Q&A: Understanding solar


I love the magazine and have been subscribing for a number of years. I would like to get some very basic information on solar from you. Although your articles about going off-grid are really interesting, I can’t follow the detail, and I suspect a lot of readers of my generation (70s) are in the same boat. I would eventually like to go off the grid and therefore would like you to answer a few very basic questions.

I live in the Canberra region which is really sunny, with an average yearly total of 246 sunny days. My average electric use is 19 kWh/day and I currently have a 2.025 kW PV system.

If we receive eight hours of sunshine in a day does this mean I would get 16.2 kWh? Regarding storage, what would be the optimum amount of battery storage required to go off the grid without having a diesel or petrol generator, in my area? Is there any course that would cover the sort of basic knowledge I require?
—Peter Fenton


Yes, solar can be confusing, and there is a lot of jargon around! Your solar panels are rated at a total of 2 kW which is the power they will generate at standard test conditions which include:

  • strong sunlight directly shining at the panel

  • panel temperature of 25 degrees.

These conditions don’t often occur in the real world. The sun moves around, and as panels heat up their output reduces. Then there are additional losses in cables, inverter and other components.

To estimate how many kilowatt-hours (kWh) of energy the system will generate in a day, we can’t rely on whether a day is described as sunny, or the number of hours of sunshine. A better measure is the average number of kWh a 1 kW solar system will generate over a whole year.

Check out the table at www.bit.ly/1NpXb5h.

Over a whole year, a 2 kW solar system in Canberra should average about 8.6 kWh of generation per day. Of course, generation will be higher in summer and lower in winter.

So if you wanted to offset your yearly electricity consumption, you would need to add many more panels.

As for going off-grid, what is your motivation to do this? Off-grid systems are expensive. We’ve analysed the economics previously and even with high grid tariffs, going off-grid isn’t attractive on a purely economic basis. As a rough rule of thumb, to supply an electricity usage of 19 kWh per day, a reliable off-grid system might cost between $57,000 and $95,000. We expect this to come down over time as batteries get cheaper, but on the other hand, the Aussie dollar is low!

There are many ATA members off-grid, but they generally are very frugal users of electricity, typically around 5 kWh per day. It’s difficult to go off-grid without a generator. The big issue is getting through a cloudy winter week. Space heating and water heating are big factors too.

Here’s a great course to learn about this: www.bit.ly/1RQBeeE. I wrote a review of this course in ReNew 132. Another information resource is Sunulator, including the user guide: www.ata.org.au/ata-research/sunulator.

And here’s a useful article to explain some important terms: www.bit.ly/1l6RQ83.
—Andrew Reddaway, ATA