Estimating your Power Demands

Convert Watts to Amps

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You need to have a good idea of how much electricity is required before you can decide on the appropriate size of your solar panel array, and the size of cables and battery bank.

There are three simple steps to determine the average daily load:

1. Select which lights and appliances will be used.
2. Find out how many amps or watts each consumes.
3. Work out how many hours each day (on average) each appliance will be used.

Since the size of your battery bank is rated in terms of amp-hours and the meter on your distribution/meter box measures the power coming in from your charging system in amps, it makes sense to convert watts to amps. I will give you some examples:

• You have a 12 volt portable radio and cassette player that has a label on the back that says 12 volts, 0.2 amps. You don't need to calculate anything for this as the current draw is already given in amps at 12 volts.
• You want to use a 12 volt 20 watt light bulb. To work out the amps you just divide 20 watts by 12 volts and you get 1.67 amps.
• You have a 240 volt juice extractor rated at 300 watts. If you have a solid state inverter rated at 400 watts you can expect 85% efficiency. So to work out the amps at 12 volts you divide 300 watts by 12 volts and you get 25 amps; on top of that you have the inverter efficiency to add to that figure. Divide 25 by 0.85 (85%) and you get about 30 amps.
• You have a 240 volt colour TV that doesn't have a watts rating but does give an amps rating. The figures it gives are 240 volts, 50 hertz, 0.3 amps. This amps usage figure is the power consumption at 240 volts. Since amps times volts equals watts, this works out at 72 watts (240 times 0.3). Now to work out the amps at 12 volts you divide 72 watts by 12 volts and you get 6 amps. If you run this off the same 400 watt inverter you can only expect 70% efficiency (refer to inverter data supplied by your dealer). Divide 6 amps by 0.7 (70%) and you get 8.5 amps.

Determine the Average Daily Load

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Now to give you an example of working out the daily power consumption:

• You listen to either the radio or cassette player for 6 hours each day. The 12 volt system you have is rated at 0.2 amps at 12 volts. Multiply the amps by the hours and you get the result of 1.2 amp-hours per day.
• You use three 20 watt 12 volt lights for about four hours each night. The power consumption for each light we worked out earlier to be 1.67 amps. So for three lights we calculate a current draw of 5 amps. So to calculate the power consumption we multiply 5 amps by 4 hours to get the result of 20 amp-hours per day.
• You use a juice extractor for 10 minutes each day. We have already calculated that the inverter draws 30 amps when the juice extractor is running. Divide 30 by 6 (because you use the juicer for 1/6th of an hour) and you get a result of about 5 amp-hours per day.
• You watch the colour TV for about 2 hours each night. We estimated before that the inverter draws about 8.5 amps when the colour TV is on. Multiply 8.5 by 2 and you get 17 amp-hours per day.

Here are those figures in tabulated form:

We can design your system for you, using a computer based Power System designing software. We will require detailed information on your envisaged power usage including power ratings and hours per day usage of lights, appliances etc. Contact us for further details or download a Power System Sizing form.

Typical Appliances Ratings A-F

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NOTE: These figures are a guide only and the wattage ratings may vary greatly from one appliance to another.

Typical Appliances Ratings G-Z

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NOTE: These figures are a guide only and the wattage ratings may vary greatly from one appliance to another.

Electric Motors

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NOTE: Brush type motors without a load do not require a significantly higher starting current than their continuous current rating.

Read all about the power requirements of various appliances run from an inverter. There are articles about the power needs of TV's, sound equipment, bread ovens, computers, washing machines, pumps, printers, fans etc.