Sizing an RV Battery Bank: How Many Amp-Hours You Need

Multiply your daily watt-hours by the number of days you want to go without sun, divide by the usable fraction of the battery, then divide by 12 for a 12V bank. That is your amp-hour target. At 1,500 Wh a day and 2 days of autonomy, it is 313Ah of lithium or 500Ah of AGM, and our RV solar calculator runs both.

The array and the bank do different jobs. The array covers the day. The bank covers the night, the storm, and the shaded campsite. Sizing one does not size the other: see how much solar you need for the array side.

The formula the calculator uses

bank watt-hours = (daily watt-hours x days of autonomy) / usable depth of discharge
bank amp-hours  = bank watt-hours / 12

Worked at 1,500 Wh a day and 2 days:

• Load to carry: 1,500 x 2 = 3,000 Wh.
• Lithium at 80% usable: 3,000 / 0.8 = 3,750 Wh, which at 12V is 313Ah.
• AGM at 50% usable: 3,000 / 0.5 = 6,000 Wh, which at 12V is 500Ah.

Note the 12V. The calculator uses 12V nominal, while a LiFePO4 battery is nominally 12.8V. That makes the amp-hour figure slightly conservative, which is the direction you want to be wrong in.

Rated capacity is not usable capacity

The 80% and 50% are not arbitrary. Depth of discharge is what sets how long a battery lasts, and the two chemistries respond very differently.

Victron’s AGM Super Cycle datasheet, a battery specifically built to tolerate deep discharge, rates it at 300 cycles at 100% depth of discharge, 700 cycles at 60%, and 1,000 cycles at 40%. Draining a lead acid battery flat is not a one-time sin, it is a shortening of every cycle you have left. The Gel and AGM documentation puts it plainly: repeated deep and prolonged discharge has a very negative effect on the service life of all lead acid batteries.

Victron’s LiFePO4 datasheet rates 2,500 cycles at 80% depth of discharge, 3,000 at 70%, and 5,000 at 50%, with capacity still at or above 80% of nominal at the end.

Some lithium manufacturers go further. Battle Born lists 100% depth of discharge and 3,000 to 5,000 cycles for their 100Ah 12V battery. Our calculator still uses 80%, deliberately. Sizing to the manufacturer’s best case leaves you no margin for a cold night, an aging bank, or the day you misjudge the weather.

What that means in weight and space

The 313Ah versus 500Ah gap is not just money. Using published weights:

• Battle Born lists their 100Ah 12V LiFePO4 at 31 lb. Four of them, to cover 313Ah, is about 124 lb.
• Victron’s AGM Super Cycle in 12V 100Ah (C20) is listed at 26 kg, about 57 lb. Five of them, to cover 500Ah, is about 287 lb.

Same job, 124 lb against 287 lb, and the AGM bank takes noticeably more floor. On a van with a real payload limit that difference is not a preference, it is a constraint. Which one is right for your rig depends on more than weight, including how cold it gets: lithium vs AGM goes through the tradeoff.

A 100Ah rating assumes a slow discharge

This is the detail that surprises people who have already bought the battery. Victron’s Gel and AGM documentation states that the rated capacity of their AGM and Gel deep cycle batteries refers to a 20 hour discharge, a current of 0.05C. Out of a 100Ah battery that is 5 amps.

Pull harder and you get less. Their table gives, for AGM:

• 20 hour discharge: 100% of rated capacity
• 10 hours: 92%
• 5 hours: 85%
• 3 hours: 78%
• 1 hour: 65%
• 30 minutes: 55%

The same document notes the capacity reduction is even faster under a constant power load, such as an inverter. So the 100Ah AGM you bought, running an inverter hard for an hour, is behaving like a 65Ah battery, and you were only supposed to use half of it anyway.

Lead acid also loses capacity in the cold. Victron’s own graph shows capacity reducing sharply at low temperatures, though they publish it as a curve rather than a table, so we are not going to quote a number off it.

Charging is the other half of the sizing job

A bank you cannot refill is decoration. Two limits matter.

What the battery will accept. Victron recommends charge current for their lead acid batteries preferably not exceed 0.2C, which is 20 amps for a 100Ah battery. That is rarely the binding constraint on a solar rig.

What the array can deliver. A 600W array at the calculator’s 70% system efficiency is about 420W, or roughly 35 amps at 12V, and only while the sun is high. Refilling 3,000 Wh of overnight and storm use out of that takes most of a good day. In a 2 sun hour December it does not happen at all.

That asymmetry is the real reason people end up disappointed. They size the bank for four days of autonomy and then discover the array needs a full week of clear weather to put those four days back. If you are planning long stays, how long you can actually boondock covers the other limits that show up first, like water and waste.

Small things that quietly eat the bank

Self-discharge is real but slow. Victron rates their VRLA batteries at less than 2% per month at 20C, doubling for every 10C increase, so a battery stored hot goes flat much faster than one stored cool.

The inverter is the bigger leak. Victron’s MultiPlus 12/1200 draws 10W at zero load, or 3W in search mode. Left switched on around the clock, 10W is 240 Wh a day out of your bank for producing nothing at all. On a 1,500 Wh budget that is 16% of your entire day. Turn it off, or use search mode.

Where to start

Get your daily watt-hour figure first, from the power budget guide, then put it into the solar calculator and switch the chemistry dropdown back and forth. The number that changes is what the chemistry decision is actually worth to you.

These are planning estimates. Have anything you intend to wire permanently checked by a qualified installer.