What size battery do I need for a solar system?

Battery size depends on how much solar energy you currently export. A typical UK home with a 4 kWp solar system exports around 6-10 kWh per day in summer. A 5-10 kWh battery captures most of this surplus for evening use. Oversizing beyond your daily export provides diminishing returns unless you use off-peak tariff arbitrage.

What is the payback period for a home battery in the UK?

At current UK electricity prices (around 24.5p/kWh) and battery costs (around £400/kWh installed), payback periods typically range from 7-12 years depending on usage patterns, solar system size, and whether you use an off-peak tariff for additional arbitrage savings. Higher electricity prices and lower battery costs improve the payback.

What is the difference between LFP and NMC batteries?

Lithium Iron Phosphate (LFP) batteries offer more cycles (6,000+), longer lifespan, and better safety but are slightly larger and heavier. Lithium NMC batteries are more energy-dense (smaller) but have fewer cycles (4,000) and higher degradation. For home storage, LFP is increasingly preferred for its longevity.

Can I use a battery without solar panels?

Yes, batteries can be used for tariff arbitrage - charging from cheap off-peak electricity (as low as 7.5p/kWh on some EV tariffs) and discharging during peak periods (24.5p/kWh+). This can save £200-400 per year. However, the economics are best when combined with solar generation.

How long do home batteries last?

Most home batteries are warranted for 10 years or 6,000-10,000 cycles. LFP batteries typically retain 80%+ capacity after 10 years of daily cycling. Actual lifespan depends on charge/discharge cycles, depth of discharge, and temperature. Many batteries will last 12-15 years with moderate use.

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Battery Storage Calculator — Home Battery Sizing

Calculate the optimal battery storage capacity for your home solar system. Compare savings with and without battery storage.

Daily Electricity Usage

Average daily consumption

Solar System Size

Installed or planned solar capacity

Electricity Rate

Current grid import rate

Export Tariff

SEG or export rate

Usage Pattern

When you use most electricity

How We Calculate This

This calculator determines the optimal battery capacity by analysing how much solar energy you currently export and how a battery could capture that energy for later self-consumption.

How battery savings work

Solar shifting: Store excess daytime solar generation for evening use, saving the difference between import rate (24.5p) and export rate (5.5p) per kWh
Tariff arbitrage: With an off-peak tariff, charge from cheap overnight electricity and discharge during peak periods
Self-consumption boost: A well-sized battery can increase solar self-consumption from 30-50% to 70-90%

Sizing approach

The recommended battery size is based on capturing the majority of daily solar export while avoiding oversizing. Diminishing returns apply - the first few kWh of battery capacity provide the most benefit.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.