LiFePO4 Battery Sizing Calculator + Complete Guide (2026)

A LiFePO4 battery sizing calculator only has to answer two questions: how much energy you use in a day, and how much power you draw at once. Get both right and you will buy neither too little nor too much. This guide walks the method with the real numbers, and flags the discharge-limit trap that capacity-only calculators miss.

I laid this method out the way I’d want it explained to me. I don’t sell batteries and I haven’t tested every pack out there. But living with a grid that cuts out unannounced makes you very precise about how many watt-hours you really have left. The two numbers below are the whole job. Get them right and picking a brand gets a lot easier.

Step 1: Daily watt-hours

List each load, its average watts and hours per day, multiply and total. Use average draw, not label peak — a fridge compressor runs about a third of the time. The full method with a worked table is in watt-hours explained.

Multiply daily use by the days you need to run without charging. LiFePO4 gives you nearly all its rated capacity, so unlike lead-acid you do not have to double the number for usable depth — a real advantage covered in the battery upgrade guide.

Step 2: The discharge limit most calculators ignore

Capacity tells you how long; it does not tell you what you can run. Your largest simultaneous load sets the current your battery must deliver, and the BMS caps that. Divide watts by system voltage: a 2,000W inverter on 12V pulls about 175A.

Now the trap: most 100Ah LiFePO4 batteries cap continuous discharge at 100A. So a battery with ample capacity can still be unable to run your inverter. A single 100Ah — or even a standard 200Ah with a 100A BMS — will not feed a 2,000W inverter. You need a 200A-BMS model, a higher-output battery like the SOK 206Ah, or two in parallel. This is the single most common sizing mistake, and the reasoning is in inverter sizing.

LiFePO4 battery sizing calculator: a worked example

A van runs 1,900 Wh a day and wants 2 days of autonomy: 1,900 × 2 = 3,800 Wh, so about 300Ah at 12V. Its biggest simultaneous load is a 1,500W induction hob plus a 200W fridge cycling — 1,700W, which at 12V is roughly 145A. A 300Ah battery with a 200A BMS (like the LiTime 300Ah) satisfies both the capacity and the current. A standard 100A-BMS battery would meet the capacity but fail on the hob.

Add margin, then choose

Add 20% for cold-weather capacity loss and future growth, then round up — the cost step between sizes is small next to running short. With your two numbers in hand, pick the battery from the buyer’s guide that meets both, and confirm capacity against 100Ah vs 200Ah vs 300Ah. For appliance figures to base the calculation on, the ENERGY STAR database lists consumption by category.

Three things that make the calculator’s answer too small

The arithmetic above gives you a floor, not a target. Three real-world factors push the number up, and sizing tools leave all three out.

Inverter losses. Every AC load runs through an inverter that is roughly 85–92% efficient at sensible loads and worse at very light ones. A 100W laptop charger pulling from a 12V bank costs closer to 110–118W of battery power. If most of your daily watt-hours are AC rather than DC, add 10–15% before you go shopping.

Cold. LiFePO4 discharges acceptably in the cold but gives up usable capacity as the temperature falls. The bigger problem is charging: standard LiFePO4 cells must not be charged below 32°F, and a decent BMS will block it — so a winter morning can leave you with solar coming in and a battery refusing to take it. If the bank lives somewhere unheated, either size up or buy a self-heating model.

Charge headroom. A bank sized exactly to your daily draw has to be fully recharged every single day just to keep pace. One overcast day and you are behind. Sizing 20–30% above the calculated figure buys a day of slack, which is the difference between a system you think about and one you don’t.

Stacked up, a 1,000Wh daily budget that “needs” 100Ah on paper is realistically a 200Ah build once the loads are mostly AC and the vehicle sees winter. If you are weighing those two sizes against each other, the practical differences are laid out in 100Ah vs 200Ah vs 300Ah, and the chemistry behind the discharge limits is covered in the LiFePO4 buyer’s guide.

LiFePO4 battery sizing calculator table: daily watt-hours to amp-hours

If you just want the answer without doing the math, find your daily use in the left column. The numbers assume one day of autonomy, nominal pack voltages of 12.8 V, 25.6 V and 51.2 V, and using 90% of rated capacity to leave margin for BMS cutoff and cold mornings. For two days without charging, double the amp-hours.

Formula behind the table: Ah = daily Wh × days ÷ (nominal volts × 0.9). Once capacity is set, check the BMS against your peak load (Step 2 above). Not sure of your daily watt-hours? The household power usage calculator walks through it, and 100Ah vs 200Ah vs 300Ah shows what each size runs in practice.

Frequently asked questions

Last updated: July 24, 2026. Figures are calculated estimates; size against your own measured consumption and verify battery specs before buying.