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Battery Size Calculator
Use this free battery size calculator to size a battery bank for a solar setup, RV, or off-grid cabin. Enter your daily energy use, how many days of autonomy you want, your battery type and system voltage, and it gives the bank size in amp-hours and kilowatt-hours — plus how many standard batteries that means.
Add up the watt-hours of everything you run in a day. A 60 W light for 5 hours is 300 Wh; a 100 W fridge running half the day is about 1,200 Wh.
Changing the battery type sets the usual depth of discharge — 90% for lithium, 50% for lead-acid — but you can adjust it yourself.
| Amount | Value |
|---|---|
| Energy needed (with autonomy) | 2,400 Wh |
| Usable energy (after inverter loss) | 2,824 Wh |
| Total stored energy (÷ DoD) | 3,137 Wh |
| Stored energy | 3.14 kWh |
| Bank capacity | 261 Ah |
| Batteries needed | 3 × 100 Ah |
How it works
Sizing a battery bank is a chain of four steps, and skipping any one of them gives a bank that runs flat too soon. First, work out how much energy you use in a day, in watt-hours. A watt-hour is one watt running for one hour, so a 60-watt light on for 5 hours uses 300 watt-hours, and a small fridge drawing 100 watts for about half the day uses around 1,200 watt-hours. Add up every load you plan to run from the batteries.
Second, multiply by days of autonomy: how many days the bank must carry you with no sun and no charging. Two days is a common choice for a weekend cabin or an RV, and three days is common for a home solar setup in a cloudy area. More days means a bigger, more expensive bank, so pick the number honestly rather than padding it.
Total stored energy = usable energy ÷ depth of discharge
Bank capacity (Ah) = total stored energy ÷ system voltage
Stored energy (kWh) = total stored energy ÷ 1,000
Third, correct for the inverter. Most loads run on AC power through an inverter, and a typical inverter is 85 to 95 percent efficient, which means 5 to 15 percent of the battery energy is lost as heat on the way out. Dividing by the efficiency, written as a decimal, adds that loss back in: 2,400 watt-hours of load through an 85 percent efficient inverter needs 2,400 ÷ 0.85 = 2,824 watt-hours from the batteries.
Fourth, correct for depth of discharge. A battery's label capacity is its total storage, but you cannot safely use all of it. Lead-acid batteries are usually limited to about 50 percent depth of discharge, because draining them further shortens their life sharply. Lithium batteries can usually go to 80 or 90 percent. Dividing by the depth of discharge gives the total stored energy the bank must hold so that the usable part covers your load.
Finally, convert energy into amp-hours by dividing by the system voltage, because batteries are sold in amp-hours at a given voltage. The same 3,137 watt-hours is 261 amp-hours at 12 volts, 131 amp-hours at 24 volts, and 65 amp-hours at 48 volts — the energy is identical, only the amp-hours change. The calculator also divides the result by the size of one battery to tell you how many batteries to buy, rounding up, since you cannot buy part of a battery.
Worked example
A small cabin uses 1,200 watt-hours a day and needs 2 days of autonomy, on a 12-volt lithium bank with a 90 percent depth of discharge and an 85 percent efficient inverter.
Step 1: Energy needed — 1,200 × 2 = 2,400 Wh.
Step 2: Usable energy after inverter loss — 2,400 ÷ 0.85 = 2,824 Wh.
Step 3: Total stored energy — 2,824 ÷ 0.90 = 3,137 Wh, or 3.14 kWh.
Step 4: Bank capacity — 3,137 ÷ 12 = 261 Ah at 12 V.
Step 5: Battery count — 261 ÷ 100 = 2.61, so 3 × 100 Ah batteries in parallel.
The result is 261 Ah at 12 V (3.14 kWh), covered by 3 × 100 Ah batteries — exactly what the calculator shows for those inputs.
FAQ
How many amp-hours of battery do I need for solar?
It depends on your daily load, how many cloudy days you want to ride through, and your system voltage. A small cabin using 1,200 watt-hours a day with 2 days of autonomy on a 12-volt lithium bank needs about 261 amp-hours. Enter your own daily load, days of autonomy, depth of discharge, and voltage in the calculator above and it sizes the bank for your numbers, not a guess.
What is depth of discharge and why does it matter?
Depth of discharge, or DoD, is the share of a battery's stored energy you can safely use. A lead-acid battery is usually limited to about 50 percent DoD, because draining it further shortens its life sharply. A lithium battery can usually go to 80 to 90 percent DoD without the same damage. A lower DoD means you need a bigger bank for the same usable energy, which is why the calculator divides by DoD.
Is a 12V, 24V, or 48V battery bank better?
For the same energy, a higher voltage means fewer amp-hours and thinner wires, because power equals voltage times current. Small systems under about 1,000 watts are usually fine at 12 volts. Medium systems up to about 3,000 watts often use 24 volts, and larger off-grid or solar systems usually use 48 volts. The energy in kilowatt-hours stays the same at any voltage; only the amp-hours change, and this calculator shows both.
What is the difference between battery size and battery life?
Battery size is how much energy the bank can store, measured in amp-hours or kilowatt-hours, and it answers the question of how big a bank to buy. Battery life is how long a battery lasts while powering a load, measured in hours, and it answers the question of how long one charge will run a device. This calculator sizes the bank; for runtime, use a battery life calculator instead.
How many batteries do I need?
Divide the amp-hours the calculator gives by the amp-hours of one battery at your system voltage. If you need 261 amp-hours at 12 volts and you buy 100 amp-hour 12-volt batteries, you need 3 of them wired in parallel, which gives 300 amp-hours. Batteries wired in parallel add amp-hours at the same voltage, while batteries wired in series add voltage at the same amp-hours, so always match the wiring to your system voltage.