Battery storage sizing is the most common point of confusion in domestic solar consultations. Customers frequently arrive with a number in mind — “I want a 10 kWh battery” — without working through whether that capacity matches their actual consumption patterns. Oversized batteries discharge incompletely each day, wasting the capital spent on unused capacity. Undersized batteries fill by midday and export the afternoon’s generation at the lower SEG rate when they could have stored it.
The right approach starts with consumption data, not with a capacity target.
The daily cycle analysis
A battery’s value comes from daily cycling — charge during solar generation hours, discharge during evening peak demand. The maximum useful capacity is the kilowatt-hours your household or business will actually consume between the end of solar generation (typically 17:00 to 18:00 in winter, 19:00 to 20:00 in summer) and the beginning of the next morning’s solar charge (08:00 to 09:00).
For a typical South Wales household consuming 12 kWh per day with 40% of that demand falling in the post-solar evening window, the useful nightly discharge is approximately 4.8 kWh. A 5 kWh battery with 90% depth of discharge (4.5 kWh usable) is close to the optimum. A 10 kWh battery would sit half-charged each morning — the second 5 kWh was paid for and never used.
Rule of thumb: 1 kWh to 1.5 kWh of battery per kWp of solar array. This ratio works well for weekday-absent households. Households with high daytime occupancy (retirees, home workers) have lower evening discharge requirements and should bias toward the lower end of the range.
Household type sizing guide
| Household type | Solar array | Recommended battery | Reason |
|---|---|---|---|
| 2-bed terrace, couple out all day | 3 kWp | 5 kWh | High export rate needs capturing |
| 3-bed semi, one home worker | 4 kWp | 5—7.5 kWh | Partial daytime use reduces surplus |
| 4-bed detached, family | 5—6 kWp | 7.5—10 kWh | Higher evening demand |
| Detached with EV | 5—6 kWp | 10 kWh | EV morning charge from stored solar |
| Rural with heat pump | 6—8 kWp | 10—15 kWh | High overnight demand for heating |
EV integration changes the calculation
Adding an EV to a solar-plus-battery household changes the optimal battery size upward. If the EV charges overnight from the battery rather than from grid, the battery needs to accommodate both household evening demand and the EV morning charge. A household charging an EV three nights per week at 10 kWh per session requires an additional 30 kWh of weekly battery throughput — roughly 4.3 kWh of additional daily average battery capacity.
A practical approach: size the battery for household load first, then review whether the EV charging demand can be partially met by solar-divert daytime charging (avoiding battery throughput entirely for midday EV charging sessions).
Blackout-rated versus non-blackout batteries
Most domestic battery systems sold in the UK are grid-tied only: they charge and discharge but cannot operate during a grid power cut. If the grid goes down, the battery shuts off alongside the solar inverter as a safety requirement under G98/G99 regulations.
Blackout-rated (backup-capable) batteries include a separate backup output circuit that can power a defined load during a grid outage. GivEnergy AIO, SolarEdge Backup Interface, and certain Solis hybrid configurations support this mode.
FLD specifies blackout-rated batteries as standard for:
- All rural SA67, SA9 and LD3 properties with elevated grid interruption frequency
- Properties dependent on well pumps, medical equipment or refrigeration for vulnerable occupants
- Farm and agricultural buildings where power interruption risks livestock welfare
For urban properties with reliable grid supply, the blackout-rated premium (approximately £500 to £1,000 over standard) is optional but often chosen by clients who work from home.
Commercial battery sizing: peak shaving and demand management
For commercial and industrial sites on half-hourly metered tariffs, battery sizing is driven by demand management economics as well as energy arbitrage. A business with a contracted import capacity of 500 kW and a regular peak demand of 450 kW can reduce demand charges significantly by discharging battery storage during peak periods to stay below a defined setpoint.
FLD’s commercial battery sizing methodology uses 12 months of half-hourly consumption data (from the site’s smart meter) to identify peak demand events, quantify the demand charge exposure, and size the battery to flatten peak events within the export limiting constraints of the DNO connection.
For most South Wales industrial sites, the demand management case alone — reducing capacity charges and triad exposure — justifies a 200 kWh to 500 kWh battery installation independent of the solar generation benefit.
Manufacturer platforms
FLD installs battery storage from three primary platforms in 2026:
- GivEnergy AIO — all-in-one hybrid inverter and battery unit, well-suited to domestic new installs. Strong UK service network and app monitoring.
- SolarEdge StorEdge — the preferred retrofit platform for existing SolarEdge solar installations. Maintains panel-level monitoring alongside battery management.
- Solis hybrid inverters with Pylontech batteries — cost-effective commercial platform for 100 kWh to 500 kWh BESS installations.
Call Paul on 01792 680611 or use the contact page to request a battery sizing assessment for your property.
