Inverter Runtime Calculator
Battery Runtime Calculator: How Long Will Your Inverter Run?
Three numbers decide how long your battery will run an inverter: voltage, amp hours, and the wattage of what's plugged in. This calculator does that math, then knocks the result down by half — because pulling a deep cycle below 50% will kill it faster than the spec sheet admits.
Enter your numbers below.
⚡ Battery Runtime Calculator for Inverters | 12V / 24V Tool
Estimate how long you can run your appliances on battery power
Select the voltage of your battery or battery bank
Enter the total Amp/Hours of your battery or battery bank
Enter the combined wattage of appliances you plan to run
💡 Important Battery Tips
- Deep cycle (marine) batteries generally have the highest reserve ratings and are capable of withstanding repeated drains and recharging
- Engine start batteries should not be discharged below 90% charge state
- Marine deep cycle batteries should not be discharged below 50% charge state
- Discharging below these levels will shorten battery life per manufacturer recommendations
🔌 Commercial Use Recommendation
If you intend to use power tools for commercial use, or any load of 200W+ for more than 1 hour regularly, we recommend installing an auxiliary deep cycle battery dedicated to the inverter. This battery should be connected to the alternator through an isolator module to prevent discharging the engine start battery when the engine is off.
Got a number you don't like?
If the runtime came back shorter than you need, the fix is almost never a bigger inverter. It's the battery, the bank size, or the wiring. Three levers, in the order most people need them.
Bigger bank
Double the usable amp hours by adding a Group 27 or Group 31 deep cycle to the bank.
Browse Group 27 batteries → Browse Group 31 batteries →Right inverter
An undersized or wrong-sine-wave inverter wastes 15–20% of your battery's energy as heat. Match the inverter to the load and you'll get the runtime the math actually predicts.
Shop power inverters →Send us the application
Not sure which one you need? Tell us the voltage, the load wattage, and how many hours per session, and we'll point you to the right setup. We're not trying to give you a smoke screen.
Chat with a tech →What size battery do I need?
Same math, run backward. Tell the calculator how long you need to run and what you're running, and it gives you the minimum bank size in amp hours — plus the standard battery group that gets you there.
📐 Reverse Sizing Calculator | What Size Battery Do I Need?
Enter your load and how long you need it to run — we'll size the bank.
Choose the voltage of the bank you'll build
Combined watts of everything you'll run at once
How many hours do you need this load to run on battery?
📏 How we round up
We match your required amp hours to the next standard deep-cycle group size — Group 22NF (50Ah), Group 24 (75Ah), Group 27 (100Ah), Group 31 (115Ah), or Group 8D (200Ah+). For banks larger than a single Group 8D, we'll show how many batteries you need in parallel.
How the math works
The runtime formula is the standard deep-cycle equation used by marine and RV installers:
12V system: runtime (hours) = (10 × amp hours ÷ watts) ÷ 2
24V system: runtime (hours) = (20 × amp hours ÷ watts) ÷ 2
That final division isn't a fudge factor. It builds in a 50% depth of discharge, the safe drawdown for marine deep cycle batteries. Pull a battery below 50% on a regular basis and you cut its service life in half, even if the spec sheet says otherwise.
One real-world example. A Vision EV27 (100Ah Group 27 AGM) powering a 600W mini-fridge through a 12V inverter gives you roughly 50 minutes of runtime, not the two hours the raw math suggests. The gap is the 50% DoD reserve plus inverter inefficiency. Both are real, and both are why "calculator says four hours" and "battery actually ran two" is the most common message we get on chat.
A note on engine-start batteries. They should never go below 90% charge, so they're not a substitute for a deep cycle bank. If you're running anything over 200 watts for more than an hour at a time, you want a dedicated auxiliary deep cycle wired through an isolator — not your starter battery.
Frequently asked questions
How long will a 100Ah battery last with a 200W load?
On a 12V system, the raw math gives you 6 hours. The real-world number, after 50% DoD reserve and around 10% inverter loss, is closer to 2.5 hours. If you need 6 actual hours at 200W, you need a 200Ah bank — two Vision EV27 batteries in parallel will do it.
How do you calculate battery run time?
For a 12V deep cycle, multiply amp hours by 10, divide by load watts, then divide by 2 for the safe-discharge reserve. So a 100Ah battery running a 100W load gives you about 5 hours. For 24V, multiply amp hours by 20 instead. The divided-by-2 step is what most online calculators skip, which is why their numbers run long.
How many hours will an 80% charged battery last?
For a deep cycle, an 80% state of charge gives you about 30% of the usable capacity before you hit the 50% safe-discharge floor. So a 100Ah deep cycle at 80% gives you the work of a 30Ah battery. For an engine-start battery, 80% is already in the do-not-drain-further zone — use the alternator or a charger, not the inverter.
Why is my battery dying faster than the calculator says?
Three usual reasons. The load is higher than rated (a "600W" fridge can pull 1,000W on compressor startup), the inverter is older or undersized and losing 15–20% to heat, or the battery isn't a true deep cycle and shouldn't be drawn past 80% charge to begin with. If you tell us the application, we can usually spot which one in a minute.
Will a deep-cycle battery run a refrigerator overnight?
A single 100Ah Group 27 deep cycle at 12V will run an efficient 80W RV fridge for 6–7 actual hours. For an overnight (10–12 hours) on a household 600W mini-fridge, you want a 200Ah bank minimum — two Vision EV27 or two Vision EV31 batteries — and you should wire it through a low-voltage cutoff so the inverter shuts down before the battery hits 50%.