Watt Hour Battery Capacity vs Watt Hour Consumption

Thread Starter


Joined Nov 5, 2015
I have various battery sizes and appliances I am trying to compare to determine the best Solar Home System for my purposes. I have converted all batteries capacities and appliances into watt hours for an apples to apples comparison. How can I determine how long it will take to deplete a battery (hours of operation) based on consumption (watts). I know resistance factors into the equation, but I am not sure how much.

IE: I have a battery that measures 4.5 Ah and 13v for 58.5 Wh and an appliance that consumes 5.5 W (12v)... How many hours of operation will I get?


Joined Nov 30, 2013
The simple answer is that hours of operation = Watt hours divided by Watts. For your example, a 58.5Wh battery delivering 5.5W will last 58.5/5.5 = 10.6 hours. Of course, that assumes a lot of ideal conditions like 100% efficient voltage conversion which you will not actually get. There are many other factors such as battery chemistry, temperature, overall load, etc. that will impact the total usable energy you get.


Joined Apr 24, 2011
One of these "many other factors" is how much do you mind replacing batteries? The more you drain a battery the more life you take away. A general rule of thumb is to drain no more than 25% of the battery capacity before a recharge. BUT that does not imply the daily drain may be 25% unless you live in an area that never gets cloudy.

The USCG specs the battery usage in aids to navigation to 1/30 th the battery capacity per day to keep their batteries working for the long term.

Your trade off is to pick a number in the middle. Keep in mind that a higher initial expense may negate a later expense when the batteries crap out during a long rainy season.