Battery Storage ROI Calculator

Battery storage saves money in two main ways: tariff arbitrage (charging on cheap off-peak electricity and discharging during expensive peak hours) and solar self-consumption (storing excess solar generation for use in the evening instead of exporting at low SEG rates — Octopus SEG is about 4.1p/kWh and Outgoing Octopus about 12p/kWh in 2026). Round-trip losses of roughly 10-12% (AC efficiency about 88-90%) mean you import a little more than you get back, so the calculator grosses up the off-peak cost. Time-of-use tariffs such as Octopus Go or Intelligent Octopus Go (about 7-8.5p overnight) maximise arbitrage savings.

Round-trip efficiency is the percentage of energy put into the battery that you get back out after inverter and cell losses. Tesla Powerwall 2 is about 90% AC, the newer Powerwall 3 about 97.5%, and LFP systems such as GivEnergy are typically 88-93%. At 88% efficiency, importing 1 kWh of cheap off-peak power only returns about 0.88 kWh at peak, so ignoring it overstates savings by 10-12%. This calculator applies round-trip efficiency to grid-charged (arbitrage) cycles.

Depth of discharge (DoD) is the percentage of total battery capacity that is actually used. Most lithium-ion batteries have a DoD of 90-95%, meaning a 10 kWh battery provides 9-9.5 kWh of usable capacity. Running to 100% DoD reduces battery lifespan.

Most home batteries come with a 10-year warranty and are expected to last 12-15 years. Capacity typically degrades by 1.5-3% per year, so after 10 years the battery may have 70-85% of its original capacity. Lithium iron phosphate (LFP) batteries degrade more slowly than NMC chemistry.

With a time-of-use tariff and solar PV, payback is typically 6-10 years in the UK. Without solar or TOU tariff, payback can be 12-15+ years. The economics improve with larger tariff differentials, more daily cycles, and falling battery costs.

Yes. With time-of-use tariffs you might charge overnight and discharge in the morning peak, then charge from solar midday and discharge in the evening peak — achieving 1.5-2 cycles per day. Some tariffs like Agile allow 2+ cycles. More cycles improve ROI but increase battery degradation slightly.