12V Battery Sizing Calculator
Add every 12 volt device you plan to run. We'll calculate your daily load in amp-hours and recommend a battery bank size for both LiFePO4 lithium and AGM lead-acid.
Your Devices
| Device | Watts | Hrs/day | Ah/day |
|---|
How the calculator works
Every 12 volt device draws a known wattage. Multiply watts by hours per day to get watt-hours, then divide by 12 (your system voltage) to get amp-hours. Add them up for every device and you have your daily load.
But a battery's rated capacity isn't all usable. Flooded and AGM batteries shouldn't be cycled below 50% state of charge without damaging them — so for lead-acid you need roughly 2× your daily load in rated Ah. LiFePO4 can safely cycle to 80% or more, which is why lithium banks are typically much smaller for the same job.
Example: a small RV
A 40W fridge running 12 hours/day = 480 Wh ÷ 12V = 40 Ah. Add a 15W LED light for 4 hours (5 Ah), a 5W fan for 6 hours (2.5 Ah), and phone charging (5 Ah) and you're around 52 Ah/day. That means a 100Ah LiFePO4 or roughly 200Ah of AGM for one day of use with safe margins.
What this calculator doesn't cover
This tool sizes your bank for a single day of use. For off-grid builds plan 2–3 days of autonomy in case of cloudy weather, and size solar panels separately — a good rule of thumb is 1.5× your daily Ah in watts of solar.