Tasman Energy
Solar power calculations

Solar power calculations

Filling Out a Load Chart

Below are a couple of load charts. You should make up your own blank example for your own system. The purpose of a load chart is to determine what you want to run and how much power it will use. Below is a pretty basic method of determining your power requirements.

The first chart below is for a small cabin with some really basic appliances. It assumes 4 sun hours per day and factors the load by 0.7 to allow for inefficiencies.

Appliance Wattage x Hours (per day used) = Electrical load
Reading light 15 x 2 = 30
Living area light 20 x 4 = 80
TV 120 x 3 = 360
Phone charger 8 x 1 = 8
Total Daily power requirement 478
Efficiency factor 0.7 (478 divided by 0.7) 682.8
Total load to give 4 days autonomy: (683 x 4) 2732
Battery voltage: 12 volts, battery capacity determined by maximum discharge of 30%. 2732 = 30%. 100 % determined by (2732 ÷ 30) x 100 9106 (watts)
Battery capacity in amp/hours = 9106 ÷ 12 758 (amp/hours)
Solar array size based on 4 sun hours per day average = factored power requirement (683 watts) ÷ 4 (sun hours per day) 171 (watts)

The following chart is for a house with a typical requirement given a set budget. With a household solar design it is far better to use the more comprehensive design criteria of Australian Standard (AS) 4509.2 - 2002 (see bottom of page) but this chart displays the basics for a house with basic (as far as Aussie homes go) appliances and typical needs

Appliance Wattage x Hours per day
(Note 1)
= Electrical Load
Living room 20 x 4 = 80
Dining room 20 x 4 = 80
Bedroom 1 15 x 1 = 15
Bedroom 2 15 x 1 = 15
Bedroom 3 15 x 1 = 15
Bathroom 15 x 2 = 30
Study 15 x 2 = 30
Outside 50 x 0.5 = 25
Others (laundry, toilets, shed etc) 60 x 1 = 60
Total lighting load 350
Refrigerator 140 x 6 (note 2) = 840
Blender 500 x 0.2 = 100
Microwave 1500 x 0.5 = 750
Juicer 600 x 0.1 = 60
Total kitchen load 1750
  Lounge Room
TV 140 x 4 = 560
DVD 25 x 2 = 50
Stereo 25 x 1 = 25
Computer 140 x 2 = 280
Total lounge room load 915
  Bedroom Appliances
Electric blanket x 1 120 x 0.5 = 60
Kids TV 80 x 1 = 80
Kids TV 2 80 x 1 = 80
Rechargeable devices 40 x 1 (Note 3) = 10
Bedside clock 5 x 24 (Note 4) = 120
Total bedroom load 380
Washing Machine 250 x 1 = 250
Iron 900 x 0.2 = 180
Total laundry load 430
Power tools 1500 x 0.1 (Note 5) = 150
Phones, toys, vacuum, tools etc. 1000 x 1 = 1000
Total household load (note 6) 4975
Factored daily load (4945 ÷ 0.7) (Note 6) 7094

Note 1: Hours per day usage

You really have to be careful with hours per day. Most people don't know. Here I have also guessed a bit.

Note 2: Refrigeration

Refrigeration is a complex issue, so much so that I am writing a page on it to include on this web site. How many hours per day a refrigerator runs is determined by lots of variables like:

  • Where is it located (hot or cold)
  • How often is the door opened
  • How old it is
  • Its design
  • Its size
  • Its contents

Note 3: Rechargeable Devices

These devices are becoming so commonplace that some houses have dedicated recharging areas. You need to look at your habits. leaving a charger plugged in to the wall will consume power even if the device is not attached. Some items are permanently left plugged in and on. Rechargeable devices can consume a surprising amount of power. This is of course not that noticeable if you like "connected to the grid". On the other hand if you are depending on a stand alone solar power system, rechargeable devices can account for a considerable amount of power.

Note 4: Bedside Clock

Here, a low power using device that runs constantly. I gave up on mine, far better to use a watch, aa battery powered device or a travel clock. A bedside clock going 24 hours per day can be a bit of power use that can be eliminated.

Note 5: Power Tools

It is hard to calculate here what the use is. A circular saw used once per day to cut one piece of wood is not a lot of power. A wood planer going for long periods is. If you use power tools it may be best to calculate the weekly use and divide by 7 for the daily use, or even calculate the monthly use and divide by 30.

Note 6: Factored Daily Load

Here a 0.7 factor has been used. With a large solar design it is far better to factor in individual inefficiencies. These are panel inefficiency, battery inefficiency, inverter inefficiency, wiring inefficiency and power factor of appliances. This method is far more accurate but would call for a huge web site. If this interests you see below! You will note that I haven't included the battery sizing in chart 2. Here, I would recommend a 48 volt battery with all appliances running off a quality inverter. The actual capacity of the battery bank and the size of the inverter is again subject to the variables listed and involve a more complex bunch of calculations.

A Better Method of Calculation

If you are serious about designing and calculating a stand alone power system, the correct calculations are specified in Australian Standard AS4509. This standard is very comprehensive and I cover AS4509 in detail in my book "Renewable Energy" including step by step instructions to help you understand the calculations required.