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Batteries · Advice

Home battery myths, checked against the evidence

Short answer

It's a quick first guess for time-of-use tariffs, but it assumes you use the same power every hour. Real UK homes use more in the evening, so about 86–91% of use falls outside a 5–6 hour cheap window, not 75%. For solar-only self-consumption, the MCS MGD 003 tables point to much smaller batteries, often 5–10 kWh.

Last checked 24 September 2026 · 12 sources

What should I know before deciding?

Heuristic: capacity = daily kWh ÷ 24 × hours outside the cheap window (e.g. 24 ÷ 24 × 18 = 18 kWh). Profile method: capacity = daily kWh × PC1 share outside the window ÷ (1 − reserve). For 24 kWh/day and IOG: 24 × 0.879 = 21.1 kWh delivered, ~23 kWh usable with a 10% reserve. Power check: charge energy = delivered ÷ RTE (21.1 ÷ 0.9 = 23.4 kWh) must fit in the window at the charge rate (23.4 kWh ÷ 6 h = 3.9 kW). A 5 kW-charge Powerwall 3 fits; a 3 kW-charge battery would not fill in a 5-hour Go window (15 kWh max). Discharge power must cover the evening peak. A 20 kWh battery on a 5 kW inverter still imports when the house draws 7 kW. MGD 003 is a solar self-consumption tool (MCS 032 5.1.7 says it is 'not ... an EESS design or sizing tool') and stops at 15 kWh and 6,000 kWh/yr.

Is a battery worth it without solar panels?

It can be, if you have a big gap between cheap and peak electricity prices and enough use in peak hours. The battery charges overnight and runs the home later. Savings depend on tariff eligibility, round-trip losses and future tariff changes, and battery-only homes cannot earn SEG export payments.

SEG eligible technologies are generation only (solar, wind, micro-CHP, hydro, AD).

Will my battery keep the lights on in a power cut?

Only if it was installed for backup. Grid-tied batteries must shut down when the grid fails unless an EPS or a backup gateway isolates the house. An EPS socket or backup circuit powers a few chosen loads, often limited to about 16 A; a whole-home gateway can run most of the house. Switchover can take milliseconds to several seconds.

No backup: battery and solar stop in a cut (anti-islanding under G98/G99). EPS: one outlet or a few circuits (myenergi libbi backup 16-20 A; GivEnergy 2023 guide ~5 s delay). Whole-home gateway: needs space near the meter, an earth electrode for island mode (MIS 3012), and the backup reserve set.

LFP, NMC or sodium-ion: which battery chemistry is best for a home?

For most homes, LFP (lithium iron phosphate). It dominates new UK home batteries, tolerates heat better, releases little oxygen in failure and is cheaper per kWh. NMC packs are more compact but run hotter in thermal runaway. Sodium-ion works in a wider temperature range and avoids lithium, but UK products are few and cost more per kWh.

BNEF (Dec 2025): average LFP pack $81/kWh vs NMC $128/kWh; stationary storage packs $70/kWh. Thermal runaway test (eTransportation 2023): NMC-811 module ran away at 371.6 C surface temperature vs 436.6 C for the first LFP cell; LFP's phosphate bonds limit oxygen release. LFP vent gas is still flammable; location and detection rules (PAS 63100) apply to all chemistries.

Does a home battery work without the internet?

Yes for basic operation: schedules, self-use and backup run on the inverter. What stops without internet is app monitoring, remote changes, dynamic tariff dispatch, AI optimisation, VPP events and firmware updates. Some warranties also shrink if the system stays offline for long, so keep it connected where you can.

Warranty effects: AlphaESS 3 yr, Growatt 3 yr, SigEnergy 5 yr minimum, Tesla 4 yr, GivEnergy may revert to 12 months. Agile or Intelligent tariffs that set charge slots from the cloud need connectivity to follow price changes. MIS 3012 requires the installer to give you network credentials for internet-connected devices.

Does a home battery need maintenance?

Very little: keep the unit clear of stored items, within its temperature range and online so firmware updates and warranty monitoring work. Tesla, for example, may only honour four of its ten warranty years if the Powerwall is left offline for long periods. Watch the app for capacity loss or repeated alerts.

Tesla (EMEA warranty Rev 1.15): 80% capacity at 10 years with unlimited cycles for self-consumption, time-based control and backup in the UK; internet connection and registration needed for full 10 years; minimum 4 years honoured otherwise. Batteries derate charging in cold and hot conditions; garages and lofts can exceed the range (see where-to-put-an-inverter). Keep ventilation clear; do not store flammables next to the battery.

Should I choose a battery based on export tariff income?

Be cautious. Export rates can fall: Outgoing Octopus Fixed dropped from 15p to 12p per kWh on 1 March 2026, its first change since 2022. Using stored energy yourself avoids the full import price, about 26p under the October 2026 cap, so self-consumption is the steadier basis for a business case; treat export and VPP income as a bonus.

Self-use value ~26.32p vs fixed export 12p (Oct 2026). Premium export deals are often tied to a supplier or installer and can be withdrawn. SEG minimum rates from some suppliers are 2-6p (existing tariff rows).

Where can a home battery be installed?

PAS 63100 says outdoors where practicable, for example an outside wall or detached garage. Batteries must not go in lofts, bedrooms, cupboards off bedrooms, hallways, landings or stairs, or basements without outside access. Outdoors they should be at least 1 metre from doors, windows and vents. Indoor spots need fire separation, ventilation and a linked smoke alarm.

Max 20 kWh per enclosure; total 40 kWh or 80 kWh (outdoors/detached/REI 60 garage). IK10 protection near driveways/garages. PAS 63100 excludes homes >200 m2 and second-life batteries.

What to do next

Use your own half-hourly electricity data where possible. Compare usable kWh, kW, tariff rules and warranty terms before buying.

Sources

Last checked: 24 September 2026