More on Power Usage

In discovering how much energy is used in your house (or premises), most energy bills will show the average daily usage per billing period:

This is the easiest way to determine your energy usage in order to make a decision about what size Solar Power system you may want to choose.


It is still worthwhile either making calculations or purchasing an energy (power) meter as previously described.  Either method will allow you to audit your energy usage.  Why audit your energy usage?  It is an effective way to find out the power usage of each appliance.  You may then identify which appliances use the least and the most power and at what time of the day.
This will then allow you to change your habits of usage, or identify which appliances could be replaced with energy efficient ones.  It changes one's mindset to be energy concious once you spend time examining these things.


Go to my Scribd page to find the spreadsheet that will help with the calculation method.  Often the power consumption is described in different ways, Watts (W), kiloWatts (kW), Amps (A) or kiloWatt hours (kWh).  Any of these can be entered into the table and will be automatically converted by the spreadsheet to kWh.

Some appliances are not used everyday, so depending on the power rating and how often you use it, you may decide to include (or not include) that item.  For example an electric can opener uses 40W and if operated for 5 minutes each week equates to .003 kWh per week.  This turns out to be a much smaller average per day and insignificant to add into your calculation.  Compare this to an electric oven that is 2kW, used for 5 hours per week equates to about 1.4kW per day.


Step by Step guide to Calculating your kWh usage per day:
1. List every electrical item that connects to an electric outlet
2. List every other electrical item not connected to an electric outlet, e.g. electric oven, air-conditioner, electric hot water service.
3. List all of the lights
4. Find out each item's consumption
5. List the times of usage of all items and then the total time used per day.  If not used daily then make an average daily use e.g. 2 hours per week is rounded to 0.3 hours per week.
6. Enter all data in the spreadsheet. You can copy down extra rows if if needed.
7. Get a total from the sum of the last column


I will examine How To's on power meters in a coming post. Also, once you know the daily power consumption, you need to understand how much energy can be generated per day, which will also change throughout the year. This will also require a whole post to itself.

More on Location

There are a few more details to cover on the location of your Solar System before moving on.

• Non ideal roof layout solutions
• General form factor of Solar Panels
• Panel layout on a given roof area

Mounting on an ideal roof slope (North Facing) is normally achieved by fixing the panels parallel (flat) to the roof surface, but with a gap between the panels to allow air to flow underneath.  This assists the panels from getting hot.


For other roof surfaces, we can use an elevating bracket mounting system.

This illustrates an elevated system on a flat roof.  It is possible to also use the elevating system on a roof that sloping east and west.

This is less than ideal as the some of the west side panels will be partially shaded in the early morning and the same will happen on the east side late in the afternoon.  This depends on the pitch angle of the roof. 

A guide to Solar Panel Form Factor.  Solar panels are generally 1.5 to 2 times longer than than they are wide.

It is important to keep this in mind when planning a layout on the roof.  Consider panels that are L= 2 x W.  Placing two panels together with ajoining long sides will form a square.  Placing another two panels with these, ajoining on the short sides of the first two panels forms a L = 2 x W rectangle twice as large as one panel and so on.  Keeping these concepts in mind will enable you to plan how to lay the panels on your given roof area.

How many panels should you use?  This depends on how much electricity you wish to generate.  Currently the most common panels are in the range of 175W to 240W each.

If you have no idea how much electricity you want to generate then a good way to make a decision is to measure the typical consumption of your house.  You will learn how to investigate this next time.