Solar Battery ROI Calculator

Annual battery benefit = avoided import cost + export-value change − charging losses − battery degradation impact − replacement cost (if within the analysis period). Both scenarios are computed for the same solar system, so solar self-consumption in the daytime stays identical - only the night economy changes.

Monthly energy model: each calendar month is computed separately. Solar generation follows a seasonal monthly yield curve around your annual average; monthly consumption follows a seasonal load shape that sums to your annual use. Each month: day load is served directly; surplus charges the battery (capped by usable capacity and max charge power × ~5 effective sun-hours per day); the battery discharges to cover night load; remaining night load imports from grid; remaining surplus exports at the buyback rate.

Round-trip efficiency convention: RTE is applied once, at discharge - energy available to the load = charged energy × RTE, and charging losses (charged × (1 − RTE)) are shown separately. There is no double counting.

Degradation: linear annual capacity loss from 100% down to the end-of-warranty floor (e.g. 2%/yr → 82% at year 10). Delivered energy and savings scale with the remaining capacity each year.

Life & replacement (annualized model): battery life = min(calendar life, cycle life ÷ (equivalent full cycles × 365)); the battery is replaced every ceil(life) years. The replacement cost is charged at the start of that year and the new battery starts at 100% capacity - degradation resets with each replacement. Real replacement timing depends on actual cycling, capacity and warranty conditions.

Financial: NPV and IRR use the discount rate you set; discounted payback is the first year cumulative discounted cash flow turns positive; LCOS = (installed price + discounted replacement costs) ÷ discounted kWh delivered over the analysis period, both discounted at your discount rate. One LCOS definition is used everywhere on this page.

Explicit assumptions: solar yield kWh/kWp/day, daytime consumption share, tariff escalation, outage hours, solar capex for comparison and battery price-per-kWh (recommend mode) are editable defaults - each is labelled in the UI as user input, manufacturer spec, calculated, estimated or assumption.

Results are engineering estimates for decision support, not a guarantee. Real-world economics sit between the Conservative and High-value sensitivity scenarios.