Battery Storage Estimator
Estimate the battery nameplate capacity needed to cover a load for a set backup period.
Battery storage systems are sized to deliver a specified amount of energy over a defined backup or discharge period. But the nameplate capacity printed on a battery — its total stored energy — is always larger than the usable energy you can actually draw from it. Two factors account for the difference: round-trip efficiency (RTE), the fraction of energy retained after a charge–discharge cycle; and depth of discharge (DoD), the fraction of nameplate capacity the battery chemistry allows you to use without degrading it prematurely.
This tool calculates the required nameplate capacity from the load, duration, RTE and DoD: Nameplate (kWh) = (Load × Hours) ÷ (RTE × DoD). The result is a simplified first estimate. Real battery system design must also account for capacity degradation over the system lifetime, temperature effects on usable capacity, peak load (C-rate) requirements, and the power electronics connecting the battery to the grid or load. Always engage a qualified storage engineer for real projects.
Battery sizing result
Nameplate capacity = (Load × Hours) ÷ (RTE × DoD). This is a simplified first-pass estimate. Real battery sizing must account for degradation over the battery lifetime, temperature effects on capacity, charge/discharge rate limits (C-rate), and peak vs. average load profiles. Always consult a qualified storage engineer for real projects.
How to use this tool
- Enter the average load the battery must supply in kilowatts.
- Enter the required backup duration in hours.
- Set the round-trip efficiency as a percentage (modern lithium-ion systems: typically 85–95 %).
- Set the depth of discharge as a percentage (typical lithium-ion: 80–95 %; lead-acid: 50–80 %).
- Press Calculate to see the usable energy required and the nameplate battery capacity needed.
💡 Good to know
All calculations use standard, published formulas and simplified assumptions so beginners can build intuition. For real projects, always consult qualified engineers and site-specific data.
Frequently asked questions
What is round-trip efficiency?
Round-trip efficiency is the ratio of energy delivered on discharge to energy consumed on charge, expressed as a percentage. A battery with 90 % RTE returns 90 kWh for every 100 kWh put in. The remaining 10 % is lost as heat during charging and discharging. Lithium-ion batteries typically achieve 85–95 % RTE; older lead-acid technologies are lower.
What is depth of discharge?
Depth of discharge is the fraction of nameplate capacity that can be routinely used before the battery must be recharged. Discharging a battery beyond its recommended DoD accelerates chemical degradation and shortens its service life. Lithium iron phosphate (LFP) cells are often rated for 90–95 % usable DoD; other lithium chemistries may be limited to 80 %.
Why does the nameplate need to be larger than the usable energy?
Both RTE and DoD reduce the effective energy available from a given nameplate. For example, with 90 % RTE and 90 % DoD, a 100 kWh nameplate battery delivers only 81 kWh of usable energy (100 × 0.90 × 0.90). To get the usable energy you need, you must install a proportionally larger nameplate.
Does this account for battery ageing?
No. Battery capacity degrades over time — typically 20–30 % over 10 years for lithium-ion under normal cycling. A real system sized for year-ten performance would start with additional margin above the values this tool gives. Consult manufacturer datasheets and a storage engineer for lifetime sizing.