12V Battery Run Time Calculator

How to calculate 12V battery run time

Run time comes down to one comparison: the energy your battery can deliver, and the power your gear consumes. A 12V battery's size is printed in amp hours, but loads are rated in watts, so the first job is putting both on the same footing. The conversion is short: 12 V x 100 Ah = 1,200 Wh.

From there the math runs in three moves:

• Multiply watt hours by the share of the battery you can safely use. Flooded and AGM lead acid should stop near 50%, while lithium accepts 80% or more.
• Subtract conversion losses. If an inverter sits between the battery and your gear, it turns 5 to 15% of the energy into heat.
• Divide the usable watt hours by the load in watts. The result is hours.

Put together, the formula is: run time in hours equals amp hours x volts x depth of discharge x efficiency, all divided by load watts. The calculator above performs each step and shows the intermediate numbers, so you can see whether a bigger battery, a lighter load or a lithium upgrade buys the most hours per dollar.

Worked example: a 100 Ah battery running a 90 W load

Picture a 100 Ah, 12 V AGM battery in a cabin, running a 60 W television and a 30 W internet router through an inverter. The two loads together draw 90 W.

Step one, stored energy: 12 V x 100 Ah = 1,200 Wh. Step two, usable energy: AGM chemistry lasts longest when cycled to 50%, so 1,200 x 0.50 = 600 Wh. Step three, inverter loss at 90% efficiency: 600 x 0.90 = 540 Wh reaches the gear. Step four, time: 540 divided by 90 gives 6 hours of television and browsing.

The same battery powering the 90 W load directly at 12 V, with no inverter in the path, keeps the full 600 Wh and lasts 6.7 hours. That 40 minute gap is the inverter earning its keep, and it runs whether the television is on or not. A typical idle draw of 6 to 10 W means an inverter left switched on overnight can quietly flatten a third of this battery by itself.

Run time by load size

Using the same 540 Wh of usable energy, every load lands somewhere on this table. The wattages are continuous draws, not nameplate peaks.

Notice the shape of the table: halving the load doubles the run time. Also notice which loads matter most. A microwave at 600 W looks alarming, but it runs for four minutes at a time, so a week of cooking costs less than one hour of the desktop computer. A fridge cycles on and off, so its average draw is often a third of its nameplate, and the honest way to size for it is watts times the duty cycle.

Depth of discharge: the biggest lever you control

Depth of discharge decides how much of the stored energy you can take without paying for it in battery lifespan. Lead acid chemistry wears out fast when pulled deep, while lithium barely notices. On the same 1,200 Wh battery, the usable energy looks like this.

The lithium row is the reason off-grid systems keep moving that direction: 1,200 x 0.80 = 960 Wh, which is 60% more working energy from a battery of the same printed size, with far more charge cycles behind it. The Department of Energy's explanation of lithium ion batteries covers why the chemistry tolerates deep cycling, and Battery University's guide to prolonging lead acid batteries documents the cycle counts at each depth. If your loads are fixed, choosing chemistry changes the answer more than any other single input on this page.

Inverter efficiency and phantom loads

Every watt that crosses from 12 V DC to household AC passes through a conversion, and conversions are never free. The table below holds the load at 100 W and varies only the path between battery and gear.

The spread between the best and worst row is a bit over an hour on this battery, and it grows with system size. The sneaky loss is not the conversion itself but the idle draw: an inverter switched on and doing nothing still pulls 6 to 15 W around the clock. Over a day that is 150 to 360 Wh, which on this 100 Ah battery is a quarter to a third of everything it can safely give. Switch the inverter off at the battery when nothing is plugged in, and the arithmetic on this page suddenly starts matching what you see on the voltmeter.

Amp hour loads, Peukert losses and cold weather

Many 12V loads are rated in amps rather than watts: a fridge compressor at 4 A, a water pump at 5 A, a light circuit at 2 A. For these, work in amp hours directly. Usable amp hours divided by load amps gives hours: a 100 Ah battery cycled to 50% has 50 Ah to give, so 50 divided by 4 is 12.5 hours of compressor time.

Lead acid adds one more wrinkle called the Peukert effect: the faster you pull current, the less total capacity the battery delivers. Discharged gently over 20 hours, a 100 Ah battery gives close to its full rating. Drained at 25 A, the same battery gives closer to 65 Ah before it hits the floor. Lithium does not behave this way, which is another reason the same printed amp hours run longer in a lithium pack under heavy loads.

Temperature stacks on top of everything else. Around freezing, a flooded battery delivers roughly 80% of its rating, and deep cold pushes that toward 60%. Size the battery for the coldest night it will work, not the datasheet's 77 F ideal.

Mistakes that make the estimate wrong

Most bad run time estimates trace back to one of five inputs.

• Reading a starter battery's rating as deep cycle capacity. A car battery's amp hour figure assumes gentle discharges it was never designed to survive, and many cranking labels quote cold cranking amps instead of amp hours entirely.
• Ignoring surge current. Compressors and pumps draw two to four times their running watts for the first second. That surge does not change total run time much, but an undersized inverter will trip on it.
• Using nameplate watts for cycling loads. Fridges, freezers and sump pumps rest between runs. Multiply by the duty cycle before dividing into capacity.
• Trusting the rating on a three year old battery. Lead acid capacity fades from the first month; a battery at 80% of its rating turns every 6 hour estimate into 4.8 hours.
• Forgetting the parasitic loads. Charge controllers, inverters on standby and monitors all sip power after the main load is switched off.

Get the inputs right and the calculator's answer usually lands within 10% of a real discharge test. When it does not, the battery itself, rather than the math, is usually the part that has changed.

Treat every result as a planning estimate. Battery condition, temperature and load behavior vary from day to day, so confirm critical systems with a real discharge test before you rely on them.