I want to use 12V to 220V inverter with 220V 18W speaker. How to choose the battery?

Firstly, the battery discharge current is calculated.

Assume that the inverter is 30W and the load rate is 60%.

Assume that at this loading rate, the inversion efficiency is 0.8.

Then the power of DC output is 18/0.8 = 23W.

The current of 12V battery is 23/ 12 = 2A.

Assuming that the battery has just been discharged for 5 hours, the current rate for 5 hours is about 1.660.

The rated capacity ratio is 83%

That is, the required battery capacity is 2 * 5/0.83 = 12ah.

Or 2/1.66 *10 =12ah.

The discharge capacity of the battery also needs to calculate the temperature influence coefficient, which is ignored here.

And the battery capacity is 100%, which is a new battery.

The actual discharge capacity can reach the rated capacity. If it's old, it won't come out. . .

The above examples refer to the parameters of lead-acid batteries.

If the parameters of lithium battery and lithium ferrous phosphate are calculated similarly.

The most important thing here is that your speaker is not necessarily 18W.

It is not necessarily constant power, and the actual power may change.

As the saying goes, thousands of miles apart, you have to measure the data to be accurate.

There is also an inverter efficiency that can also be measured.

So the conclusion is > 12AH.