Use this RV battery runtime calculator to see how long your battery can power a load. Enter your battery’s amp-hours, pick lithium or lead-acid, and type the watts you draw to see your runtime in hours, from a weekend light load to a heavy inverter load.
How to Use the Calculator
Enter your battery’s amp-hours, pick lithium or lead-acid, and type the watts you are drawing. Then choose 12-volt DC or 120-volt AC. The result shows how long the battery lasts before it reaches its usable limit, plus its total and usable watt-hours.
How RV Battery Runtime Is Calculated
The formula has three steps. Multiply your amp-hours by 12 volts to get watt-hours, then by the usable percentage, and divide the result by your watts. The answer is your runtime in hours, so 200 Ah, 50 percent, and 100 W gives you 12.
The calculator’s starting numbers show how it works. A 200 Ah lead-acid battery holds 2,400 Wh, of which 50 percent, or 1,200 Wh, is usable. At 100 W, that lasts 12 hours.
Find Your Battery’s Amp-Hours
The amp-hour (Ah) rating is on the battery’s label. Most RV house batteries are 12 volts, so multiply Ah by 12 to get watt-hours, which means a 100 Ah battery holds about 1,200 Wh. Batteries wired in parallel add their amp-hours, so two 100 Ah batteries make 200 Ah.
Lead-Acid vs. Lithium Runtime
RV Travel says lead-acid and AGM batteries should not be drained much below 50 percent if long life is the goal, so the calculator counts 50 percent for them. For lithium, it suggests stopping at about 20 percent remaining, which makes 80 percent usable.
That is why a lithium battery gives more runtime from the same amp-hour rating. At the same size, lithium has 60 percent more usable energy than lead-acid in this calculator.
Find Your Watts
A device’s watts are on its label. If it lists amps instead, multiply by 12 for a 12-volt device. Add up everything that runs at the same time, since the calculator needs your combined draw. For example, a 10 W light and a 60 W laptop together draw 70 W.
Battery Runtime at a Glance
Here is how long a 200 Ah lithium battery runs a few DC loads, with the lead-acid runtime in brackets. For a 100Ah battery, divide those hours by two, so a 100Ah lithium battery runs a 100 W load for about 9.6 hours and a 100Ah lead-acid battery for about 6.
- 50 W: 38.4 hours (24 hours lead-acid)
- 100 W: 19.2 hours (12 hours lead-acid)
- 300 W: 6.4 hours (4 hours lead-acid)
- 600 W: 3.2 hours (2 hours lead-acid)
AC Loads Use More
Anything plugged into an inverter loses some power as heat. The calculator adds 10 percent for this, so a 1,000 W microwave pulls about 1,100 W from the battery. A 200 Ah lithium battery runs that load for about 1 hour and 44 minutes.
Ten percent is on the friendly side. RV Travel says modified sine wave inverters run at around 85 percent efficiency and high-end pure sine wave models reach about 90 percent. In one test, RVelectricity found a fridge averaged about 28 W on 12-volt power and about 45 W through an inverter and power brick.
Why Actual Runtime Is Shorter
The result is a planning number. Heavy draws, cold weather, and an aging battery all cut runtime. RV Travel gives an extreme example, where a typical 100 Ah travel trailer battery pushed at 100 amps delivers only about 35 percent of its rated capacity.
Leave yourself a cushion. Watch your battery monitor on your first few trips to see how your own numbers compare, and adjust your plan from there.
How to Make Your Battery Last Longer
Cut your biggest loads first. LED lights, fans instead of the A/C, and turning off the inverter when you do not need AC power all stretch your runtime. Solar panels fill the battery back up during the day, so try our solar calculator to see how many you need. Need more power than a battery can give? Our generator size calculator shows what size to buy.
Your Next Step
Add up the watts you run on a typical evening, enter them with your battery size, and compare lithium with lead-acid. If the runtime is shorter than you hoped, cut a load, add battery capacity, or add solar before your next dry-camping trip.