Work out how big a battery bank you need: total watt-hours, amp-hours at your system voltage, and how many 12 V 100 Ah batteries that means.
Your daily loads
Days without any recharging.
Plans on using 90% of capacity to protect battery life.
Inverter
Daily use
1,800 Wh
Battery needed
2,222 Wh
At 12 V
186 Ah
12 V 100 Ah batteries
2
in parallel
1,800 Wh/day × 1 day(s) ÷ 90% usable ÷ 90% inverter efficiency = 2,222 Wh. Add 10–20% if the batteries will sit in the cold, or if you expect to add loads later.
Daily Wh is each load's average watts × hours per day, added up. The usable fraction protects battery life: 90% for LiFePO4, 80% for NMC lithium and 50% for lead-acid. If loads run on AC, we also divide by 0.9 for inverter losses.
Then convert to amp-hours at your system voltage:
Ah = Wh ÷ system voltage
Worked example
A fridge (60 W × 24 h = 1,440 Wh), four LED bulbs (36 W × 5 h = 180 Wh) and a laptop (45 W × 4 h = 180 Wh) need 1,800 Wh a day. For one day on LiFePO4 through an inverter: 1,800 ÷ 0.9 ÷ 0.9 ≈ 2,222 Wh, or 185 Ah at 12 V. That is two 12 V 100 Ah LiFePO4 batteries in parallel.
Tips for sizing
Measure real usage with a watt meter where you can. Guessing is the biggest source of error.
Add 10–20% if batteries will be cold: lithium delivers less below about 50°F (10°C), and most LiFePO4 can't charge below freezing.
Wiring batteries and inverters involves high currents. Use correctly rated cables and fuses, and get a qualified electrician for anything connected to home wiring.
Frequently asked questions
How many 100Ah batteries do I need?+
Divide your required watt-hours by about 1,280 for LiFePO4 (12.8 V × 100 Ah) or 1,200 for lead-acid, then round up. For 24 V or 48 V systems, round up to a multiple of 2 or 4, because 12 V batteries are wired in series strings.
Why does lead-acid need a battery twice as big?+
Regularly draining lead-acid or AGM batteries below about 50% shortens their life dramatically. LiFePO4 handles 80–90% discharge for thousands of cycles, so you need far less rated capacity for the same usable energy.
Should I choose 12 V, 24 V or 48 V?+
12 V suits small systems (vans, boats, under about 1,500 W of inverter). 24 V or 48 V halves or quarters the current for the same power, allowing thinner cables and more efficient larger inverters: 48 V is standard for home and off-grid systems over about 3,000 W.