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Battery Runtime Calculator: UPS, Power Station and Battery Bank
Estimate runtime or reverse-size battery capacity in Wh or Ah at a nominal voltage. Add device loads, separate depth of discharge from inverter efficiency, and optionally model lead-acid Peukert behaviour.
Battery Runtime Calculator: UPS, Power Station and Battery Bank: Starts with a 1000 Wh battery and a two-device AC load. Standard runtime is rated Wh times depth of discharge times inverter efficiency divided by watts. Reverse sizing solves that equation for rated Wh. The optional lead-acid estimate changes capacity with discharge rate. For a single small battery with mAh and current, use the [battery life calculator](/battery-life-calculator). Runs 100% locally in your browser with zero server file uploads.
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Typical appliance examples are illustrative planning values. Replace them with nameplate ratings and measured starting watts; they are not product specifications.
| Appliance | Running W | Starting W | Add to list |
|---|---|---|---|
| Refrigerator | 150 | 600 | |
| Water pump | 800 | 2400 | |
| Lighting | 60 | 60 | |
| Kettle | 1500 | 1500 | |
| Laptop | 60 | 60 |
Formula: Wh = Ah × V; standard runtime = Wh × DoD × inverter efficiency ÷ watts. Reverse capacity = hours × watts ÷ (DoD × efficiency). Percentages are divided by 100.
Rated capacity: 1000 Wh / 83.33 Ah at 12 V. DoD 80%; efficiency 90%. Starting surges, inverter standby consumption, ageing, cold temperatures and voltage cutoffs are excluded. Add standby draw to the load and check the inverter's power limit separately.
Peukert is a constant-current approximation for lead-acid, using nominal voltage and an entered rated discharge duration (often 20 hours). High current reduces available capacity; low-current extrapolation can overestimate it. DoD is applied as a planning share of predicted full-discharge time. Use the manufacturer's discharge curves for final sizing; lithium normally uses k = 1.
Energy and runtime
Rated Wh equals Ah times nominal volts. Standard t = C × d × η / P. Reverse C = t × P / (d × η), with percentages entered as fractions. The model excludes standby draw unless included in the load, and does not check inverter power ratings. SI unit background: https://www.nist.gov/pml/special-publication-811
Lead-acid discharge rate
Doerffel and Abu Sharkh, A critical review of using the Peukert equation for determining the remaining capacity of lead-acid and lithium-ion batteries, Journal of Power Sources 155 (2006), 395–400. The rate model here is t = d × H × (C × η / (P × H))^k; C is rated Wh at H hours. Variable current and temperature are outside this approximation. Source: https://doi.org/10.1016/j.jpowsour.2005.04.030
How to use it
- Choose runtime or reverse capacity, then enter Wh or Ah and nominal voltage.
- Enter the load directly or add devices, depth of discharge and inverter efficiency.
- For lead-acid, enter the Peukert exponent and capacity-rating duration; read the result and assumptions.
Privacy & limitations
Battery and appliance data are calculated locally in your browser.
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Frequently asked questions
Does this include inverter idle draw and starting surges?
No. Add standby draw to the running load and check the inverter's continuous and surge ratings separately. Cold, ageing, voltage sag and shutdown thresholds can shorten runtime.
What does the Peukert option assume?
A constant load, nominal voltage and capacity specified at the entered rating duration, often 20 hours. It applies depth of discharge as a share of predicted full-discharge time. Use the battery maker's curves for final sizing, especially at very high or low currents.
How is this different from the battery life calculator?
This worksheet targets UPS and bank planning with multiple devices, separate discharge and inverter allowances, reverse capacity and lead-acid rate effects. The battery life page handles a single device in mAh/mA or Wh/W.
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