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Batteries

What Size Battery Bank for 125 Amp-Hours a Day? (LiFePO4)

Researched from published standards and manufacturer specifications. Updated .

Quick answer

Using 125 amp-hours a day, a LiFePO4 bank needs about 156Ah of nameplate capacity for one day off grid, 313Ah for two days and 469Ah for three. That is because only about 80 percent of a LiFePO4 battery is usable, so nameplate capacity is always larger than the energy you actually get.

This page answers one question: if your RV uses 125 amp-hours a day, how much LiFePO4 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 125Ah daily load does not need a 125Ah 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 LiFePO4 battery for 125Ah 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 125Ah a day at 80 percent depth of discharge.

Battery bank size for 125 amp-hours a day, LiFePO4
Days off gridEnergy neededNameplate capacityStored energy at 12V
1 day125 Ah156 Ah1,498 Wh usable
2 days250 Ah313 Ah3,005 Wh usable
3 days375 Ah469 Ah4,502 Wh usable
4 days500 Ah625 Ah6,000 Wh usable

Convention Source: Common planning figure. Individual LiFePO4 manufacturers publish their own usable depth and cycle life.. 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 313Ah bank for two days off grid is 4 batteries of 100Ah, or 2 of 200Ah. At roughly 25 pounds per 100Ah LiFePO4 battery, that is about 78 pounds.

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 lead-acid be a better choice at this load?

For the same 125Ah daily load over two days, lead-acid needs about 500Ah of nameplate capacity against 313Ah for LiFePO4. Lead-acid is cheaper per nameplate amp-hour and considerably heavier per usable amp-hour, and it will not accept a fast charge the way lithium does. See LiFePO4 vs AGM for RV house batteries and the usable capacity calculator.

How much charging does a bank this size need?

Replacing 125Ah a day means putting back about 1,500 watt-hours. At a 0.75 system derate, that needs roughly 500W of solar at four peak sun hours, or about 364W 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.

LiFePO4 batteries that suit a 125Ah daily load

Frequently asked questions

How many batteries do I need for 125 amp-hours a day?
For two days off grid at 125 amp-hours a day you need about 313 amp-hours of LiFePO4 nameplate capacity, which is 4 batteries of 100Ah each. For a single day between charges you need about 156 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 125Ah battery for a 125Ah daily load?
Because you do not get to use all of a battery's nameplate capacity. Planning LiFePO4 at 80 percent depth of discharge means a 125 amp-hour battery only delivers about 100 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 125Ah per day load need?
Replacing 125 amp-hours a day at 12 volts is about 1,500 watt-hours. At four peak sun hours and a 0.75 system derate that needs roughly 500 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.