Most domestic hot water systems I have seen in Canada are designed for year-round use. I don't think the added cost over seasonal systems is worth it.
I'm assuming a 5:12 pitch roof-mounted install of a Thermo-Dynamics G32 collector in Ottawa. An inlet water temperature half way between the cold(10C) and hot (50C) will be used. Converting the data in CBD-39 to metric, at summer solstice 1087 watts/m^2 will fall on the collector vs. 538 watts at equinox. According to Environment Canada, the June & December average temperatures are 18.4 and -6.6C. For insolation, 253 hours of bright sunshine are normal for June vs. 82 hours for January.
Summer solstice efficiency works out to 68% vs. 38% for the winter equinox. Factoring in the number of hours of bright sunshine, the panel will collect over five times as much energy in June than in December. To save more on a seasonal system, an unglazed absorber such as the AG32-P could be used instead of a glazed panel.
I've written a guide to installing an inexpensive solar hot water system to help people do it themselves.
