Start with battery energy in watt-hours
A 12 V 160 Ah battery has 12 × 160 = 1,920 Wh of nominal stored energy. This does not mean all 1,920 Wh should or can be delivered to an appliance.
Account for usable capacity and losses
If 80% of capacity is usable and the inverter/system efficiency is 90%, a 12 V 160 Ah battery supplies an estimated 1,382 Wh to the load: 1,920 × 0.80 × 0.90.
Example with a 400 watt load
Using the 12 V 160 Ah example with 80% usable capacity and 90% efficiency: 1,382 Wh ÷ 400 W ≈ 3.46 hours. A simple calculation that ignores losses would show 4.8 hours, which is usually too optimistic.
What changes real battery runtime
- Battery chemistry and recommended depth of discharge.
- Battery age, temperature and state of charge.
- High discharge rate, especially for lead-acid batteries.
- Inverter efficiency and its own idle consumption.
- Motor starting surge and varying appliance power.
- DC wiring losses and voltage cutoff settings.
Series and parallel battery banks
In series, battery voltages add while amp-hour capacity stays the same. In parallel, amp-hours add while voltage stays the same. Two identical 12 V 100 Ah batteries can form 24 V 100 Ah in series or 12 V 200 Ah in parallel. Both arrangements have about 2,400 Wh nominal energy, assuming identical batteries.
AC appliances and inverter loads
For an AC fan, television, motor or office load running from a DC battery, use the appliance watts on the AC side and include inverter efficiency. If the appliance power varies, use a measured average rather than only the nameplate maximum.
Motor and compressor loads
Motors can have a high startup surge even when average running watts are moderate. A battery/inverter system must be able to supply both the energy over time and the short-term peak power. Runtime alone does not confirm that the inverter can start the load.
Practical runtime range
Battery-backup calculations are estimates. For planning, test the real system under normal load and record voltage, power and cutoff behavior. Use a reserve margin when backup time is operationally important.
Frequently asked questions
How do I calculate battery backup from Ah?
Convert Ah to watt-hours using volts × Ah, adjust for usable capacity and efficiency, then divide by load watts.
How long will a 12 V 160 Ah battery run a 400 W load?
A simple ideal estimate is 4.8 hours. With realistic usable capacity and inverter losses, the runtime can be substantially lower; for example about 3.46 hours at 80% usable capacity and 90% efficiency.
Does connecting batteries in series increase Ah?
No. Series increases voltage; parallel increases amp-hour capacity.
Why is actual backup time less than the formula?
Losses, depth-of-discharge limits, battery age, temperature and high discharge rates reduce usable energy.
Enter your own values, keep the assumptions visible and use this guide to understand the formula behind the result.