 8,000-mile EV saves £300–£500/yr on Octopus Go or Agile vs the flat Ofgem cap .
  • Order matters. Install solar first, layer an EV, then a battery if the peak/cheap spread justifies it. This calculator lets you see the effect of each on the others.

Frequently asked questions

Why model solar, battery and EV together?
Because the savings interact. A battery on its own, bought purely for Agile arbitrage, typically takes 10–20 years to pay back. Stack it with solar and it dramatically lifts solar self-consumption (from 35% to 65–70%), shortening solar payback too. Add an EV that overwhelmingly charges overnight on off-peak, and the household's marginal electricity price drops to 8–12p/kWh blended — which is the real argument for the whole setup. Calculators that model each in isolation miss the compounding.
How is battery arbitrage modelled?
Simple but honest: one full cycle per day at 85% round-trip efficiency, discharging during your supplied peak window price and charging at the supplied cheap window price. Effective charge cost per useful kWh = cheap price ÷ 0.85. If that exceeds peak, the battery loses money and the savings clamp to zero. Real-world arbitrage can cycle 1.1–1.3× per day with smart software, and can also stack solar-surplus charging in summer — we don't model those uplifts in v1.
What counts as the 'peak' and 'cheap' window?
If you're on a flat tariff, both are the same and battery arbitrage saves nothing — it only makes sense on a dynamic or time-of-use tariff. Sensible defaults: Octopus Agile 4–7pm averages 35–45p; overnight trough averages 10–14p. Octopus Go: 28.5p peak / 8.5p cheap. Octopus Cosy: three cheap windows around 13p. Put whatever your actual tariff shows.
Is the payback realistic?
Payback is on the total stack cost — solar install + battery install + EV charger install — against the combined year-1 saving, with your electricity-inflation assumption compounding. For a typical 4 kWp + 5 kWh + EV stack with a decent off-peak tariff, 8–11 years is typical. Much longer if you overspec the battery and pay flat tariff rates; much shorter if you layer Agile + a big EV driver.
What's not in this v1?
Heat pump load (model it separately with our heat-pump calculator), regional PVGIS refinement by postcode (we use a UK-average yield you can override), solar-to-battery charging that doesn't hit the cheap/peak arbitrage model, and the battery's value during grid-outage backup. All on the v2 roadmap.

How this calculator works

The stack calculator composes three calculations you'd normally do separately. For solar it reuses the same math as our Solar Panel Savings Calculator — annual generation from kWp × yield, self-consumption scaled by battery size, SEG export for the rest. For battery arbitrage it models one full daily cycle at 85% round-trip efficiency, with savings equal to cycled kWh × (peak price − cheap price ÷ 0.85); if that spread is negative the saving clamps to zero. For EV it splits annual EV kWh into an off-peak share (priced at your supplied off-peak rate) and the rest (priced at your flat tariff), and reports the gap against a flat-only baseline.

Payback is on the total stack cost (solar install + battery install + EV charger install) against the combined year-1 cashflow, with your electricity-inflation assumption compounding. The 15-year view applies the same inflation curve to the combined saving. That's more honest than the usual "each piece pays back separately" framing, because in real life you spend the money in one lump and the savings arrive in one lump.

What v1 doesn't model

  • Heat pump load. Use our heat-pump calculator separately. Adding one would change the peak/cheap window allocation enough to deserve its own modelling path.
  • Solar-to-battery direct charging. In summer, surplus solar charges the battery at zero marginal cost — we don't separately credit that yet, so sunny-summer numbers are conservative.
  • Backup / outage value. A battery during an outage is genuinely valuable; we don't price it.
  • Live regional yield. We take annual kWh/kWp as an input rather than looking it up by postcode. Our postcode solar calculator has regional precision for the solar piece.

When this isn't the right answer

If you don't have off-street parking, skip the EV piece — public-charger economics are completely different (see our EV charging calculator ). If you're on a flat tariff and unwilling to switch, skip the battery — arbitrage only works on a dynamic tariff like Agile . And if you're targeting MEES compliance as a landlord, start with the MEES calculator — solar usually isn't on the cheapest-first stack.

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