Determining how long your solar or backup system’s batteries will last depends on a number of things; the total battery capacity, the amount of power being drawn, the inverter efficiency and the depth of battery discharge. These tables will assist you in estimating how long your batteries will last after they have been fully charged.
The following table gives you an idea of how long your power system will be able to provide power, in minutes. This table assumes an inverter efficiency of 95% (pure sine wave), the inverter is providing its rated output power and that the batteries are discharged to 30% of their original capacity. To use this table, the system voltage does not matter as long as you add up the amp hours of all of the 12V supply batteries. For example if you have two 105Ah batteries in series providing 24 volts for your system, you would look at the 210Ah column in the table. All times shorter than 10 minutes and longer than 1440 minutes (a 24hr day) have been removed.
This table gives you an idea of how long your power system will last operating at 60% of the inverters rated power output. The efficiency is reduced to an average of 90% working at a lower power output. All other parameters are the same as in the above table.
Alternately you can use these tables backwards to determine how many batteries you need. Find your inverter size in the column on the left, then the column with the numbers of minutes you require it to run for and then read up the total amp hours which will give you the total battery capacity required. A standard 12V deep cycle battery is usually 105Ah so divide the total required capacity by 105 and that will give you the number of batteries required.
Here is a guide on selecting the correct inverter size for your solar or backup power system.
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