A 12V 7Ah battery has 84 watt-hours available in storage. However, up to 20% of the available charge can be lost in conversion from DC to AC to power an AC device. Furthermore, many batteries prefer an 80% depth of discharge, leading to about 54 watt-hours of actual capacity.
A 12V 7Ah Battery has (12v x 7Ah = 84) 84 watt-hours available in storage. But as we now know, an 84 watt-hour battery is, in reality, a 54 watt-hours battery that will power a 54w device for one hour or a 27w device for 2 hours.
|54 watt-hours will typically power a 27-watt device for 2 hours (54 watt-hours/2=27)|
Discharging a battery at half its Watt-hour charge (54 watt-hours/2=27) or less is highly recommended to avoid damaging your battery, shortening its lifespan, and overheating it.
But here is the real reason why you can only use half of the available charge at once:
|Unless stated on the battery, a 12v 7Ah battery prefers to give only 3.5 amps to stay healthy.|
Most Batteries don’t like their maximum current to be drawn at once, so you’ll need to multiply your battery voltage by half its Ah to get the maximum amount of watts you can simultaneously use.
12v x 3.5Ah = 42 watts continuously. Now we still need to deduct a discharge depth of 80% and inverter losses of about 80%.
42 watts x 0.8 x 0.8 = 26.88 watts
We are back to the same 27-watt maximum simultaneous and continuous usage mentioned above.
A 12v 100Ah battery will give you:
12v*100*0.65= 780 watt-hours are available for use.
You should use those 780 watt-hours at a maximum rate of 400-watt devices connected simultaneously to optimize your battery usage.
12v 100ah battery will power a 400w device for 1 hour and 54 minutes and could go a bit longer but at the expense of the battery’s health.
780 watt-hours / 400watt = 1.95 hours i.e. 1 hour and 95% of an hour which is 54 minutes.
Below is a general approximation to quickly figure out what you can get out of a battery
How many hours will my battery last?
- Multiply the Voltage and the Ah of your battery to get the Watt-hours rating,
- Multiply your Watt-hours rating by 0.65 to get the discounted rate (this is the real-life situation available charge).
- Divide the discounted watt-hours rating by the watt rating of all AC devices connected to the battery.
- The result of the above division is the number of hours available to run your devices on that battery.
- Read through the begining of the article for a calculated example.