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Batteries

What Size Battery Bank for 300 Amp-Hours a Day? (lead-acid)

Researched from published standards and manufacturer specifications. Updated .

Quick answer

Using 300 amp-hours a day, a lead-acid bank needs about 600Ah of nameplate capacity for one day off grid, 1,200Ah for two days and 1,800Ah for three. That is because only about 50 percent of a lead-acid battery is usable, so nameplate capacity is always larger than the energy you actually get.

This page answers one question: if your RV uses 300 amp-hours a day, how much lead-acid battery do you need to buy? The arithmetic is short. Multiply daily use by the number of days you want to go without charging, then divide by the fraction of the battery you are willing to use.

That last fraction is the part people get wrong. A 300Ah daily load does not need a 300Ah battery. Run the numbers for your own use with the battery bank calculator, and if you have not measured your daily draw yet, build it up load by load with the daily energy budget calculator.

How many amp-hours of lead-acid battery for 300Ah a day?

Nameplate capacity is what the label says. Usable capacity is what you get before you are into the part of the discharge curve that shortens the battery's life. These are the sizes for 300Ah a day at 50 percent depth of discharge.

Battery bank size for 300 amp-hours a day, lead-acid
Days off gridEnergy neededNameplate capacityStored energy at 12V
1 day300 Ah600 Ah3,600 Wh usable
2 days600 Ah1,200 Ah7,200 Wh usable
3 days900 Ah1,800 Ah10,800 Wh usable
4 days1,200 Ah2,400 Ah14,400 Wh usable

Convention Source: Widely repeated planning guideline, not a published specification. Depth of discharge is a planning convention rather than a published specification. Battery manufacturers publish cycle life against depth of discharge curves instead of a single usable percentage.

What does that look like in real batteries?

A 1,200Ah bank for two days off grid is 12 batteries of 100Ah, or 6 of 200Ah. At roughly 65 pounds per 100Ah AGM battery, that is about 780 pounds, which is a real payload consideration.

Batteries wired in parallel add capacity at the same voltage. Keep the interconnect cables identical in length and gauge, or the nearest battery does more of the work and ages faster than the rest.

Would LiFePO4 be a better choice at this load?

For the same 300Ah daily load over two days, LiFePO4 needs about 750Ah of nameplate capacity against 1,200Ah for lead-acid. Lithium costs more up front and weighs far less for the same usable energy, and it accepts charge much faster, which matters when your only charging window is a few hours of sun. See LiFePO4 vs AGM for RV house batteries and the usable capacity calculator.

How much charging does a bank this size need?

Replacing 300Ah a day means putting back about 3,600 watt-hours. At a 0.75 system derate, that needs roughly 1,200W of solar at four peak sun hours, or about 873W at five and a half. Size it against your own region with the solar array calculator.

Charging from the tow vehicle is the other option. A DC-DC charger is what makes that safe on a lithium bank, and the DC-DC charger calculator sizes one against your alternator.

lead-acid batteries that suit a 300Ah daily load

Frequently asked questions

How many batteries do I need for 300 amp-hours a day?
For two days off grid at 300 amp-hours a day you need about 1,200 amp-hours of lead-acid nameplate capacity, which is 12 batteries of 100Ah each. For a single day between charges you need about 600 amp-hours. The difference between nameplate and usable capacity is why the bank is always larger than the daily figure.
Why can I not just buy a 300Ah battery for a 300Ah daily load?
Because you do not get to use all of a battery's nameplate capacity. Planning lead-acid at 50 percent depth of discharge means a 300 amp-hour battery only delivers about 150 amp-hours before you should recharge it. Discharging deeper than that repeatedly shortens the battery's life, and on lead-acid it does so sharply.
How much solar does a 300Ah per day load need?
Replacing 300 amp-hours a day at 12 volts is about 3,600 watt-hours. At four peak sun hours and a 0.75 system derate that needs roughly 1,200 watts of panel. Peak sun hours vary a great deal by region and season, so check your own figure rather than assuming four.
Does cold weather change the bank size I need?
It changes what the bank can do rather than the arithmetic. Lead-acid loses usable capacity as temperature falls, so a cold bank delivers less than its rating. LiFePO4 holds capacity better but must not be charged below freezing, so a cold-weather lithium bank needs either a low temperature cutoff in the BMS or a self-heating battery. Furnace blower run time also rises in cold weather, which increases the daily load itself.

Lithium safety. A LiFePO4 bank needs a charger with a genuine lithium profile, and that means the converter, the solar controller and any DC-DC charger all have to be set correctly. A stock lead-acid converter will never bring a lithium pack to full. Charging a lithium battery below freezing damages the cells permanently, so the pack needs either a low temperature charge cutoff in its BMS or a self-heating design if you camp in the cold.