Energy from roof top solar systems

May/June 2011

How much energy do you get from a roof top solar system?

by Ray Prowse (convenor Inner North Group)

The number of roof top solar systems is ever increasing. Many people have the expectation that they are going to get sufficient free electricity from the sun to run their house.

The amount of electricity needed to run a whole house depends on what the electricity is used for, and the efficiency of the electrical items in the house.

The typical electricity consumption of an all-electric home in Canberra is in the vicinity of 20 kilowatt hours (kWh) a day. This means that to offset all the electricity used, the roof top solar system would need to produce, on average, 20 kilowatt hours per day.

However offsetting the electricity requirement of the house and offsetting the cost of electricity are two different things, brought about by the difference between what we pay for the electricity we use and what we are paid for the electricity that our solar system produces.

For example if we pay 18 cents per kilowatt hour (the tariff) for the electricity that we buy and we are paid 50 cents per kilowatt hour for the electricity that our solar system produces, which is fed back to the electricity grid (feed-in tariff), then we only need to produce approximately one third of the amount of electricity that our house uses to break even financially.

You may want to sell more electricity to cover the costs of purchasing and installing the solar system. Just how quickly you want to pay the system off and how much income you want to make from the system in the future will determine the size of the system you need to buy.

The output of most solar roof top systems will be less than the electricity consumed in the house. However, even if you don’t break even in terms of energy and costs of the system, you will still reduce your electricity bill by a significant amount.

What are the factors which effect how much electricity your system will produce?

The most significant issue is the size of the system. The larger the system the more electricity it will produce.

The next most significant issue is the amount of sunshine the system receives. This depends on:
1. The geographic location of your system, i.e. Canberra is approximately 35 degrees south of the equator. Generally, the closer to the equator the more sun you will receive. Local factors like clouds and shade from trees and hills will also reduce the amount of sun on your system.

2. The orientation of the solar panels. For maximum output they should face solar north (true north). Note that this is different to magnetic north. In Canberra magnetic north is currently approximately 11 degrees east of true north. The easiest way to find north is to look at the grid lines in the street directory. They run north–south and east–west.

3. The inclination (tilt) of the solar panels. The panels can lay horizontal, vertical or any angle in between. For maximum annual output, without having to change the tilt of the panels for each season, the best angle is the latitude angle of the location, i.e. in Canberra they should be tilted at 35 degrees to the horizontal. Most roof pitches are less than 35 degrees and, because the panels are installed flat on the roof in most cases the tilt angle is not ideal. This is a compromise most people tolerate because the extra cost required to install a frame to tilt the panels to 35 degrees is not readily offset by the extra output you would get if the tilt was ideal.

These three factors can be represented by a quantity known as peak sun hours or PSH. Peak sun hours are the number of hours per day when solar irradiance averages 1 kilowatt/m². For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1 kilowatt/m².

The number of peak sun hours will depend on the location of your system, and the orientation and tilt of the panels. They will also vary according to your location as well as the season, time of day and weather conditions. Solar installers will be able to tell you the number of peak sun hours for your location, orientation and tilt of the solar panels.

The following chart shows the variation in peak sun hours for Canberra for an orientation of 0 degrees (solar north) and a tilt angle of 18 degrees (typical of many roofs).

Note how the peak sun hours vary from a maximum of 7.53 in January to a minimum of 2.63 in July. The annual average is 4.76.

Estimating the electrical output from a system

The electricity produced by a system is measured in kilowatt hours (kWh) and can be worked out by multiplying the power rating of the system by the number of peak sun hours, i.e. energy = power x peak sun hours

For example

System size:  2 kilowatts
Location:  Canberra, facing north at a tilt angle of 18 degrees, peak sun hours = 4.76
Annual average energy = 2 x 4.76  =  9.52 kilowatt hours per day
In January, energy = 2 x 7.53  =  15.06 kilowatt hours per day
In July, energy = 2 x 2.62  =  5.24 kilowatt hours per day

From these estimations it looks like you would expect 9.52 kilowatt hours per day on average for a 2 kilowatt system. However, the reality is that what is exported to the electricity grid will be less than this. Remember that the solar system comprises an array of solar panels on the roof, connected via electrical cables to an inverter, which in turn is connected to the meter box and ultimately the electricity network. 

Energy is lost in the cables because they are not perfect conductors and they heat up. The thinner the cables the more energy is lost as heat. Energy is also lost in the inverter, which transforms direct current (DC) electricity from the panels into alternating current (AC) electricity in the same form as the electrical mains.

Between the cable losses and the inverter losses there can be as much as 10–15 % of the energy lost.

Rule of thumb

A conservative estimate of the electricity exported from your system to the electricity grid is the power multiplied by 4 peak sun hours. You may do better but efficiency losses will keep your output less than the theoretical maximum.

Of course, some days will be better and some days will be worse and summer will be better than winter. But, you can also get a greater output from your system on a sunny day in winter compared with a cloudy day in summer!

Income from the sale of electricity

This depends on your feed-in tariff. The following table shows estimates of the average daily income from different sized systems for two different feed-in tariffs, based on 4 peak sun hours per day.

In spite of these estimates, for both energy produced and income from the sale of electricity, you will still get days when the output is very low (close to zero) and you will get days when the output is very high.

How to tell if the system is not working

Because of the variability of the sun from day to day it will be hard to determine if you are getting the output that should be expected. You will need to keep records for a long time to find out if it is performing as expected. However, after the system is first installed, the installer will check your system to make sure that it is working and producing the optimum output.

If you want to satisfy yourself that every panel in the system is contributing to the output then try the following. You will need two people, one to keep an eye on the reading on the display of the inverter and the other on the roof with a blanket. Cover each panel one by one and have the person on the ground note the reading on the display. Covering a panel should reduce the output of the system so if there is no change to the output, then that panel isn’t contributing to the total output of the system.

Because of the way the panels are connected together in series and parallel strings, if there is a fault, you might find that by covering several panels there is no change to the output of the system. This does not necessarily mean that there are several faults. A fault in one panel (or cable, or fuse box) will most likely affect the output of others as well. So, if the reading on the inverter display doesn’t change, contact the installer and let them know what you have found.

Next Newsletter

In the next newsletter I will look at the differences between mono-crystalline, poly-crystalline and thin-film solar panels and what to look for when making the decision to choose one or the other.

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