Key numbers
- 3 h 14 min: 12 V 100 Ah LiFePO4 battery running 300 W through a 90% inverter
- 1,200 Wh stored, 972 Wh usable in the default example
- Usable share assumed: 90% lithium, 80% small UPS battery, 50% lead-acid/AGM
- The same battery as lead-acid (50%) lasts only 1 h 48 min at 300 W
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How to use the Battery Runtime Calculator
- 1Enter your numbersFill in load (W), battery voltage (V), battery capacity (Ah), number of batteries, battery type and inverter efficiency (%). Every field starts with a typical value you can overwrite.
- 2Check the assumptionsRuntime = V × Ah × batteries × usable % × inverter efficiency ÷ load. Lead-acid batteries deliver less at high loads (Peukert effect), so short, heavy loads run shorter than this estimate.
- 3Read the resultsThe calculator shows estimated runtime, usable energy, total stored energy and battery current at this load, and updates as you type.
- 4Use the resultCompare it with your bill, a quote or a product spec sheet before you buy or install anything.
Results update as you type. Back to the calculator ↑
This battery runtime calculator estimates how long a battery bank can power a load through an inverter or UPS. It works for a solar battery, an RV house battery, a portable power station or the battery inside a desktop UPS. Enter the load, battery voltage, amp-hours, how many batteries and the chemistry, and it returns the runtime, usable energy and battery current.
How battery runtime is calculated
- Stored energy (Wh) = battery voltage × amp-hours × number of batteries
- Usable energy (Wh) = stored energy × usable share × inverter efficiency
- Runtime (hours) = usable energy ÷ load watts
- Battery current (A) = load watts ÷ inverter efficiency ÷ battery voltage
The usable share depends on battery type: 90% for lithium iron phosphate (LiFePO4), 80% for small sealed UPS batteries, and 50% for flooded lead-acid or AGM. Shallower cycling extends battery life, as Battery University explains for lithium cells.
Worked example with the default values
- Battery: 12 V × 100 Ah × 1 battery = 1,200 Wh stored
- LiFePO4, 90% usable: 1,200 × 0.90 = 1,080 Wh
- Inverter 90% efficient: 1,080 × 0.90 = 972 Wh delivered to the load
- Load 300 W: 972 ÷ 300 = 3.24 hours = 3 h 14 min
- Battery current: 300 ÷ 0.90 ÷ 12 = 27.8 A
Swap to lead-acid and usable energy drops to 540 Wh, or 1 h 48 min. A small sealed UPS battery at 80% gives 864 Wh, about 2 h 53 min.
Battery runtime reference table
Runtime of one 12 V, 100 Ah battery through a 90% efficient inverter (before Peukert losses):
| Load | LiFePO4 (90%, 972 Wh) | UPS battery (80%, 864 Wh) | Lead-acid/AGM (50%, 540 Wh) |
|---|---|---|---|
| 50 W | 19 h 26 min | 17 h 17 min | 10 h 48 min |
| 100 W | 9 h 43 min | 8 h 38 min | 5 h 24 min |
| 300 W | 3 h 14 min | 2 h 53 min | 1 h 48 min |
| 500 W | 1 h 57 min | 1 h 44 min | 1 h 5 min |
| 1,000 W | 58 min | 52 min | 32 min |
For two batteries, double the times; for a 24 V bank of the same amp-hours, also double them.
What affects real runtime
- Peukert effect. Lead-acid capacity is rated at a slow (often 20-hour) discharge. At high loads the battery delivers less, so short, heavy loads run shorter than the table (Battery University BU-501). Lithium is far less affected (Doerffel & Sharkh, 2006).
- Temperature. Cold batteries deliver less energy. Store and use them at moderate temperatures where possible.
- Battery age. Capacity fades with cycles. A five-year-old UPS battery may hold much less than its label.
- Inverter idle draw. Inverters use some power even with no load. On small loads this can be a big share.
- Cycling loads. Fridges and pumps cycle on and off, so use their average draw, not the peak.
Do not run to empty
History of Peukert’s law
In 1897 the German scientist Wilhelm Peukert published “Über die Abhängigkeit der Kapazität von der Entladestromstärke bei Bleiakkumulatoren” (on the dependence of capacity on discharge current in lead accumulators) in the Elektrotechnische Zeitschrift. He found that a lead-acid battery’s usable capacity falls as the discharge current rises, and expressed it as an empirical power law: Cp = I^k × t, where I is the discharge current, t the discharge time and k the Peukert exponent, which is above 1 for lead-acid cells.
The law has outlived the batteries it was written for, but with limits. A 2006 review in the Journal of Power Sources found that it can mislead under variable loads and changing temperatures, and that it is a poor fit for lithium-ion cells (Doerffel & Sharkh, 2006). That is why this calculator applies no Peukert correction and instead warns that heavy loads on lead-acid will run shorter than shown.
| Year | Milestone | Source |
|---|---|---|
| 1897 | Peukert publishes his capacity-versus-current law for lead-acid cells | Elektrotechnische Zeitschrift |
| 2006 | Review tests Peukert’s equation on lead-acid and lithium-ion | J. Power Sources 155:395 |
Sizing the battery instead
Turn the formula around to size a battery for a target runtime: required Wh = load W × hours ÷ usable share ÷ inverter efficiency. For a full system with solar panels and days of autonomy, use the off-grid solar calculator. Choose the UPS itself with the UPS calculator, and convert battery labels with the amp hours to kWh converter.
Our off-grid solar system guide explains how batteries, charge controllers and inverters fit together to store clean, renewable energy for when the grid is down. For the units themselves, see what a kilowatt-hour is.
Quick rule of thumb
For a 12 V battery, divide the load in watts by 12 to get amps, then divide the usable amp-hours by that current. A 300 W load draws about 25 A at 12 V before inverter losses (27.8 A after them). A 100 Ah lithium battery at 90% usable gives 90 Ah, so 90 ÷ 27.8 ≈ 3.2 hours, the same answer as the calculator.
References
- Peukert, W. (1897). “Über die Abhängigkeit der Kapazität von der Entladestromstärke bei Bleiakkumulatoren.” Elektrotechnische Zeitschrift (1897).
- Doerffel, D., & Sharkh, S. A. (2006). “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(2), 395–400. doi:10.1016/j.jpowsour.2005.04.030
- Battery University. “BU-501: Basics about Discharging.” batteryuniversity.com
FAQ
Frequently asked questions
How do I calculate battery runtime?
Multiply battery voltage by amp-hours to get watt-hours, multiply by the share you can safely use and by inverter efficiency, then divide by the load in watts. A 12 volt 100 Ah lithium battery stores 1,200 Wh; at 90% usable and 90% inverter efficiency that is 972 Wh, or about 3.2 hours at 300 watts.
How long will a 100Ah battery run a 1000W load?
A 12 volt 100 Ah lithium battery provides about 972 usable watt-hours through a 90% efficient inverter, so it runs 1,000 watts for roughly 58 minutes. A 12 volt lead-acid battery limited to 50% depth of discharge gives about 540 Wh, roughly 32 minutes, and less in practice because of the Peukert effect.
Why use only 50% of a lead-acid battery?
Deep discharges shorten the life of lead-acid and AGM batteries dramatically. Limiting use to about half the rated capacity is a common rule of thumb to get a reasonable number of cycles. Lithium iron phosphate batteries tolerate much deeper discharge, so the calculator uses 90% for them.
How do I estimate UPS runtime?
Choose the small sealed UPS battery option, then enter the UPS battery voltage and amp-hours from the battery label or manual, and your load in watts. Many desktop UPS units use one or two 12 volt batteries of 7 to 9 Ah. Manufacturer runtime charts are the best check.
What is the Peukert effect?
It describes how lead-acid batteries deliver less total capacity when discharged quickly. A battery rated 100 Ah at a 20-hour rate may provide noticeably less at a 1 or 2-hour rate. Lithium batteries are much less affected, so their estimates are closer to reality.
Do batteries in series or parallel last longer?
Total energy is the same either way. Two 12 volt 100 Ah batteries store 2.4 kWh whether wired as 24 volt 100 Ah or 12 volt 200 Ah. Enter the voltage of one battery and the number of batteries, and the calculator multiplies the energy.
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Last updated October 25, 2026. Part of our renewable energy toolkit. Found an error? Tell us.