What size battery do I need for my solar panels?

The optimal battery size depends on your daily solar surplus — the energy your panels generate that you don't use directly. For a typical UK home with a 4kWp system, daily generation averages 10.8kWh, self-consumption is about 35% (3.8kWh), leaving 7kWh surplus. A 7-8kWh battery would capture most of this. Going larger than your daily surplus provides diminishing returns unless you're using tariff arbitrage. Most UK homes suit a 5-10kWh battery.

What is depth of discharge (DoD) and why does it matter?

DoD is the percentage of battery capacity that is actually used. A 10kWh battery at 90% DoD provides 9kWh usable energy. LFP (lithium iron phosphate) batteries can safely cycle to 90-100% DoD. NMC batteries are typically limited to 80-90% DoD to preserve longevity. A higher DoD means more usable energy from the same rated capacity, but may reduce cycle life for some chemistries.

Is tariff arbitrage worth it without solar panels?

Yes — if you're on a time-of-use tariff like Octopus Agile or Intelligent, you can charge the battery during cheap off-peak periods (e.g., 00:30-04:30 at 7-10p/kWh) and discharge during peak periods (4pm-7pm at 30-40p/kWh). A 10kWh battery cycling daily could save £600-900/year on tariff arbitrage alone. The round-trip efficiency (typically 90-95%) means you lose some energy in the process, but the price differential more than compensates.

How long do home batteries last?

Modern LFP batteries are rated for 6,000-10,000 cycles, which at one cycle per day equates to 16-27 years. NMC batteries typically offer 3,000-5,000 cycles (8-14 years). Most manufacturers warranty their batteries for 10 years at 70-80% of original capacity. Real-world degradation depends on temperature, cycling depth, and charge/discharge rates. LFP chemistry is now preferred for home storage due to longer life, better safety, and lower degradation.

Do I need a special electricity meter for battery storage?

If you're claiming Smart Export Guarantee (SEG) payments for exported solar energy, you need a smart meter (SMETS2) that can measure import and export separately. For tariff arbitrage, you need a smart meter and a compatible time-of-use tariff. The battery inverter must be configured not to export grid-charged energy if your SEG contract prohibits it (most do). Some battery systems like Tesla Powerwall and GivEnergy can be configured to handle this automatically.

TradeCalculator

Battery Storage Calculator — UK

Calculate optimal battery storage size based on your solar generation, daily usage pattern, and electricity tariff. Includes tariff arbitrage savings for time-of-use tariffs.

Daily Electricity Usage

Annual kWh ÷ 365

Solar PV System Size

Installed array capacity

Usage Profile

Affects self-consumption rate

Battery Chemistry

LFP preferred for longer life

Import Electricity Rate

Your standard unit rate

Export Rate (SEG)

Smart Export Guarantee rate

How We Calculate This

This calculator determines the optimal battery storage capacity by analysing your daily solar surplus and electricity usage pattern.

Key Calculations

Daily surplus = Solar generation × (1 - Self-consumption rate)

Optimal battery = Surplus ÷ DoD

Saving = Captured kWh × (Import rate - Export rate)

UK Averages

Daily solar: ~2.7 kWh per kWp (annual average)
Self-consumption (no battery): 30-50% depending on lifestyle
Self-consumption (with battery): 70-90%
Round-trip efficiency: 90-95% for lithium batteries
SEG export rate: ~12p/kWh (varies by supplier)

Frequently Asked Questions

Related Calculators

Cable Sizing Calculator

Calculate the correct cable size based on BS 7671 18th Edition regulations.

Voltage Drop Calculator

Check voltage drop complies with UK wiring regulations.

Maximum Demand Calculator

Calculate maximum electrical demand for domestic installations.

Circuit Breaker Sizing Calculator

Determine the correct MCB rating for your circuit.

Socket Spacing Calculator

Plan socket layouts with Part P guidance on spacing and positioning.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.