Solar Battery Bank Sizing Calculator

Free UK solar battery bank sizing calculator. Size your battery in Ah and kWh for off-grid cabins, motorhomes, or grid-tied storage based on daily load and autonomy.

Solar Battery Bank Sizing Calculator

Daily load (Wh) System voltage (V) Days of autonomy Battery chemistry Depth of discharge (%) Inverter efficiency (%) Battery round-trip efficiency (%)

Usable energy needed

10,000 Wh

Gross capacity needed

12,713 Wh

Bank capacity (Ah)

530 Ah

Bank capacity (kWh)

12.7 kWh

How to use this calculator

Enter your average daily load in watt-hours (Wh), choose your system DC voltage (12V, 24V, or 48V), set days of autonomy you need without solar input, then pick your battery chemistry. The tool returns capacity in both Ah and kWh.

If you don’t know your daily load, take your monthly bill in kWh, multiply by 1000, then divide by 30. For grid-tied backup, only count the loads you’d run in an outage (fridge-freezer, broadband, lights, boiler controls, perhaps the heat pump’s controls but not the compressor) — usually 3–8 kWh/day, not the Ofgem TDCV medium-household figure of around 7.4 kWh/day total.

The formula

Usable energy needed (Wh) = Daily load × Days of autonomy
Gross capacity (Wh)       = Usable / (DoD × Inverter eff × Battery eff)
Bank capacity (Ah)        = Gross capacity / System voltage

DoD is depth of discharge (90% LiFePO4, 50% AGM/Gel lead-acid). Inverter efficiency is typically 92% for a quality pure-sine inverter under load. Battery round-trip efficiency is 95% for LiFePO4 and 80–85% for lead-acid.

Worked example — 5 kWh/day off-grid cottage

Grid-isolated cottage in Cumbria with 5,000 Wh daily load, 3 days of autonomy, 48V LiFePO4:

Usable energy: 5,000 × 3 = 15,000 Wh
Gross capacity: 15,000 / (0.90 × 0.92 × 0.95) = 19,065 Wh
Bank size: 19,065 / 48 = 397 Ah at 48V (or 19 kWh)

That maps to roughly two 5kWh GivEnergy or Pylontech rack batteries plus one extra module, or four 100 Ah 48V LiFePO4 modules. With AGM lead-acid the same load needs about 800 Ah at 48V — twice the cells and triple the floor area in a typical battery cabinet.

Battery chemistry — the dominant cost driver

Per Energy Saving Trust and Solar Energy UK 2025 cost guides, the typical 2026 installed costs in the UK are:

LiFePO4 lithium (rack-mount): £700–1,000 per kWh installed (Pylontech, GivEnergy, Sunsynk, BYD)
AGM lead-acid: £350–550 per kWh installed, but you need 1.8–2× the kWh to match usable energy — net cost similar or higher
Flooded lead-acid: cheapest per nameplate kWh (£200–350) but requires ventilation per BS 7671 and only 1,000–1,500 cycles vs 4,000–6,000 for LiFePO4

Most MCS-certified installers in 2025–2026 quote LiFePO4 by default; the wholesale price on rack-mount lithium fell below £500/kWh during 2024 making lead-acid uneconomic for new installs.

BS 7671 and MCS compliance

The calculator handles the energy maths. BS 7671 (the IET Wiring Regulations, 18th Edition Amendment 2:2022) and MCS 020 add the regulatory layer:

Battery enclosure ventilation (lead-acid: hydrogen; lithium: thermal containment per BS EN 62619)
DC isolation and overcurrent protection per BS 7671 Section 712
Cable sizing per BS 7671 Appendix 4 — see the Solar Panel Wire Size Calculator for sizing
MCS 020 limits for residential storage installations
DNO notification under ENA G99 for grid-tied systems above 3.68kW per phase

For domestic installations the battery should be MCS-certified and installed by an MCS-registered contractor to qualify for the 0% VAT relief, the SEG export rate, and most home insurance schemes.

How to estimate daily load

A typical UK load table for a small off-grid or backup setup:

Fridge-freezer (A++ rated): 0.8–1.0 kWh/day
LED lighting (8 fixtures × 5 hours × 8W): 0.32 kWh/day
Laptop + broadband router: 0.4 kWh/day
Boiler controls + heat pump compressor: 1.5–4 kWh/day in winter (often the dominant load)
Kettle/microwave (intermittent): 0.5 kWh/day
TV + games console: 0.5 kWh/day

Add 15% for inverter standby, ghost loads, and BMS self-consumption. Real-world UK cabin baselines run 3–6 kWh/day; full-house off-grid is rare below 12 kWh/day given UK heating loads.

Sources cited

Energy Saving Trust storage sizing guide (2025)
Solar Energy UK Domestic Battery Storage Code of Practice
BS 7671:2018 Amendment 2:2022, Section 712 and Appendix 4
MCS 020 Battery Storage Standard
Ofgem TDCV (Typical Domestic Consumption Values), 2024 publication
HMRC Notice 708/6 (0% VAT on energy-saving materials)
DNO ENA Engineering Recommendation G99

Cross-check your design with an MCS-certified installer (the gov.uk Trustmark search lists active firms) before purchasing. The 0% VAT relief and SEG eligibility both require MCS certification — DIY installs forfeit both.

Frequently asked questions

What size battery bank do I need for a UK off-grid cabin?

For a typical small UK cabin or shepherd's hut running 3–5 kWh/day with two days of autonomy, you need around 250–400 Ah at 24V (or 125–200 Ah at 48V) of usable lithium capacity. With AGM lead-acid you need roughly double that because safe depth of discharge is 50% rather than 90%.

Is a battery worth it on top of UK rooftop solar?

With the SEG export rate at roughly £0.08/kWh in 2026 versus a tariff cap import rate near £0.27/kWh, every kWh you self-consume saves about £0.19. A 5 kWh battery cycling daily can save £200–350 per year — payback on a £3,500 install is 12–18 years, close to the warranty horizon. Worth it for resilience and bill stability rather than pure ROI.

Should I pick 12V, 24V, or 48V?

12V suits small motorhomes and narrowboats below 1500W of inverter load. 24V suits a typical garden room or off-grid cabin at 3–5 kWh/day. 48V is required by most home battery products (Tesla, GivEnergy, Sunsynk) and makes sense above 5 kWh/day because it cuts cable current by 4× compared to 12V — important for BS 7671 compliance.

How many days of autonomy should I plan for in the UK?

Two days is standard for grid-tied backup. Off-grid sites in Scotland or northern England typically plan for 3–5 days because winter solar yield can drop to 10% of summer. Above 5 days, a small petrol or LPG generator becomes more economical than buying additional cells.

Do home batteries qualify for the 0% VAT rate?

Yes — since February 2024, batteries installed alongside or retrofit to existing solar PV qualify for 0% VAT under the energy-saving materials relief, extended through March 2027. This applies to the supply and install. MCS-certified installers handle the paperwork; DIY installs do not get the relief.

Solar Battery Bank Sizing Calculator