Free tool

RV battery runtime calculator

The math is battery watt-hours divided by the watts you draw. What is hard is knowing the usable part of a battery, which depends on its chemistry, so the tool asks for what is printed on the label and applies the right fraction.

The answer updates as you type. Nothing is sent anywhere.

Your battery bank

How to get it: printed on the battery label as Ah. Two batteries in parallel add their amp-hours; two in series add volts, not amp-hours.
Sets how much of the amp-hours you can use. The label says the chemistry: LiFePO4, AGM, or nothing (flooded). Lead-acid chemistries give up about half their amp-hours before the recommended recharge point; lithium gives most of them, and 80 percent is the conservative default.

What you are running

How to get it: add the watts printed on each item's label. The common 12-volt draws: an LED bulb is about 2, the furnace fan 45 to 60 while it runs, the water pump 50 to 110 while it runs, a 12-volt television 35 to 50. A 60-watt load is a few LED lights and the television, which is a normal evening.
Optional. Leave empty to see hours of runtime directly. Fill it in to see how many days the bank carries that schedule.

Why chemistry decides the answer

The amp-hours on the label are the whole tank. How much of the tank you may use depends on the chemistry, because discharging a lead-acid battery deeply costs it cycle life. Battle Born's comparison gives lead-acid chemistries roughly 50 percent usable capacity and lithium 80 to 100 percent, and Renogy states the same split as a sizing rule: for lead-acid, the standard depth of discharge is 50 percent, and for lithium it is 80 to 100 percent. This tool uses 50 percent for flooded and AGM, and 80 percent for lithium, which is the conservative end of the 80-to-100 range both makers publish.

The other half of the math is the watt-hours, and the honest convention for a 12-volt lithium battery is 12.8 volts nominal, so 100 amp-hours is 1,280 watt-hours, not 1,200. Dakota Lithium prints exactly that on its 100 amp-hour battery. The tool uses 12.8 volts for lithium and 12.0 for lead-acid, and says so in the result.

What the answer means

The result is hours of runtime, not a promise. Cold weather cuts lead-acid capacity, high discharge rates cut it further, and an inverter running 120-volt gear from the batteries adds its own losses: Victron's datasheets put small 12-volt inverters at 87 to 92 percent at their best, and real-world average efficiency is lower than peak. If your gear runs through an inverter, take the answer as optimistic and add margin. The watts to amps calculator converts any appliance label into the 12-volt draw it becomes through an inverter.

Where to go next

The solar sizing calculator answers the follow-up question: how many panel watts it takes to put back what you use each day. The watts to amps calculator handles the conversion side.

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