Battery Bank & Backup Time Calculator

How many batteries does a 500-watt load need to run through a four-hour power cut? The answer depends on more than watts and hours. Inverters lose some energy, and lead-acid batteries should not be drained much past half. The batteries also have to be wired so their voltage matches the inverter. This calculator handles all of it and gives you a count and a wiring layout, not just an amp-hour figure.

It answers the question the other way round too. Choose How long will my bank last?, enter the batteries you already own, and it tells you the runtime for your load. That works for an IPS at home, a solar system, a campervan or a UPS cabinet.

Worked Example: Four Hours of 500 W on a 24 V Bank

The calculator opens with a 500 W load for 4 hours through a 90% inverter, on a 24 V lithium iron phosphate bank used to 80% depth of discharge, built from 12 V 100 Ah batteries. It reports:

• Energy drawn: 2,222.22 Wh, since the inverter wastes a tenth
• Capacity needed: 2,777.78 Wh nominal, which is 115.74 Ah at 24 V
• Batteries: 4 × 12 V 100 Ah, 2 in series × 2 parallel strings = 24 V 200 Ah (4.8 kWh)
• That bank actually runs the load for 6 h 55 min, drawing 23.15 A at about C/9

Load Sample continues with a 300 W IPS backup on 12 V tubular batteries, the runtime of four 200 Ah lithium batteries in a 48 V bank feeding a 150 W fridge and router (2 d 6 h 43 min), and a 60 W campervan load that runs on DC with no inverter.

How the Math Builds Series and Parallel Strings

Energy drawn is the load times the hours, divided by the inverter efficiency (skipped for DC loads). Dividing by the usable depth of discharge gives the nominal capacity, and dividing that by the system voltage gives amp-hours. Batteries go in series until their voltages add up to the system voltage. Whole parallel strings are then added until the amp-hours cover the need, so the bank always rounds up.

Picking a battery type fills in a typical safe depth of discharge: 50% for lead-acid and 80% for lithium. A system voltage that the batteries cannot add up to is refused: “System voltage 24 V must be a whole multiple of the 3.2 V battery voltage, so batteries can be wired in series to reach it.” In runtime mode the battery count must likewise fill whole series strings.

Accuracy Limits: Where Real Batteries Fall Short of the Label

Lead-acid capacity is rated over 20 hours. Drain it faster and you get less. That is why the tool warns when the discharge rate passes C/5. A 1,500 W load on a 12 V lead-acid bank draws 138.89 A and triggers both that warning and a tip that the same load at 24 V needs half the current. Cold weather, battery age and inverter standby draw all cut runtime further, so leave headroom for a bank you depend on. Lithium batteries hold their capacity at high discharge rates far better, but check the battery management system’s current limit.