Why divide by depth of discharge?

You can only safely use part of a battery's rated capacity. A lead-acid bank should not go below about 50 percent, so to deliver a given usable energy you must install roughly twice that nominal capacity. Dividing by the DoD fraction grosses the bank up to its rated size.

What days of autonomy should I pick?

Two to three days is common for sunny climates, and four to five for cloudy regions or critical loads. More autonomy means a larger, costlier bank, so balance resilience against budget and consider a generator backup for rare long outages.

How is the amp-hour figure calculated?

Required Ah equals daily Wh times days of autonomy, divided by system voltage, divided by the depth-of-discharge fraction. Energy in kWh is simply daily kWh times days of autonomy divided by the DoD fraction, independent of voltage.

What depth of discharge is realistic per chemistry?

Flooded lead-acid is typically limited to 50 percent, AGM and gel to about 50 to 70 percent, and lithium iron phosphate to 80 to 90 percent. Lithium's deeper usable range is why it needs far fewer rated amp-hours for the same usable energy.

Does this include charging or inverter losses?

No. This sizes storage only. Add inverter and round-trip charging losses, typically 10 to 20 percent, to your daily load before entering it, and size your panels and charge controller separately so the bank can actually be refilled.

Battery Bank Sizing Calculator (Off-Grid Solar)

An off-grid or backup battery bank must store enough usable energy to carry your loads through several days without sun. This calculator turns your daily watt-hour consumption, target days of autonomy, depth of discharge, and system voltage into the rated amp-hours and kilowatt-hours you need to install.

How it works

The required usable energy is your daily load multiplied by the days of autonomy. Because you can only safely use a fraction of a battery’s rated capacity, you divide by the depth-of-discharge fraction to get the rated size:

Required Ah = (Daily Wh × Days) ÷ System Voltage ÷ DoD fraction
Required kWh = (Daily Wh × Days) ÷ 1000 ÷ DoD fraction

A 2000 Wh/day load at 48 V, 3 days autonomy, and 50 percent DoD needs (2000 × 3) ÷ 48 ÷ 0.5 = 250 Ah of rated capacity.

Sizing strings

Batteries are sold in fixed amp-hour ratings. To reach the required Ah at your system voltage you wire batteries in parallel into strings. The tool divides the required Ah by your selected battery’s Ah rating and rounds up, so you always have at least enough capacity.

Tips

Add inverter and charging losses to your daily load before entering it.
Lithium iron phosphate tolerates 80 to 90 percent DoD, shrinking the bank.
Never mix battery ages or chemistries in one bank — they will not balance.