A household appliances’ wattage can be determined by:

a)     Checking the appliance’s rear or power supply for the figures.

b)    Referring to the product manual.

c)     Using a watt meter to measure the wattage.

d)    Checking similar appliances for the wattage.

A power supply tends to show amps as output current instead of showing the wattage of the device. A power supply with an output voltage display can be used to calculate the wattage by multiplying current (amps) with voltage.

Power (watts) = Volts x Current (amps)

P = VxI

A watt meter can measure the energy requirements of an electric appliance that uses electricity from the mains. Just plug the appliance into the watt meter, which is plugged into the wall socket, to check the wattage. It is displayed on the LCD watt meter display.

Taking the wattage of another similar electric appliance is just an estimate, and generally proves to be much less accurate for determining the power requirement of the device in question. Appendix C [V1]  shows the power ratings of some frequently used household appliances.

With the completed list of the electrical devices’ power requirements, and using a table (or spreadsheet) that you have created for this purpose (that is, 12V versus mains voltage), note the type of voltage for each electrical device along with the wattage.

Add a column to your table for the estimated daily usage of each device and multiply the number of hours of use by the number of watts to determine each device’s total watt-hour energy requirement.

“Phantom loads” – that is, electricity leaks or electronics that use energy while in standby mode - must be factored in if they are going to be used on the solar system. These devices are typically identified by their consumption of electricity even when not in use; devices such as chargers for cell phones, laptop computers and other rechargeable devices, televisions, microwaves, coffee makers, etc. An electrical appliance that is plugged-in will produce a phantom load. It is most efficient to get into the habit of unplugging or powering off these devices when they are not in use. Otherwise, you must factor them in as additional appliances that will be using power from your solar system on a regular basis.

Of course, there will be some devices which will be in standby mode – things like alarm clocks and the like – and there will be days when you may forget to unplug a device and leave it running. These intended and unintended phantom loads will need to be accounted for as well.

You will also need to remember that a central heating system that uses gas comes equipped with an electric pump and controller – both of which consume electricity. The electric pump generally consumes a daily power of 60 watts, while the controller consumes 2 - 24 watts daily.

Your completed power analysis will look something like this: