Determine the current (ampere) specification of the electronic devices and appliances that will be connected to your power supply. To calculate the amount of amp hours that your power supply is required to deliver, you will have to find out the amperes that the electronic devices connected to it use.
Electronic manufacturers sometimes list the current that their electronics products require. And with that information you can easily calculate ampere-hours. But often, manufacturers don't list current requirements. Most often manufactures just list the wattage rating.
Change the wattage rating to amperes. If the appliance rating is given in watts, you can convert watts to amperes by dividing the watt value by the power supply voltage. If the device uses a 2-Volt supply, which is the battery used in many laptop computers, and the computer is rated at 40 watts, the ampere rating would be 40 watts divided by 2 Volts, or 20 Amperes.
Sometimes the wattage rating may not be in watts, but in milliwatts or microwatts. The Watt is the standard unit of electrical power measurement. A microwatt is one-millionth of a watt and a milliwatt is one thousandth of a watt. To convert microwatts to watts, divide the microwatt value by one million. To convert a milliwatt rating to watts, divide the milliwatt value by one thousand.
For example, if the milliwatt rating was 50 milliwatts then the equivalent watt rating would be 0.05 watts. If the microwatt rating was 100 microwatts, the watt rating would be 0.0001 Watts. .
Change the units of the given ampere specifications. If you are given a current specification instead of a watt specification, you may notice that the current is not given in amperes. Instead it may be given in microamperes or milliamperes. The ampere, also known as an amp, is the standard unit of current measurement. A microampere is one-millionth of an ampere and a milliampere is one thousandths of an ampere.
To convert microamperes to amperes, divide the microampere value by one million. To convert a milliamperes rating to amperes, divide the milliampere value by one thousand.
For example, if the current rating was 50 milliamperes, then the equivalent ampere rating would be 0.05 amperes. If the microampere rating was 100 microamperes, the ampere rating would be 0.0001 amperes.
Calculate the ampere-hours for each electronic device connected to the power supply. The ampere-hours that an electronic device uses are equal to its ampere rating multiplied by the number of hours it is on. For example, if a laptop computer rating is 20 amperes and it is on for 4 hours, it would need an 80 ampere-hour battery. If the battery within your computer was rated at 60 ampere hours, it would, however, only be able to supply a 20 ampere current for three hours.
Sum the ampere-hours for each electronic device connected. This will give you the total ampere-hours that your power supply will need to deliver. If the sum was 20 ampere-hours and your power supply was rated at 40 ampere-hours, your power supply could deliver power for two hours before it needed to be recharged.