LiFePO4 Battery Size Calculator: What battery capacity do I need in my RV?
How big does the lithium battery have to be so you can get by two days without sun and shore power? This calculator derives the required LiFePO4 rated capacity from your daily consumption, accounts for depth of discharge and safety reserve, and shows which AGM capacity and weight would mean the same usable energy.
Result
Required LiFePO4 rated capacity
187 Ah
next standard size: 200 Ah (2,560 Wh)
- Energy need over the period
- 1,600 Wh
- Stored energy (rated)
- 2,400 Wh
- AGM equivalent (50% DoD)
- 320 Ah
- for the same usable energy
- Weight LiFePO4 (typical)
- 50 lb
- AGM: approx. 205 lb
Show calculation
- 1Energy need: 800 Wh/day × 2 days = 1,600 Wh
- 2Rated capacity: 1,600 Wh ÷ (12.8 V × 80%) × (1 + 20%) = 187 Ah
- 3Lead-acid comparison (AGM, 50% DoD, 12 V): 1,600 Wh ÷ (12 V × 50%) × (1 + 20%) = 320 Ah
How it's calculated
The starting point is the daily energy need, multiplied by the days the battery should bridge without recharging. Since only part of the rated capacity should be used, it is divided by the depth of discharge; a reserve margin covers cold, aging and estimation errors.
Rated capacity [Ah] = daily consumption [Wh] × off-grid days ÷ (nominal voltage [V] × depth of discharge) × (1 + reserve)The AGM comparison calculates the same usable energy with 50% depth of discharge and 12.0 V – that's why lead-acid needs about 1.7 times the rated capacity and weighs roughly four times as much.
| Daily consumption | 2 days off-grid (LiFePO4, 80% DoD, 20% reserve) | AGM equivalent |
|---|---|---|
| 400 Wh (frugal) | ≈ 94 Ah → 100 Ah | ≈ 160 Ah |
| 800 Wh (compressor fridge, lights, pump, laptop) | ≈ 188 Ah → 200 Ah | ≈ 320 Ah |
| 1,500 Wh (plus inverter, coffee maker) | ≈ 352 Ah → 400 Ah | ≈ 600 Ah |
Worked example
A couple in a camper van uses 650 Wh per day (compressor fridge, lights, pump, phones, laptop) and wants to be able to stay three days without recharging.
Inputs
- Daily power consumption: 650
- Unit of consumption: Wh per day
- Off-grid days without recharging: 3 days
- Depth of discharge (DoD): 80 %
- Safety reserve: 20 %
- System voltage: 12 V (12.8 V LiFePO4)
Result
229 Ah
Required LiFePO4 rated capacity
- Energy need over the period
- 1,950 Wh
- Stored energy (rated)
- 2,925 Wh
- AGM equivalent (50% DoD)
- 390 Ah
- Weight LiFePO4 (typical)
- 60 lb
Calculation
- Energy need: 650 Wh/day × 3 days = 1,950 Wh
- Rated capacity: 1,950 Wh ÷ (12.8 V × 80%) × (1 + 20%) = 229 Ah
- Lead-acid comparison (AGM, 50% DoD, 12 V): 1,950 Wh ÷ (12 V × 50%) × (1 + 20%) = 390 Ah
The variables explained
- Daily power consumption (Wh)
- Energy need of all loads per day including inverter losses.
- Off-grid days
- Days to bridge without solar, driving or shore power. Two to three days is a common planning figure.
- Depth of discharge (DoD)
- Share of the rated capacity that is used. 80% is a good compromise between usefulness and battery life.
- Safety reserve
- Margin for cold (less capacity), aging and misjudged consumption.
Common mistakes
- Estimating consumption too optimistically – fridge and inverter standby are often underestimated.
- Ignoring charging power: a big battery is of little use if solar and the DC-DC charger never fill it. The solar panel size and DC-DC charger calculators complete the planning.
- Overlooking BMS limits: the battery's maximum discharge current must match the inverter (2,000 W ÷ 12 V ≈ 170 A).
- Not checking weight and space: the battery compartment is often designed for a 95 Ah lead-acid battery.
Assumptions and limits
- Nominal voltage 12.8 V (or 25.6 V) for LiFePO4, 12.0 V and 50% DoD for the AGM comparison.
- Weights are typical values of commercially available batteries (LiFePO4 ≈ 26 lb/100 Ah (12 kg), AGM ≈ 64 lb/100 Ah (29 kg)) and vary by model.
- The calculator only considers the storage size, not the charging sources.
Frequently asked questions
Is a 100 Ah LiFePO4 enough for an RV?
For frugal users (lights, pump, phones, absorption fridge on propane) with 300–400 Wh per day it lasts two days. With a compressor fridge and laptop, 200 Ah makes more sense; with an inverter for a coffee maker or induction cooktop, 300 Ah and more.
Why can't I simply replace my 100 Ah AGM with 100 Ah LiFePO4?
You can – and you even gain: 100 Ah LiFePO4 delivers about 1,000 Wh usable, 100 Ah AGM only about 600 Wh. Check the charger (LiFePO4 charge profile), the DC-DC charger and the temperature range.