This guide walks through the calculation logic UK homeowners should use, with worked examples for common household profiles.
The three questions battery sizing answers
Before looking at kWh numbers, establish what you need the battery to do:
- Cover overnight demand — shift daytime solar into evening and morning consumption
- Grid arbitrage — buy cheap off-peak (7p/kWh on Octopus Go) and use at peak rates (30p+)
- Backup power — keep essential circuits running if the grid fails
Most domestic batteries do all three; the sizing logic differs depending on your primary driver.
Step 1: Establish your daily consumption
Pull last 12 months from your supplier's app. Divide annual kWh by 365. A typical UK home uses:
| Household type | Average daily consumption |
|---|---|
| Small home, 1–2 occupants | 4–7 kWh/day |
| Average family, 3–4 occupants | 8–12 kWh/day |
| Large home or home with ASHP | 12–18 kWh/day |
| Home with ASHP + EV charging | 18–30 kWh/day |
Step 2: Calculate your overnight demand
The battery needs to cover consumption from sunset to ~7am. Overnight demand ≈ 50–65% of daily consumption for households out during the day.
Example: a 4-person family on 10 kWh/day needs approximately 5–6.5 kWh of usable battery overnight.
Note on 'usable' capacity
Step 3: Factor in your solar array size
A larger array fills the battery faster. A 6 kWp array generating 5,000–5,500 kWh/year produces 20–25 kWh/day in summer — far more than a 5.76 kWh battery absorbs in one day.
| Array size | Summer daily gen | Ideal battery | Alliant package |
|---|---|---|---|
| 3–4 kWp | 12–16 kWh | 5–7 kWh | 5.76 kWh (£5,999 pkg) |
| 4–6 kWp | 16–24 kWh | 7–10 kWh | 5.76 kWh (captures ~60–70%) |
| 6–8 kWp | 24–32 kWh | 10–14 kWh | 11 kWh (£9,000 pkg) |
| 8 kWp+ | 32+ kWh | 14+ kWh | 11 kWh + grid optimisation |
Want sizing based on your smart meter data?
We pull your actual half-hourly consumption (with permission) and build the calculation around real usage — not averages.
Step 4: Add EV and heat pump loads
An EV adds 8–15 kWh of daily charging demand. Options:
- Charge from solar during the day via a solar-divert charger (Zappi) — no battery impact
- Charge from battery overnight — requires 8–15 kWh extra storage most domestic batteries can't supply
- Charge from grid at off-peak (Octopus Go: 7p/kWh) — cheapest option, not a battery function
Optimal for solar + EV: solar charges battery by day, battery powers home in evening, EV charges from grid at 7p/kWh overnight via smart charger.
An ASHP draws 1–3 kW continuously during heating periods. 8 hours at 2 kW = 16 kWh — beyond typical battery capacity. For ASHP homes, the battery covers general loads while the heat pump draws from grid at off-peak.
Step 5: Consider grid arbitrage
- Octopus Go: 7p/kWh from 00:30–05:30 (4 hours). A 10 kWh battery charges in ~3 hours at max rate.
- Net saving per cycle: 10 kWh × (30p − 7p) = £2.30/day. Annual: £840.
- Payback on a £2,000 battery upgrade: ~2.4 years from arbitrage alone.
Battery sizing shortcut
Alliant's approach to battery sizing
Our standard packages include 5.76 kWh (packages 1 and 2) or 11 kWh (package 3):
- 5.76 kWh — households under 10 kWh/day; overnight cover for most 3-person homes
- 11 kWh — larger households, ASHP homes, or solar over 6 kWp where daily generation justifies it
Where usage falls between packages, we show the actual numbers — not a generic recommendation.
Frequently asked questions
Is it better to get a bigger battery or more solar panels?
Generally, more panels first — generation creates the value, storage just shifts it. Once your array produces more than you can self-consume, additional storage extends the self-consumption window.
Can I add more battery storage later?
Yes — Fox ESS batteries are modular (5.12 kWh increments). Adding a second module to an existing Fox ESS hybrid is a half-day job. You don't need to over-specify on day one.
What size battery do I need for overnight backup?
Essential loads only (fridge, freezer, lights, phone, router): 3–5 kWh is enough for most homes. Whole-home backup including heating: 10–15 kWh minimum, with careful management of high-draw appliances.
Do batteries lose capacity over time?
Yes — LFP batteries typically lose 1–2% per year, reaching the warranted 80% around year 10. After year 10, degradation slows. A 5.76 kWh battery after 10 years provides ~4.6 kWh usable — still enough for most overnight loads.
