How Long Will a 100Ah Battery Last? The 20-Hour Rating Catch

2026-10-24

How long will a 100Ah battery last? Multiply the amp-hours by the voltage to get watt-hours, keep only the share you're allowed to use, take off the inverter loss and divide by the load in watts. By that formula, a 12 V 100Ah lead-acid battery used to 50% through a 90% efficient inverter runs about 5.4 hours at 100 W, and a 12.8 V 100Ah LiFePO4 battery used fully runs about 11.5 hours. Treat both as upper limits, though. The heavier the load, the further real runtime falls below them.

The reason is in small print on the label. The 100 Ah is measured at a slow, 20-hour discharge: a steady 5 A. Almost every guide in search results applies the full 100 Ah at 500 W or 1000 W as well, and at those loads a lead-acid battery delivers noticeably less. Below are the simple formula, the correction for discharge rate, and the numbers behind both.

How long will a 100Ah battery last: the simple formula

Battery and inverter makers use the same estimate:

runtime in hours = Ah × V × depth of discharge × inverter efficiency ÷ load in watts

Each factor knocks something off the ideal number:

• Ah × V turns the rating into energy. A 12 V 100Ah battery holds 1200 Wh. A 12.8 V LiFePO4 battery of the same capacity holds 1280 Wh.
• Depth of discharge is the share you can take out without shortening battery life. Victron's handbook says never to leave a lead-acid battery more than 50% discharged. LiFePO4 makers quote 80–100%.
• Inverter efficiency covers the loss in turning 12 V DC into mains AC. 90% is the usual planning figure, and 85% is safer for small or cheap units. A device that runs on 12 V directly, like a car fridge, avoids this loss altogether.

With those defaults, a 12 V 100Ah lead-acid battery runs 5.4 hours at 100 W, 1.8 hours at 300 W, 1.08 hours at 500 W and 0.54 hours at 1000 W, which is barely half an hour.

A 12.8 V 100Ah LiFePO4 battery used to 100% through the same inverter runs 11.5 hours at 100 W, 1.15 hours at 1000 W and 0.77 hours at 1500 W. Most of that advantage comes from depth of discharge. At 12 V and 100%, the lithium figure for 100 W is 10.8 hours, exactly double the lead-acid one, because the lead-acid battery is only allowed to give half of what it stores.

What "100Ah" actually means

A capacity rating is always tied to a discharge time. For most lead-acid batteries that time is 20 hours, written as C20. A 100Ah C20 battery delivers 100 Ah when it's drained at 5 A over 20 hours. Victron, which makes battery monitors, describes the other end of the same battery in its manual: discharged in 2 hours, it may give only 56 Ah.

The simple formula skips that factor. It assumes 100 Ah whether you draw 5 A or 90 A. At light loads that's close enough. At the loads people actually search for, like a 1000 W or 2000 W inverter, it isn't, so the half hour at 1000 W above is already the optimistic case.

Peukert's law: the correction in numbers

Peukert's equation describes how capacity drops as current rises. In the form that uses the rated hours: time to empty = rated hours × (capacity ÷ (current × rated hours)) raised to the power k, where k is the Peukert exponent. An exponent of 1 means no loss. The higher it is, the faster capacity disappears as current rises.

Here are two worked examples from published sources, both reproduced by our calculator:

• A 100Ah C20 battery with k = 1.2, drawn at 10 A, lasts about 8.7 hours. It delivers about 87 Ah, not 100.
• A 100Ah battery with k = 1.26, drawn at 15 A, empties in 5 hours and delivers 75 Ah. This is Victron's own example of a battery rated 100 Ah over 20 hours and 75 Ah over 5 hours.

Depth of discharge applies on top of the correction. Stop the 15 A example at 50% and you get 2.5 hours. Note that the correction uses the DC current leaving the battery. The same 15 A from a 12 V battery through a 90% inverter means a 162 W load on the AC side, and the battery still empties in 5 hours.

Typical exponents if the datasheet doesn't list one:

• lead-acid in general: about 1.1–1.3, with 1.25 as Victron's default for most lead-acid batteries;
• AGM: about 1.05–1.15;
• LiFePO4: close to 1. Battle Born uses 1.05 for its batteries, and Victron suggests 1.01.

That's why lithium holds up so much better under heavy loads, and why a comparison done with the simple formula makes lead-acid look better than it is.

The correction doesn't reward light loads. Below the rated current the equation formally predicts more than the label capacity. No source shows that happening, and at very low currents self-discharge starts to matter. So our calculator caps it: a 200Ah C20 battery lasts 20 hours at its rated 10 A and 40 hours at 5 A, not longer.

Why 100 W already counts as a heavy load

Most guides get this case slightly wrong. A 100 W load through a 90% inverter pulls 9.26 A from a 12 V battery. That's nearly twice the 5 A at which a 100Ah C20 battery earns its rating. With a lead-acid exponent of 1.25, the calculator therefore returns less than the simple formula's 5.4 hours. It also shows the effective capacity as a percentage of the label, so you can see what the current costs you.

At 500 W and 1000 W the current is many times the rated value, and the gap widens. There, even Peukert's equation turns optimistic. As the research summarised on Wikipedia notes, at very high currents real batteries deliver less than it predicts. If a load drains a lead-acid battery in under an hour, the manufacturer's discharge table is a better guide than any formula.

The practical fix is to lower the current. Two 12 V 100Ah batteries in series make a 24 V 100Ah bank: twice the energy, and half the current for the same load. Without any correction, the 24 V bank used to 50% runs 100 W for 12 hours, against 6 hours for one 12 V battery. With the correction the 24 V bank gains a little more on top, because its current sits closer to the rated value. Wired in parallel, the same two batteries make 12 V 200Ah with a rated current of 10 A, which also makes the same load relatively lighter.

What the formula still leaves out

Even with the rate correction, the result assumes a healthy battery and a steady load at a steady temperature. Real setups break those assumptions in a few predictable ways:

• The inverter draws power just by being switched on. That idle draw is in its spec sheet. Add it to the load, because it runs the whole time.
• Fridges, freezers and boiler pumps switch on and off. Use the average power over a few hours, not the nameplate figure. Motors also pull a start-up surge that the inverter has to handle.
• Cold cuts capacity. Victron's handbook puts a lead-acid battery at about 80% of its rated capacity at −10 °C.
• Age cuts it too. A battery counts as worn out at 80% of its original capacity, and the Peukert exponent grows as it ages.

Each of these makes runtime shorter, never longer. If something important depends on the battery, such as a heating boiler in winter or a medical device, treat the result as a ceiling. Plan with a margin and check the equipment maker's runtime tables.

What to calculate next

The battery runtime calculator below runs the whole chain. Enter the capacity and the hours it's rated at, the voltage, battery type, depth of discharge, Peukert exponent, inverter efficiency and the load in watts. It shows the runtime in hours and minutes, the current the battery has to supply, the capacity it will actually deliver at that current, and the total energy stored. Set the exponent to 1.00 to get the simple formula and see how much runtime the discharge rate costs you.

If you're still working out the wattage, the appliance electricity cost calculator uses the same watts and hours of use and shows what running the load costs.