# Sizing the Solar System (PV Array)

One of the most difficult parts of deciding to go off-grid has been the sizing of the solar system we will be using.  This is a pretty time consuming and sometimes difficult question to answer but in general what you are shooting to do is figure out how much power all the electrical devices in your house use, and use that as a starting point for all your calculations.

In some cases this is as simple as looking at your monthly power bill and it will generally tell you exactly how many kWh per month you are consuming.  If you are anything like us, in our city house we were using around 1000kWh/month or ~30kWh / day.  Of course this is in a large 4bdrm house with all electrical appliances, double oven, lots of computer gear running, and not a whole lot of attention paid to saving power.

Lets just go with the value of 30kWh / day for the sake of discussion here, though, we will be hoping to cut that considerably in our new house.  One of the more surprising things I’ve learned while embarking upon this house building journey is the large amount of government resources readily available on the web, free to use!  You can find all sorts of great information such as water well registrations (gives an idea how far we’ll need to drill to hit good water) but also information for renewable resources such as solar and wind.

For the DC Rating, you are basically going to play around with some values here to try to cover the 30kWh/day we talked about above.  So lets put 20kW as an array size / DC Rating and hit calculate; you’ll get back a list like the following:

 Month Solar Radiation (kWh/m2/day) AC Energy (kWh) Energy Value (dollars CAN) 1 3.05 1044 89.99 2 4.91 1499 129.21 3 5.19 1690 145.68 4 4.59 1340 115.51 5 4.58 1325 114.21 6 4.72 1270 109.47 7 4.81 1324 114.13 8 4.52 1238 106.72 9 4.35 1208 104.13 10 3.55 1082 93.27 11 2.35 685 59.05 12 2.78 927 79.91

If we examine the third column, AC Energy (kWh), we can see that for every month except November and December we exceed our target of 30kWh / day or ~1000kWh / month.  November is grim up here with an average of only 2.35 hours of sun per day – that’s not a lot of time to get a battery bank fully charged!

There’s a lot to talk about here – first off is that a 20kW array would be substantial – both physically and cost wise.  If you went for the cheapest / most readily available panels in Ontario right now, the Canadian Solar 250W 60cell panels, you’d be looking at around 80 panels and with a cost of around 76cents per watt, the array would cost around \$15,000CDN.  Consider also, that decent racking for your solar panels costs about the same as the panels themselves so you would be up to near \$30,000 for panels and mounting alone, with none of the other necessary equipment!

This also doesn’t factor into account that we ran the calculations at a fixed 60 degree array tilt to optimize our winter solar collection, but if we run it again using 45 degrees for the summer months you can see we are well in excess of our goals.  Just oversizing a system like this will cost an extremely prohibitive amount of money and will lead to a needlessly complex and large system.  We’ll talk a little more about this in another post but for now this one is getting a bit long, so I’ll leave it here.

## 3 thoughts on “Sizing the Solar System (PV Array)”

1. Ted says:

How would deal with snow cover from December to March?

1. At a steeper 60 degree “winter” angle, most of the snow will shed naturally. When it doesn’t, a walkabout with a broom should make short work of it!

2. Ted says:

Oops…how would YOU deal…….