100 Amp Hour Battery Run Time Calculator

What 100 amp hours means in practice

A 100 Ah battery stores 100 amp-hours of charge, which at 12 volts is 1,200 watt-hours, or 1.2 kilowatt-hours. That is the raw energy in the box. How long it lasts depends on three things: the load you put on it, how far down you are willing to drain it, and how the battery behaves at the current you draw.

The 100 Ah figure itself comes from a test. Manufacturers rate capacity at a fixed discharge time, usually 20 hours, so a 100 Ah battery is the battery that delivers 5 amps for 20 hours before it hits its cutoff voltage. That rating is the anchor for every run time estimate. Drain it at 5 amps and the label math works. Drain it at 20 amps and the battery delivers less than its label says, because the chemical reaction inside cannot keep up.

For a 12 volt pack, 100 Ah is the size of a common deep-cycle marine battery, a mid-size RV house battery, or a small solar storage bank. In lithium, the same 100 Ah label fits in a much lighter case and usually gives up more of its rated capacity before the low-voltage cutoff. Both are called 100 Ah; they do not behave the same way.

The run time formula, with the Peukert correction

The plain estimate is hours = usable amp-hours divided by load amps. Usable amp-hours is capacity times depth of discharge. A 100 Ah battery drawn to 50 percent gives you 50 Ah, so a 5 amp load runs 10 hours and a 10 amp load runs 5 hours.

The plain estimate breaks down at high currents. Peukert's law, published in 1897 and still the standard first correction, says the capacity a lead-acid battery can deliver falls as the discharge current rises. The Department of Energy storage guide describes the effect directly: as the discharge rate increases, the battery's available energy capacity decreases. The usual form, as detailed in a Stanford battery modeling paper, is I raised to n, times T, equals a constant, where I is current, T is time and n is the Peukert exponent. Solved for time at a rated capacity C and rated hour rate H, it becomes T = H times (C divided by (I times H)) raised to n.

The exponent is the tuning knob. A value of 1 removes the correction and returns the label math. Flooded lead-acid sits around 1.2 to 1.3, AGM and gel around 1.1 to 1.2, and lithium iron phosphate close to 1.05, which is why lithium run times track the simple division far more closely. Then multiply by depth of discharge to get the time you can use in practice.

Worked example: a 5 amp load on a 100 Ah battery

Take a 100 Ah flooded lead-acid battery at 12 volts, rated at the 20 hour rate, with a 5 amp load. That 5 amps is exactly the rated current, so the Peukert correction does nothing: 20 times (100 divided by 100) raised to 1.25 is 20 hours to a full discharge. At 50 percent depth of discharge you get 10 hours of run time, using 50 Ah and 600 Wh of the 1,200 Wh stored.

The same 5 amp load on a 100 Ah lithium iron phosphate battery at 80 percent depth runs 16 hours, because the full-discharge time is still 20 hours but you are allowed to spend 80 Ah instead of 50. That is 960 Wh delivered from the same nominal capacity, a 60 percent gain in usable energy from chemistry and depth of discharge alone.

Put a number on the load and the picture gets concrete: 5 amps at 12 volts is 60 watts, about a laptop charger, a small fan or a few LED lights. A 100 Ah lead-acid battery runs that for a 10 hour night and still has half its charge in reserve. Add a 60 W fridge cycling on and off and the average draw rises, so the run time falls in proportion.

Worked example: a 10 amp load, where Peukert bites

Double the load to 10 amps on the same lead-acid battery. Label math says 50 usable amp-hours divided by 10 amps is 5 hours. The Peukert form says 20 times (100 divided by 200) raised to 1.25, which is 8.41 hours to a full discharge, so the usable time at 50 percent is 4.20 hours, or 4 hours 12 minutes. The gap between 5 hours on paper and 4.20 hours in practice is the effect in one line.

Push harder and it widens. At 30 amps, label math promises 1.67 hours; Peukert gives 2.13 hours to full discharge and 1.06 hours usable, about 1 hour 4 minutes. At 50 amps the battery is down to 0.56 hours usable, roughly 34 minutes, against a paper figure of 1 hour. A trolling motor, a microwave through an inverter or a winch all sit in this range, which is why owners who size a bank from label amp-hours come up short.

At low currents the correction runs the other way. A 2 amp load gets 31.44 usable hours, not the 25 hours label math predicts, because a slow discharge lets the chemistry keep up and recovers capacity that a fast draw would leave behind. Peukert is a curve, not a penalty.

Depth of discharge: how much of the 100 Ah you can spend

Depth of discharge is the share of capacity you take out before recharging. A 50 percent limit on a 100 Ah battery means you treat it as a 50 Ah battery and leave the other half in place. That habit is what separates a battery that lasts several hundred cycles from one that fails inside two years.

The Department of Energy's primer on lead-acid storage batteries notes that discharges beyond about 80 percent of capacity can be expected to shorten battery life, and the same document puts flooded battery life at 10 years or longer with proper maintenance. The practical numbers most off-grid users settle on are 50 percent for flooded lead-acid, 50 percent for AGM, and 80 percent for lithium iron phosphate, whose battery management system usually enforces the floor for you.

Depth of discharge is a trade, not a rule handed down. Drawing to 80 percent instead of 50 percent gives 60 percent more run time on the same battery, at the cost of fewer cycles before replacement. If the battery is cheap and cycles are rare, that trade can win. For a daily solar cycle, the 50 percent habit usually costs less per kilowatt-hour over the life of the bank.

The table converts those limits into usable hours on one 100 Ah battery at a 5 amp load, using the Peukert exponents above and a 20 hour rating.

Two rows deserve a second look. Lithium at 100 percent is a spec sheet figure rather than a plan, because most LiFePO4 banks ship with a management system that stops discharge near 90 percent and daily trips to the floor cost cycles. AGM at 50 percent matches flooded on hours, and the real difference between the two shows up in price and cycle life, not in run time.

Run time by load on a 100 Ah battery

The table shows one 100 Ah flooded lead-acid battery at 12 volts, rated at the 20 hour rate, with a Peukert exponent of 1.25 and a 50 percent depth of discharge. The label column is the simple division most people do in their heads. The last column is what the battery gives up in test conditions.

Notice where the label math is honest: at 5 amps, the rated current, the two agree. Everywhere else they part company, and the error grows with the load. A 50 amp load gets 34 minutes of usable time, not the hour the label implies, which is a 44 percent shortfall on a load a small inverter can pull without warning.

Watts, inverters and real loads

Most loads are labeled in watts, not amps, and the calculator converts for you. On a DC circuit the current is watts divided by volts, so 60 watts at 12 volts draws 5 amps. Through an inverter the math picks up one more term: current from the battery is watts divided by inverter efficiency divided by volts. A 90 percent efficient inverter running a 60 watt AC load pulls 5.56 amps from a 12 volt battery, and the Peukert penalty at that slightly higher current trims the usable time to 8 hours 46 minutes instead of 10 hours.

Large inverter loads get ugly fast. A 1,000 watt load through a 90 percent inverter draws 92.59 amps at 12 volts, close to a 1C rate on a 100 Ah battery. Peukert time to full discharge is 0.52 hours, and at 50 percent depth you get about 16 minutes. A 100 Ah battery is not a serious match for a 1,000 watt inverter, no matter what the label suggests.

Two more habits keep estimates honest. Use the average draw for cycling loads, not the peak, since a fridge that pulls 5 amps while running and runs a third of the time averages 1.7 amps. And keep the inverter's idle draw in the total: a 20 watt standby load left on all night spends 160 watt-hours overnight, which is over a quarter of the 600 Wh a 12 volt, 100 Ah lead-acid bank gives up at 50 percent depth.

The table runs one 12 volt, 100 Ah lead-acid battery at 50 percent depth through a 90 percent efficient inverter at common load sizes.

Read the last row as a warning. A 1,500 watt load flattens the usable half of this battery in about nine minutes, well before the inverter reaches its own low voltage cutoff, and the current draw sits above 1C. Size the battery bank from the loads you run in practice, and treat the inverter label as a ceiling rather than a target.

How to use this calculator

Enter the capacity, the pack voltage and the load. Switch the load between amps and watts depending on the label; in watts mode the inverter efficiency input converts to battery current. Pick a battery type to load its usual depth of discharge and Peukert exponent, or set both by hand. The rated hour rate defaults to 20, which is what most 100 Ah labels use.

The results give the usable run time in hours and minutes, the time to a full discharge, the label-math estimate for comparison, the amp-hours and watt-hours you spend, and the current drawn from the battery. The label-math row is there so you can see how much the derating changed the answer.

What the model leaves out: temperature (a cold battery gives up less), age (a three-year-old bank may hold 80 percent of its rating), the low-voltage cutoff of your inverter, and loads that vary minute to minute. Treat the result as a planning figure with a margin, not a promise, and check the battery manufacturer's data sheet for the Peukert exponent and the recommended depth of discharge for your exact model.