Power Usage vs. Power Generation

You may have taken the time to look at your energy bill, do some calculations of your usage, but how does all that relate a kW rating given for a system?  We have all seen the ads on TV "buy a 1.5kW system...buy a 3kW system..." etc. etc.

So now for some information on how much electricity you can generate for a 1.5kW and 3 kW system, and then from these examples examine the reduction in grid energy usage these will achieve.

This will vary upon how close you live to the equator and the (average or mean) number of sunny days per year your region receives.  So this depends upon weather conditions and is therefore dependent on an average prediction.  There is a very good website to help with solar power prediction and in fact will give you comprehensive information. http://www.nrel.gov/rredc/pvwatts/grid.html

Experiment with this website if you feel ambitious.

I will use Melbourne as the example location.  Melbourne probably has some of the lousiest weather in Australia, so therefore less sunny days for generation.  Almost anywhere else in this country, the result will be higher.  I live there so I know the syndrome of "four seasons in one day".
Here is the results for a 3kW system:

And the results for a 1.5kW system:

The results give total predicted kWh generated per month. 
If we go back and examine the chart of average daily usage, our consumption for Dec, Jan, Feb was approximately 13kWh per day (peak reading only). Multiplied by 30 days is 390kWh per month, so a 3kW system would provide about the same or more energy than we consume over the Summer months.  Winter is a little more challenging at about 11 kWh per day, 330kWh per month where solar generation would drop to 140kWh for the month of June.  Having said that, average usage in Spring per month is 300kWh so a 3kW solar power system would generate more than we would consume, hence no power bill for that quarter and a return on grid feed in.

So check you bill.  What is your average usage and how much can you cut your bill by installing a solar system? Compare your usage to the 1.5kW and 3kW calculations above.  I would love to hear some feedback, leave a comment below and please share this if you think its worthy.

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.

Size and Power

The most common size Solar Systems commercially sold are around 1.5 or 3 kW.

If you don't know what size you want or need then its best to figure out the power consumption of the house as a basis.  You can then decide to go higher or lower, depending on whether you want to generate and excess to put back into the grid, break even or just save on your electricity bill. The power consumption can be figured out two ways:

• By Calculation or,
• By Measurement

Measurement is the easiest method but means you need to connect a metering device to the supply wires, which normally needs to be done by an electrician.  It's reasonable to make an investment in a power meter at this stage because you will make use of it once you have your solar array and system in place.  It may be possible to get an idea of daily consumption by reading your meter daily, at the same time each day for some days - maybe a week.  This will give you the consumption for each day, but will not tell you the times during that 24 hour period when the electricity is being used the most, or the least.  A power meter designed especially will give you lots of information and log the power usage data too.

Using calculation is tedious because you have to figure out the power consumption of every electrical device in the house. This can be done by reading the information from specification plates on appliances or from the appliance's specifications in the user manual. Some things don't have such specification, for example lights, but you can read it from the light bulb.

ASKO washing Machine Specification Plate

I shall publish a spreadsheet to help with calculation, I haven't yet figured a format I am happy with. I will also go deeper into power measurement.