If you've been quoted a 10kWh battery and you're wondering whether you actually need that much, the honest answer is: probably not. The average home battery sold in the UK in 2026 is 10kWh. The average home actually needs 6–8kWh. That 2–4kWh gap is pure salesperson margin — it costs you £700–£1,500 of upfront battery that never gets used over its lifetime. Sizing properly from your real smart-meter data, before anyone quotes, is the single biggest saving you can lock in.
Here's how we do it on every OMEGA survey — and how you can do it yourself in 15 minutes with your Octopus or British Gas account open.
The proper sizing method
- Pull 12 months of half-hourly electricity data from your supplier (Octopus: in-app CSV export; British Gas / EDF: request via customer services — it is your data and they must provide it within 30 days under GDPR).
- Filter to the 4pm–midnight window — that is when battery discharge matters most, because daytime self-consumption comes straight from the panels.
- Take the average kWh across that 8-hour evening window across the year — typically 3.5–5.5 kWh for a no-EV, no-electric-heating home, 6–9 kWh with an EV charging overnight.
- Multiply by 1.3 — a 30% buffer for winter peak weeks and unusual days.
- Cap at 2.5x your solar array size — oversizing the battery beyond what the panels can refill is wasted kWh.
Where customers get it wrong
Using daily total instead of evening-window
The battery only matters from about 4pm onwards — daytime demand is already offset by the panels. Homeowners who size from "my annual bill is 3,800 kWh ÷ 365 = 10.4 kWh/day, so I need a 10kWh battery" end up with roughly double the capacity they actually use. A 5-bed family with no EV rarely needs more than 8kWh.
Ignoring the solar refill constraint
A 4kW solar array generates ~3,600 kWh/year — roughly 10 kWh/day averaged annually, but as low as 2 kWh/day in December. A 15kWh battery paired with a 4kW array is empty every January. The battery can only store what the panels can produce — add too much capacity and the excess is dead weight.
Not accounting for tariff structure
If you are on Octopus Flux or Agile, the battery is doing two jobs: (1) storing solar for evening use, (2) buying cheap grid at 3am and selling at 4pm peak rates. Job (2) means you size bigger than just evening consumption — closer to 12–15 kWh for a family on Flux. For a fixed-rate tariff without arbitrage, 6–8 kWh is plenty.
Typical ranges by household type
| Household | Evening use | Solar array | Recommended battery | Annual value on Flux |
|---|---|---|---|---|
| 2-bed flat, no EV | 2.5 kWh | 2.8 kW | 4 kWh | £320 |
| 3-bed semi, gas heating | 4.2 kWh | 4.0 kW | 6 kWh | £620 |
| 4-bed detached, gas heating | 5.5 kWh | 5.5 kW | 8 kWh | £820 |
| 3-bed with EV (overnight charge) | 7.8 kWh | 4.5 kW | 10 kWh | £1,070 |
| 4-bed with EV + heat pump | 10.2 kWh | 6.0 kW | 13.5 kWh | £1,450 |
How OMEGA actually sizes your battery
- On survey day, if the customer can pull their smart-meter CSV, we ingest it into our sizing script on the spot. 5-minute job.
- If not, we pull average-UK figures for the property type then adjust for the customer's reported evening lifestyle (working from home, kids, EV, etc.).
- We model three battery sizes against the 12-month data — usually the recommended size, +2 kWh and -2 kWh — and show the customer the trade-offs in £-per-year.
- We price each option transparently so the commercial decision is on the customer.
- We do NOT default to the biggest battery on the shelf. That is how the industry earned its oversizing reputation.
Battery sizing FAQs
- Where do I get my smart-meter data?
- If you are on Octopus, it's in the app under "Energy History → Download CSV". Other suppliers: request by phone or email — they are legally required to provide under 30 days under your data-access rights. If you cannot get it, we can install a clamp-on CT meter for a week to measure.
- Should I just buy a bigger battery to be safe?
- No — oversizing costs ~£800 per extra kWh and only pays back if you use it. A 10kWh battery in a home that needs 6kWh loses you roughly £800 upfront and never recovers it. Size to 30% buffer, not 100%.
- Can I expand the battery later if I get an EV?
- Yes — most modern home batteries (GivEnergy AIO, BYD HVM, Tesla Powerwall 3 stacked) support adding modules. We typically recommend sizing for your current situation and expanding in 2–3 years if/when an EV arrives. Cheaper than pre-buying capacity.
- Does battery size affect solar generation?
- Not directly. Panels generate whatever sunlight delivers. The battery just decides what happens to the surplus — stored or exported. A too-small battery means more SEG export (3p–40p/kWh depending on tariff) instead of self-consumption (28p/kWh saved). A too-big battery means the opposite problem — unused capacity.
- What if my home use changes after installation?
- Nearly every 2026 battery is software-upgradable for new tariff integration, and the physical capacity is fixed but you can add modules if the brand supports it. We build that expansion path into the initial quote so no surprises.
Let us size yours properly
Book a free home energy survey online or ring 0800 229 4094. Bring your 12 months of smart-meter data if you have it, or we will pull it together for you. We will size the battery on your actual data — not a default — and quote the right size in writing. No cost. No obligation. No pressure.
Want to find out how much you could save?
Book a free 45-minute survey. We'll check every active grant, measure your home, and give you a written quote — with no obligation to proceed.
No cost. No obligation. No pressure.