EngineerHub.orgWhat this calculator does
Estimate whether a home battery earns value by shifting surplus solar into later household use, charging from a low time-of-use tariff and discharging during expensive hours, or both. It also compares an existing battery with a proposed capacity expansion so you can see the marginal value of adding another module instead of assuming that a larger battery always saves proportionally more.
Inputs
Battery
Electricity prices
Long-term & expansion
Results
Solar + battery interaction
| Metric | Calculated result |
|---|---|
| PV used directly before battery | — |
| PV surplus available | — |
| PV energy sent into battery | — |
| Useful battery output from solar | — |
| Solar self-consumption | — |
| Solar share of household load | — |
| Solar-shifting value | — |
| Grid-arbitrage value | — |
Should I add another battery?
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Why larger batteries show diminishing marginal value
Background & method
The calculator keeps four different questions separate.
Solar self-consumption is the share of PV generation kept on site instead of exported. Solar self-sufficiency is the share of household electricity demand supplied by direct PV plus useful battery discharge from PV. A battery can increase both, but they are not the same metric.
Solar energy charged into the battery would otherwise have been exported. The model therefore values useful battery discharge at the avoided import price, then subtracts the export revenue forgone for the charging energy. Round-trip efficiency is applied explicitly: solar value = battery output × import price − battery input × export price.
Optional off-peak grid charging uses battery throughput that remains after solar charging. Savings are useful discharge × peak price − charging energy × off-peak price. If the price spread is small compared with efficiency losses, arbitrage can have little or even negative value.
Storage has diminishing marginal utility when the first battery already captures much of the available solar surplus or expensive-period load. The expansion comparison reruns the same model with the larger total battery and reports only the incremental energy shifted and savings created by the added capacity.
This is an annual screening model, not an hourly dispatch optimizer. It cannot capture cloudy sequences, seasonal PV surplus, dynamic tariff rules, inverter limits, standby power, battery temperature, warranty throughput limits, taxes, financing or demand charges. Entered energy tariffs are held constant through the NPV period; the capacity-degradation assumption changes battery throughput over time. Use actual interval-meter data and the battery manufacturer's usable-energy and warranty data before making a purchase decision.
- U.S. DOE — Solar Energy and Storage Basics: storage shifts energy between times and energy capacity (kWh) is distinct from power capacity (kW).
- NREL/NREL — Technology Tips for Solar + Storage (DOI): sizing, autonomy and diminishing marginal utility of additional storage.
- PVWatts Calculator — NREL or JRC PVGIS for site-specific PV production.
FAQ
Common questions about battery economics.