How Much Battery Storage Do You Actually Need? A Cornwall Sizing Guide
Battery storage capacity is often misunderstood. "Usable" capacity is not the same as "rated" capacity. And the right size depends on your solar system, your overnight demand, and whether you plan to arbitrage time-of-use tariffs. We explain the numbers for Cornwall households.
Rated vs Usable Capacity: The Key Distinction
Every home battery has two capacity figures. The rated capacity (or nominal capacity) is the total energy storage the battery cells contain. The usable capacity is the amount you can actually draw from it in normal operation. The difference matters enormously when comparing products.
Most lithium iron phosphate (LFP) batteries — including the GivEnergy, Fox, and Sigenergy units we install — have a Depth of Discharge (DoD) of 90–100%, meaning usable and rated are nearly identical. The Tesla Powerwall 3 at 13.5kWh rated has 13.5kWh usable. A Fox Pro 10kWh has 10kWh usable. Older NMC (lithium nickel manganese cobalt) batteries sometimes have DoD of 80–85%, meaning a "10kWh" battery might only deliver 8–8.5kWh. Always compare usable capacity figures, not rated.
How Much Battery Do You Need Without Solar?
If you're installing battery storage purely for tariff arbitrage — buying cheap overnight electricity and using it through expensive peak hours — the right size is your daily household demand that falls in peak hours (typically 4pm–7pm on a tariff like Octopus Agile or Flux).
For a typical Cornwall 3-bed house using 10–14 kWh/day total, roughly 3–5 kWh falls in the peak period. A 5–6kWh battery is the minimum for pure tariff arbitrage. A 10kWh battery provides comfortable headroom, especially for evening EV charging.
If you also have an EV, budget for the EV's typical daily charge in the calculation. Adding 5–10 kWh of EV overnight charging to household demand on peak tariff hours pushes optimal battery size toward 10–16kWh.
Sizing Battery to Your Solar System
When solar is in the picture, the battery needs to capture what the panels generate during the day for use at night. Cornwall's solar profile produces most generation between 9am and 4pm — a household that isn't home during the day exports most of this without a battery.
A rough sizing rule: battery capacity (kWh) = 50–80% of daily solar generation in your worst capture months (spring and autumn). For a 5kWp system generating 20–25 kWh/day in May, a 10–13kWh battery captures most of the day's generation and runs the home into the late evening.
| Solar system size | Recommended battery size | Products we supply |
|---|---|---|
| 3–4kWp | 5–8kWh | Fox Eco 5kWh, GivEnergy 5.2kWh |
| 5–6kWp | 8–13kWh | GivEnergy 9.5kWh, Fox Pro 10kWh |
| 6–10kWp | 13–16kWh | Tesla Powerwall 3 (13.5kWh), Sigenergy 15kWh |
| 10kWp+ | 16kWh+ | GivEnergy stacked, Sigenergy expandable |
The Case for Larger Batteries in Cornwall
Cornwall's solar irradiance peaks sharply in May–July and drops significantly in November–January. The most valuable battery days are the long, sunny summer days when a 10kWh battery fills by 2pm and the surplus generates SEG income instead of being stored. A 16kWh battery delays this cutoff to 5–6pm, capturing three extra hours of surplus into storage.
In winter, larger batteries provide more resilience during poor weather. For properties with heat pumps running electric heating, a 13.5kWh+ battery meaningfully reduces the heat pump's peak electricity draw from the grid.
For off-grid and hybrid properties where resilience is a priority, size to 3–5 days of minimal winter demand. See our off-grid solar page for full guidance on autonomy-focused battery sizing.
Popular Battery Sizes Compared
GivEnergy Giv-Bat 9.5kWh — our most popular residential battery. 9.5kWh usable, 100% DoD, 10-year warranty, expandable to 19kWh. Excellent app and monitoring. Best value at this capacity for most Cornwall homes.
Tesla Powerwall 3 (13.5kWh) — integrated inverter/battery, clean installation, very good smart tariff integration. Premium price for premium aesthetics and Tesla ecosystem.
Sigenergy SigenStor — modular and expandable, available from 5kWh to 30kWh+. Best for homes planning future capacity expansion. Also available in 3-phase for larger properties.
Fox ESS Pro 10kWh — strong performance, good app, 10-year warranty. Slightly lower profile than GivEnergy in Cornwall market but technically comparable.
Our Sizing Recommendation by Household Type
- Small household, 3–4kWp solar, no EV — 5–8kWh. GivEnergy 5.2kWh or Fox 5kWh.
- Average household, 5–6kWp solar — 9.5–10kWh. GivEnergy 9.5kWh or Fox Pro 10kWh.
- Larger household, 6–10kWp solar, or EV — 13.5kWh. Tesla Powerwall 3 or Sigenergy 15kWh.
- High-energy household, heat pump, large solar, EV — 16–20kWh. Sigenergy expandable or stacked GivEnergy.
- No solar, tariff arbitrage only — 5–10kWh depending on peak demand and EV use.
Contact us for a battery sizing consultation. We'll review your solar system, energy bills, and tariff options to recommend the right capacity for your home — at no cost. See our battery storage installation page for full details.
Need personalised advice?
Our MCS certified engineers can answer your questions and provide a free, no-obligation assessment for your property.