How do you allocate roof space for both a solar PV system and a solar hot water system? True solar enthusiasts occasionally run into this type of dilemma. It's most common to see one or the other on an individual roof, but the space requirements for these complementary technologies add another dimension to choosing between photovoltaics (PV) and solar hot water, or going with both.
For PV systems -- the sort that generate electricity -- a generic rule of thumb is that you need 100 square feet (visualize 10 feet by 10 feet) for each kilowatt (kW) of solar PV panels you install. This means that to install a 5-kW system -- which is roughly the current national average -- you'd need approximately 500 feet of usable roof space.
When it comes to installing a solar hot water system, the space requirements vary more by region. Location matters for hot water systems more because outdoor temperatures determine how much the water must be heated.
These size estimates are for a family of four, assuming that each person requires 20 gallons of hot water per day. The hot water system is designed to meet 100 percent of summer hot water demand and 40 percent of winter demand. Meeting these specific levels of seasonal demands will avoid wasting energy and incurring excess cost. For more information on this sizing process, refer to this explanation provided by the USDA.
For both solar PV and solar hot water systems, south facing roofs will allow the system to be most efficient, and shading from nearby trees can prove problematic even for those buildings with an ideal roof orientation.
It is certainly possible for an average home to have ample roof space for both systems. Assume a 2,400 square-foot home (40 ft x 60 ft) with a roof pitch of 30 degrees. The half of the roof that faces south would have dimensions of approximately 23 ft x 60 ft, giving you about 1,380 square feet on which to install solar PV and/or a hot water system.
The fight for roof space definitely arises for some buildings nonetheless. Deciding how to allocate that precious south-facing roof area comes down to how much hot water you use and how much you're willing to spend. Like so many issues related to solar installation, solar roof space is site specific.