Solar Battery Size Calculator
Estimate a practical solar battery size from daily electricity use, the share used after sunset, backup reserve and system efficiency.
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How to choose the inputs
This is a conservative energy-budget heuristic, not precise storage engineering. Round-trip efficiency describes a full charge/discharge cycle, so using it as a capacity divisor can overstate the stored capacity needed compared with a model using discharge efficiency separately.
Formula and measurement boundary
Three worked scenarios
These are fixed hypothetical examples, not live tariffs or verified product data. They use EUR and kilometres where applicable and do not change with the form settings. Only the table input changes.
Unchanged inputs
- Daily household use: 20 kWh
- Use outside solar hours: 60 %
- Usable battery capacity: 90 %
- Round-trip efficiency: 90 %
| Backup reserve | Estimated result |
|---|---|
| 0 % | 14.81 kWh battery |
| 20 % | 18.52 kWh battery |
| 40 % | 24.69 kWh battery |
Each example shifts 12 kWh. More backup reserve leaves less capacity for daily use and raises the nominal estimate. Here reserve is a fraction of the usable window, not necessarily the same definition as a manufacturer’s state-of-charge setting.
What this result does not establish
- The tool does not check that enough daytime solar surplus exists to charge this battery.
- Reserve and usable-share restrictions must not describe the same reduction twice.
- Hourly production, seasonal demand, power limits, economics and off-grid reliability need a separate model.
A battery needs both spare daytime energy and later demand
The following are additional hypothetical decision examples. Separately discussed fees, reserve allowances or running costs are not automatically added to the form result.
Suppose measured daytime export is 8 kWh and later household demand is 5 kWh. With 90% round-trip efficiency, delivering 5 kWh requires about 5.56 kWh of daytime charging energy. If only 3 kWh is exported on a dull day, no larger battery can create the missing energy: at most 2.7 kWh can be delivered from that surplus under the same efficiency assumption.
Avoid sizing solely from annual solar generation or a daily average. Make a table of daylight export and evening/night demand for representative summer, shoulder-season and winter days. Count how often the proposed capacity can actually be filled and emptied. A larger battery may sit partly empty in winter and have unused capacity on low-demand summer days.
This calculator is an energy-sizing aid, not an hourly dispatch model or an economic recommendation. A full assessment needs charge/discharge power, usable state-of-charge range, inverter compatibility and export compensation. Value a shifted kWh by the avoided import cost minus the export income forgone and losses; it is not automatically worth the full retail tariff.
Sources and their scope
- European Commission JRC: PVGIS — Location-specific PV estimates; not a tariff or payback guarantee.
These reference pages were accessed on 21 September 2026. The equations and examples are the site’s own model; these sources do not certify its defaults or results.
Frequently asked questions
Is this the battery size I should buy?
Not on this result alone. Check measured hourly demand, available solar surplus and the exact system definitions with a designer.
Does it simulate winter autonomy?
No. One typical daily energy balance does not model a sequence of low-generation days.
Read the model and verification limits