How long can I run appliances on my batteries?

The easy steps to answer this question are:

  1. Calculate battery bank capacity.
  2. Determine total wattage of draw of your equipment.
  3. Use this calculator to calculate  how long your loads will operate from your battery bank.

Step 1: Calculating battery capacity

In order calculate battery capacity, you will need to collect a few figures:

Amp-hour capacity of the battery bank  
First, enter the amp-hour capacity of your battery bank:

Most batteries express their capacity in terms of amp-hours, batteries may be wired in parallel to increase capacity. Simply add the individual capacity's together to gain the bank capacity.
Planned depth of discharge of batteries:  
Second, enter the planned depth of discharge of your batteries: %

Since your system is probably using deep-cycle lead-acid batteries, maximum longevity is achieved by discharging batteries to a maximum of 50% of capacity.

Discharges as high as 80% are acceptable, but we recommend the 50% figure. Think of the 50% figure as the normal discharge, with up to 80% on rare occasions.
Temperature de-rating factor:

Third, enter your de-rating factor due to temperature: %
The chemical reaction that takes place in a battery to produce electricity is dependent on the temperature of the battery. Most batteries perform best under conditions that resemble room temperature. In colder situations, battery capacity can be reduced as much as half. To get your "de-rating" factor due to temperature, consult the table below.

Battery Temp. (Fahrenheit)

Battery Temp. (Celsius)

Capacity de-rating based on average expected weekly temp.

80

27

100%

70

21

96%

60

16

90%

50

10

84%

40

4

77%

30

-1

71%

20

-7

63%


The total de-rated amp-hour capacity of your battery bank:

(Rated Amp hr capacity x % depth of discharge x
% temp de-rating:

Surprised at the "real-life" capacity of your batteries? Many retailers don't build in the proper safety factors for battery bank sizing. Now that you've got "real world" conditions, you're ready to move on to step 2.

Step 2: Determine total wattage draw of your equipment

Enter the total wattage of the equipment you wish to run:

Step 3: Now that you've got all your figures, you're ready to calculate your battery's capacity under real conditions:

Calculate how long a given load will run on my batteries:  
Calculated "real world" capacity from step 1:
Total maximum wattage draw from step 2:
enter system voltage here: Volt x capacity ÷ load = hrs

Example: I've go 500 amp-hours of "real world" capacity in my 12V battery bank and wish to run five 100 watt lights.

(12V x 500 amp-hours) ÷ 500 watts (5 lights x 100 watts) = 12 hours.