Battery Sizing Calculator

Understanding the value

What a battery actually does for you

Solar generates most power at midday. Most households use most power in the morning and evening. A battery bridges that gap — capturing surplus midday generation and releasing it when you need it most.

Without a battery

→Panels generate surplus power at midday

→Surplus is exported to the grid at your SEG rate (typically 12–20p)

→In the evening you import electricity at the full unit rate (26p+)

→Typical self-consumption: 30–45% of generation

You're effectively selling cheap and buying expensive — the gap between SEG rate and import rate is the opportunity cost.

With a battery

✓Surplus midday generation charges the battery (free)

✓Battery discharges in the evening — avoids importing at 26p+

✓Self-consumption rises from ~40% to 65–75%

✓Can also charge from cheap overnight tariffs (e.g. Octopus Go)

The value of each kWh shifted = import rate minus SEG rate. At 26p import, 15p SEG, each shifted kWh is worth 11p.

Popular options

UK home battery options (2026)

Battery technology has matured significantly. Most systems now use lithium iron phosphate (LFP) chemistry — longer lifespan, safer, more charge cycles than older NMC batteries.

Battery system	Capacity	Installed cost	Warranty	Notes
Tesla Powerwall 3	13.5 kWh	£8,500–£10,500	10 years	Includes integrated inverter. Tesla-certified installers only.
GivEnergy All-in-One	9.5 kWh	£5,500–£7,000	10 years	Popular UK-focused brand. Expandable. Good monitoring app.
SolarEdge Home Battery	9.7 kWh	£6,000–£7,500	10 years	LFP chemistry. Best with SolarEdge inverter. DC-coupled.
Solis RHI + BYD	5.1–15.4 kWh	£4,500–£9,000	10 years	Modular BYD battery. Flexible sizing. Very competitive pricing.
myenergi Libbi	5–20 kWh	£5,000–£11,000	10 years	Made in UK. Integrates with Zappi EV charger and eddi diverter.

Prices and availability change frequently. VAT is currently 0% on battery storage installed alongside solar. All prices are approximate installed costs including labour, 2026.

Retrofitting vs. installing at the same time

If you're installing solar now, fitting a battery at the same time saves £500–£1,000 in scaffolding and wiring costs vs. retrofitting later. Even if you're unsure, ask your installer to specify a "battery-ready" inverter (hybrid inverter) — it costs little extra and makes future battery addition much cheaper.

AC vs DC coupling — which is better?

DC-coupled batteries (charged directly from panels) are ~5% more efficient. AC-coupled batteries (charged via an inverter) are easier to retrofit to any existing system. For new installations, DC-coupled hybrid inverters are generally preferred. For retrofits, AC coupling (e.g. Tesla Powerwall) is simpler.

Tariff optimisation

Getting more from your battery with the right tariff

The calculator shows savings from solar self-consumption alone. A time-of-use tariff can add £100–£400/yr on top by charging your battery cheaply at night.

Intelligent Octopus Go

Charge overnight, save on daytime use

6 hours of cheap overnight rate (~7p). Battery charges at night, discharges during the day — doubling the effective storage cycle. Works well with most smart batteries.

Octopus Flux

Import cheap, export expensive

Three-rate tariff: cheap overnight (7–10p), standard midday, premium evening rate. Solar export in the peak evening window earns more. Pairs excellently with battery for "buy low, sell high" arbitrage.

Agile Octopus

Half-hourly wholesale pricing

Prices change every 30 minutes, tracking wholesale electricity markets. Can give negative rates (you get paid to use electricity) on very windy nights. Highest complexity, highest potential saving for active managers.