How much power can our system produce?
The short answer is:
During its first two years, our solar array produced an average of about 10.3 kilowatt hours per day (What’s a kilowatt hour?). For us, that works out to about half of the power we use in our house each day. The peak power our system can produce at any one moment is 2,879 watts.
You can look up data from the federal government about the amount of sun we get annually in Toronto, and how much power you can generate from it. According to their numbers, we’re a tiny bit above average.
In southern Ontario, an unshaded system that faces mostly south should produce at least three times its nameplate capacity (ie: the manufacturer’s rating for its maximum output). So a south-facing array rated for 5,000 watts should produce at least 15,000 watt hours of electricity per day, on average, if it is unshaded for the whole day and at a good angle.
Snow in the winter is also a noticeable factor. If you can clear your panels of snow, you’ll get a small bump in your annual numbers. It’s not a major issue though; we can’t clear snow from our panels, and we still get good production for the year (January 2011 was really lousy though!).
The long answer is:
Our array has 18 solar panels, and each one is rated by the manufacturer as outputting 175 watts. That means, the theoretical maximum power that our array can produce at any one moment is 3,150 watts (18 x 175).
However, in practice, even on a sunny day, the peak output is usually between 2,600 and 3,000 watts. On a cloudy day, it could be less than 2,000 watts, and on a very cloudy or rainy day it would be less than 1,000 watts.
The power output starts out small, after sunrise, and peaks around the middle of the day when the sun is highest in the sky. You can see a textbook example of this on the power generation graph for April 28, 2010.
There are many factors which affect the actual output of a solar power system, most importantly its orientation towards the sun. Our array faces about 160° south, on approximately a 30° angle, and gets no shade during the day. This excellent orientation is the main reason our array produces slightly more power than average for Toronto, even though we can’t clear off the snow in the winter.
Our inverter will accept a maximum input of about 3,030 watts. It then converts that from direct current (DC) electricity into alternating current (AC) electricity. It usually does that conversion with around 95% efficiency. The maximum recorded AC power we have ever got from the inverter is 2,879 watts.
When you measure electricity, you measure the amount of watts that are used or generated in an hour. If you generate 1,000 watts for two hours straight, that’s “2,000 watt hours.” That’s how the electric company determines how much they’re going to bill you. It’s the same for measuring the power production of our solar array.
On a sunny day, our system will produce over 20,000 watt hours. On a cloudy day it can vary a lot between 10,000 and 20,000 watt hours. On a very overcast or rainy day, it will produce less than 10,000 watt hours, sometimes less than 2,000, especially in the fall and winter when the days are shorter.
On average, it produces 10,345 watt hours each day. In our house, we use an average of 21,110 watt hours of electricity each day, so the solar array provides nearly half the electricity we need.
However, our peak usage does not correspond with the peak output of the array. Our peak usage is usually around dinner time, and for much of the year, the sun is setting or has set by then. So when we say the array provides half our electricity, that is simply a way of comparing what it outputs to what we use. It puts the overall amount of electricity into a real context. But at any one moment, the array may be producing more electricity than we are using in the entire house, and at other times, it produces no electricity (like after the sun sets).
You can see this fluctuation on the Historical graphs and logs page. We have graphs that show the balance of grid power versus solar power for each day, plus how much solar power is exported from our house to the grid. You can click on specific days to see how the balance changes over time.
We have a page on the site that explains watt hours in more detail, if you’re interested.