How to calculate how long a 12v battery will last when working through an inverter?

Question

Rechargeable batteries

Hello. Learn how to correctly calculate how long the battery lasts.

Let's say I buy a 12v 4Ah car battery and buy a 220v inverter. I take a light bulb for 220v 20 watts, and connect it to the inverter.

So we have: 1. battery 12v 4Ah 2. bulb 220v 20 watts 3. inverter 12v 220v
How to calculate how long a full battery charge will last with these parameters?

Answer 1

Here we must understand that such a calculation cannot be accurate, but only an estimate, because the initial data is fundamentally inaccurate. The battery has a capacity not exactly 4 Ah, but from ... to ..., and even reduces it as it wears out. A light bulb too: if you buy a hundred pieces and measure their power, you will see a percentage spread of 20 ... 30.
Therefore, it is quite enough to estimate through the efficiency of the inverter, and it is unlikely to be better than 70%. And then a trivial calculation follows: if the lamp is 20 watts, then the inverter will consume 20 / 0.7 = 29, rounded 30 watts. To find out the current, we divide 30/12 \u003d 2.5 amperes.
Well, the final - 4 Ah divided by 2.5 A and we get 1.6 hours of lamp glow until fully discharged. Of course, plus or minus half an hour.