LiFePO4 Battery Sizing Calculator + Complete Guide (2026)
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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.
The two-number method
1. Daily watt-hours × days of autonomy = the capacity you need. 2. Largest simultaneous load in watts ÷ system voltage = the continuous current your battery’s BMS must supply. Capacity picks the battery size; the BMS limit picks the battery model.
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.
| Use case | Typical daily use | Battery (2 days autonomy) |
|---|---|---|
| Weekend camper | ~800 Wh | 100Ah (1,280 Wh) |
| Boondocker w/ fridge | ~2,000 Wh | 200–300Ah |
| Full-timer | ~3,000 Wh | 300–600Ah |
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.
Frequently asked questions
How do I size a LiFePO4 battery?
Two steps: multiply your daily watt-hours by days of autonomy for capacity, and divide your largest simultaneous load by system voltage for the continuous current the BMS must supply. The battery you buy has to satisfy both, not just capacity.
Do I need to derate LiFePO4 like lead-acid?
No — LiFePO4 gives you roughly 90–100% of rated capacity, versus about 50% for lead-acid. You do not double your capacity target the way you would with lead-acid, which is part of why lithium is worth the higher upfront cost.
Why does my big battery still not run my inverter?
Because capacity and output are different specs. A high-capacity battery with a 100A BMS still cannot supply the ~175A a 2,000W inverter needs. Check the continuous discharge rating, not just the amp-hours, and choose a 200A-BMS model if you run a big inverter.
How much margin should I add?
Around 20% for cold-weather capacity loss and future growth, then round up to the next available size. The price difference between adjacent sizes is small compared with the cost of running out of power, so err on the generous side.
Last updated: July 24, 2026. Figures are calculated estimates; size against your own measured consumption and verify battery specs before buying.