Estimating your Power Demands

Convert Watts to Amps

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. This can be done with pen and paper (in which case please read on), or using our online calculator.

Online Calculator

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 230 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 230 volt colour TV that doesn't have a watts rating but does give an amps rating. The figures it gives are 230 volts, 50 hertz, 0.3 amps. This amps usage figure is the power consumption at 230 volts. Since amps times volts equals watts, this works out at 69 watts (230 times 0.3). Now to work out the amps at 12 volts you divide 69 watts by 12 volts and you get 5.75 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 5.75 amps by 0.7 (70%) and you get 8.2 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:

ApplianceAmpsHours UsedAmp-hoursradio/cassette0.206.001.203 lights5.004.0020.00juicer30.000.175.00colour TV8.502.0017.00TOTAL43.20

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. Please fill in and submit a quote request for a residential solar system.

Typical Appliances Ratings A-F

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POWER CONSUMPTION GUIDE(230V)APPLIANCESSTARTWATTSAir Conditioner(Single Split System)(evaporative - mobile)-500 - 2500275 - 1000Alarm/security system-6Blanket (under)-60 - 120Blanket (over)-150 - 350Can Opener-100Cassette (Tape) Player Deck-30CB (receiving)-10CD Player-30Circular Saw (small)-1350Clothes Drier-2400Coffee Grinder-75Coffee Maker-300-1500Cordless phone (use or charge)-2-3Computer (Laptop or Notebook)-40-60Computer (Desktop + Screen)office usegaming-150-200500-1000Computer Printer-30-50Digital Video Recorder-20-50Disposal Unit-650Drill-250 - 500Dishwasher-1200 - 2500Domestic Water Pump2000500Electric Toothbrush (charging stand)-6Exhaust Fan-40 - 75Fan-20 - 100Fax (standby)-10Fax (printing)-120Food Mixer & Whiz-500Floor Polisher-350Freezer2500500Frypan-1400

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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POWER CONSUMPTION GUIDE(230V)APPLIANCESSTARTWATTSHair Dryer, heated-800 - 1800Heater-500 - 2400Hotwater Service-2500 - 5000Infra-red Grill-2000Iron-800-2000Juicer/Blender-350-550Kettle or Jug-1600 - 2400Lighting LED-3-15Lighting Fluoro-10-20Microwave oven-600 - 1800Mobile phone (charging)-5-15Modem/Router-5-15Modem NBN Satellite-35Radio-15 - 60Radiator-1000 - 2500Record Player-75Refrigerator1500300Sewing Machine-60Space Heater-2000Stove-5000 - 10000Tablet (charging)-10-25Television LED-30 - 120Toaster-500 - 1500Vacuum Cleaner-700 - 1800Washing Machine2500600Welder - 140A-4000

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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Electric Motors - Starting Current-Type of Motor-WattsInductionCapacitorSplit-Phase1/6 hp27560085020501/4 hp400850105024001/3 hp450975135027001/2 hp6001300180036001 hp110019002600-

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.