To determine how long your battery will last before running down, you need to know the “Reserve Capacity” of your battery. Reserve Capacity is a specification provided by the battery manufacturer. It is specified in minutes. If you have more than one battery, simply add up the reserve capacity of all the batteries.
Look on your battery for the Reserve Capacity specification. If it is not listed, then you will need to check with the battery manufacturer. If your battery is more than 3 years old, multiply the reserve capacity by 0.85 to allow for aging.
If you cannot determine the reserve capacity, then use a value of 120 minutes per battery. If your battery is more than 3 years old, then use 102 minutes. These values will give you a rough estimate, but the reserve capacity of your particular battery may be higher or lower.
The next step is to look at your appliance to see how many watts it uses. Try checking in the manual. You can also look on the back of the appliance near the power cord. It should either list the watts or the amps. If it lists the amps, then multiply by 115 to get watts. For example if it says 1.5A on the back of the appliance, then 1.5 x 115 = 173 watts.
Now that you have the reserve capacity of the battery (in minutes) and the watts of the appliance, plug these values into the following formula to determine battery life:
Battery Life (in hours) = 4.348 x Reserve Capacity in minutes / Watts
How long will the inverter run my TV?
Lets say you look on your battery and it does indeed list the reserve capacity, which is 130 minutes. You look on the back of the TV and it says 87 watts. Using the formula:
Battery Life = 4.348 x 130 / 87
Battery Life = 565.24 / 87 Battery Life = 6.5 hours
Please note: The formula will give you an approximation for determining how long your battery will last under load, running the battery down to 10.5 volts. Doing so will deplete the battery to a state where it will not have enough power to start the engine!
Longer operating times can be obtained by either using more batteries (connected in parallel) or running the engine. If you decide to run the engine, set it at a high idle so that the alternator produces adequate output.