What happens to the backup time when batteries are connected in series?

Question

I have a backup power system consisting of a charger, a battery and an inverter. The inverter powers my load (a few mains appliances). Currently I am using a 12V 200 Ah battery which gives me a backup of say 2 hours for my load.

I am upgrading to an inverter that requires 24V, so I will need to use two batteries in series. To save on the cost of the batteries I am thinking of buying two smaller batteries so that it costs me the same as the larger battery I originally had.

My question is: If I use two 12V 100 Ah batteries in series, will I still have the same backup time (2 hours) for the same load?

I am confused about this because from what I remember connecting in series will only double the voltage and retain the rating at 100 Ah, however what is the benefit of 24V, should that not translate to the same backup time since at the end of the day the load consumes the same power?

Answer 1

If you think of it in terms of watt-hours, the two are the same.

Assuming you use all of the charge in the battery, a 12V 200Ah battery has 12x200 = 2400Wh.

Two 12V 100Ah batteries in series gives you 24V 100Ah, which is still 2400Wh.

A 24V inverter might be more efficient. The currents on the DC side will be halved. But on paper, there's no difference.

Answer 2

Putting two batteries in series doubles the power available over that of one battery.

Whether your load is capable of using that power efficiently is another matter.

In the case of an inverter designed to run from a specific voltage, you can probably assume it will use that input power efficiently. For the same load, for equal efficiencies, a 24v input inverter will draw half the current from the batteries that a 12v input one would.