The £6,000 decision installers won't show you the maths on.

A home battery has exactly three jobs: store excess solar so you can use it after dark, charge from the grid at cheap overnight rates and discharge at peak, and (sometimes) keep the lights on in a power cut. The economics of each are completely different.

Storing solar saves you the gap between buying electricity (~28p/kWh) and the export tariff (~5–15p/kWh); call it 15–22p per stored kWh. A 10kWh battery, cycled once a day, saves £550–£800 a year if you have enough solar to fill it. Without enough solar, the battery sits empty.

Charging from the grid on a time-of-use tariff saves the gap between peak (~35p/kWh) and off-peak (~7p/kWh on Intelligent Octopus). A 10kWh battery cycled once a night saves £900–£1,000 a year; but only if you're on a tariff that allows it and only if you actually consume that much during peak.

Without a time-of-use tariff a battery is a hobby, not an investment. The flat-rate 28p/kWh tariffs that most UK households still use leave no arbitrage gap for a battery to exploit.

Octopus Flux, Intelligent Octopus Go, and Cosy Octopus all offer overnight rates between 7p and 12p/kWh, with peak rates of 30–40p. That gap is what makes batteries pay back. Switch to the tariff first; size the battery to the tariff second.

If you have an EV, Intelligent Octopus Go gives you 5 hours of 7p/kWh electricity every night. A 10kWh battery on that tariff functionally halves your evening electricity cost without any solar involvement.

A 5kWh battery covers a typical evening's consumption (TV, lights, kettle, dishwasher) for a small household and pays back fastest per kWh installed. A 10kWh covers a normal-sized family's evening with headroom. A 13–15kWh battery starts to push past most households' realistic daily discharge; meaning the extra capacity sits idle.

The curve is steep. Doubling the battery from 5kWh to 10kWh might add 60% to savings. Doubling from 10kWh to 20kWh might add 15%. Beyond about 1.5–2kWh per occupant, the extra capacity rarely earns its install cost.

AC-coupled batteries (Tesla Powerwall, GivEnergy AC) have their own inverter and connect to your house's AC wiring. Easy to retrofit alongside an existing solar inverter. Slightly less efficient at storing solar (two AC↔DC conversions). Best when the solar's already in.

DC-coupled batteries (hybrid inverter setups; SolarEdge, Sunsynk, Sofar) share a single inverter with the solar panels. Slightly more efficient. Cheaper when installed at the same time as new solar. Hard to retrofit if you've got an existing string inverter you don't want to replace.

Pick AC-coupled if you're adding to existing solar. Pick DC-coupled (hybrid) if you're installing solar and battery together.

Most home batteries only provide backup power if specifically configured with a backup gateway or transfer switch; and even then, only to circuits you've designated as essential. A standard battery install does not power your home through a grid outage by default.

Adding backup capability costs £1,200–£2,500 extra at install. For most UK households (where outages are rare and short) the maths doesn't justify it. For rural homes, home offices, or anyone with medical equipment, it absolutely does.