Will adding a battery increase my solar savings?
Short answer
Usually yes, by shifting surplus solar into the evening. MCS tables show a 4–5 kWh battery lifts self-consumption from about 25% to 63% for a typical 3,500 kWh home that is out half the day. The gain shrinks as batteries get bigger. EST puts batteries at £5,000–£8,000 installed (August 2026), so payback depends heavily on tariffs.
Last checked 24 September 2026 · 5 sources
In more detail
- Diminishing returns: for in-half-day, 3,500-3,999 kWh use, 3,300-3,599 kWh generation: PV-only 25%, 4-5 kWh 63%, 9-10 kWh 75%.
- A battery on a time-of-use tariff can also charge cheaply overnight in winter; MCS values that separately (usable kWh × 730/yr cap).
- MCS assumes round-trip efficiency ≥80% and a battery that fully charges/discharges within 6 hours.
- Battery life per EST 10-12 years; warranties often 10 years (Which?) - may need replacing within panel life.
The facts, dated
- 01MGD003 in-half-day 3,500-3,999 kWh, gen 3,300-3,599: PV only / 4.1-5.1 kWh / 9.1-10.1 kWh — 25 / 63 / 75 % self-consumption
standard · UK · checked 24 September 2026 · MCS
- 02EST battery cost range — 5,000-8,000 GBP
market_data · UK · checked 24 September 2026 · Energy Saving Trust
- 03EST 5 kWh battery system — 4,600 GBP
market_data · UK · checked 24 September 2026 · Energy Saving Trust
- 04EcoFlow STREAM Ultra X 3.84 kWh retail (no install) — 1,299 GBP
manufacturer · UK · checked 24 September 2026 · EcoFlow UK
- 05MCS cap on assumed cycling for non-self-consumption use — 730 cycles/yr
standard · UK · checked 24 September 2026 · MCS
What we saw in the field
[QA review 2026-09-24] I-10 CONTRADICTS: MCS 032 values unused battery capacity at usable kWh x 730 (arbitrage). | Earlier notes: Installer claim 'batteries only help with a surplus or for backup' PARTLY CONTRADICTS the evidence. MCS 032 explicitly values grid-charged arbitrage (usable kWh × 730), and all three customers used Go/Flux/Intelligent Go to charge cheaply in winter. Customer 2's 'battery before solar' view is consistent with arbitrage value on time-of-use tariffs. It can't be tested with MCS tables, which assume solar self-consumption mode. Customer 1's 24 kWh and Customer 2's 40.5 kWh are far outside MGD 003's 15 kWh range, so MCS lookup values don't apply. Self-consumption gains above ~10 kWh are small in the tables.
Sources
- MCS — mcscertified.com/wp-content/uploads/2025/02/MGD003-LookupTables-FINAL.xlsx
- Energy Saving Trust — energysavingtrust.org.uk/advice/solar-panels/
- Energy Saving Trust — energysavingtrust.org.uk/advice/solar-panel-battery-storage/
- EcoFlow UK — uk.ecoflow.com/products.json
- MCS — mcscertified.com/wp-content/uploads/2025/02/MCS-032-2025-V1.0.pdf
Last checked: 24 September 2026