Sizing a battery bank or UPS? Estimate how long it will run a load from its capacity, voltage, usable depth of discharge and system efficiency.
Quoting a battery or off-grid job? traqR takes your sizing through to a polished quote, then a scheduled job, then the invoice.
Start free trialCapacity (Ah) times voltage gives total watt-hours. Multiplying by the depth of discharge gives the energy you can actually use without over-draining the battery.
Inverters and wiring aren’t perfect, so multiply by system efficiency (around 90%) to get the energy actually delivered to the load.
Delivered watt-hours divided by the load in watts gives the runtime in hours. A bigger load or deeper-but-limited discharge shortens it.
Multiply capacity (Ah) by voltage to get watt-hours, apply the usable depth of discharge and system efficiency, then divide by the load in watts. For example, a 200 Ah 48 V lithium bank at 90% DoD and 90% efficiency delivers roughly 7,800 Wh — about 9.7 hours running an 800 W load.
Depth of discharge (DoD) is how much of the battery’s capacity you use before recharging. Lithium (LiFePO₄) batteries commonly allow 80–90% DoD, while lead-acid batteries last much longer if kept to around 50%. Regularly discharging deeper than recommended shortens battery life.
Energy is lost as heat in the inverter, wiring and the battery itself, so not all the stored energy reaches the load. A good inverter system is around 90% efficient; factoring that in gives a realistic runtime rather than an optimistic one.
Yes. Cold reduces the usable capacity of most batteries, and high temperatures shorten their life. Manufacturers rate capacity at a reference temperature (often 25°C), so real-world runtime in a hot roof space or a cold morning can differ. Size with some margin.
This calculator is a general estimate. Real runtime depends on battery chemistry, age, temperature, discharge rate and inverter efficiency, and manufacturers’ limits should always be followed. Battery and off-grid systems must be designed and installed to the relevant standards by a licensed installer.
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