Calculate the watt-hours (Wh) and amp-hours (Ah) needed for your load and runtime
Total Energy Needed
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Wh (before losses)
Battery Capacity Required
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Wh (with efficiency)
Amp-Hours Needed
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Ah (with DoD applied)
Usable Capacity
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Wh (after DoD)
Energy Loss (Efficiency)
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Wh wasted as heat
Recommended Battery Size
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Formula Used:
Wh = (Load × Runtime × Devices) ÷ Efficiency
Ah = Wh ÷ (Voltage × DoD)
Recommended Size = Ah × 1.2 (20% safety margin)
To calculate battery capacity in watt-hours, multiply the total load in watts by the desired runtime in hours. Then divide by the battery efficiency (typically 80–90%) to account for energy losses. For example, a 100W device running for 5 hours at 85% efficiency requires approximately 588 Wh.
Divide the total watt-hours needed by the battery voltage to get amp-hours. For instance, if you need 600 Wh from a 12V battery system, you would need 50 Ah. Always add a 20% buffer for depth of discharge limits to extend battery lifespan.
Lithium-ion (LiFePO4) batteries can safely discharge to 80–90% of their rated capacity, while lead-acid batteries should only be discharged to 50% to avoid damage. This means a 100Ah lead-acid battery effectively provides only 50Ah of usable capacity, whereas a 100Ah lithium battery provides 80–90Ah.