Heat or Power: Choosing Between Two Kinds of Solar

What is the difference between solar thermal and solar PV?

A thermal collector absorbs sunlight on a dark plate or tube and passes that heat to a fluid, usually water mixed with antifreeze. The fluid carries the heat to a tank or a heating loop. There is nothing electronic in the collector itself.

A PV panel uses semiconductor cells that release an electric current when light hits them. The current goes through an inverter and into the house wiring. Both are called solar, and both can sit on the same roof, but they solve different problems.

Which is more efficient?

Per square metre of roof, a good thermal collector captures a considerably bigger fraction of incoming sunlight as useful energy than a PV panel does, because heat is an easier product to make than electricity. That advantage shrinks when the water you want is very hot, since losses rise with the gap between collector and outdoor temperature. It also depends on whether the heat has somewhere to go. A collector delivering heat to a full tank produces nothing useful.

Efficiency is therefore the wrong single test. Ask what the energy is worth. A kilowatt-hour of heat replaces gas or electric resistance heating. A kilowatt-hour of electricity can run a heat pump, which turns it into several kilowatt-hours of heat. For many homes that second route closes the gap.

A worked comparison helps. Suppose 10 square metres of roof. Thermal might deliver useful heat at several times the energy per square metre that PV delivers as electricity, but only while the tank can accept it. Cover that same roof in PV and the electricity could drive a heat pump at a seasonal efficiency of 2.5 or more, so each kilowatt-hour yields two to three kilowatt-hours of heat, or it could feed lights, appliances and a vehicle. The figures here are illustrative assumptions, not measurements, and an installer should model both options for your address.

Which suits a Canadian home better?

The answer turns on what you buy most of. A household with large hot-water use and a plain gas or electric tank may see a good fit with thermal. A household with high electricity use, an electric vehicle or a heat pump usually gets more flexibility from PV. PV also needs no plumbing, no freeze protection and almost no moving parts, which keeps maintenance simple. The explanation of how solar water heaters work shows what a thermal system adds in the way of pumps and fluid.

Net metering and micro-generation rules decide how much a PV surplus is worth, and they differ by province. Cost-wise, see the hedged ranges in the solar hot water cost guide and compare them with a PV quote for the same roof.

What about the seasonal mismatch?

Both technologies produce most in summer, yet Canadian homes need the most heat in winter. Thermal systems handle that badly unless there is storage that can hold summer heat for months, and a household tank cannot. PV sidesteps the problem by exporting surplus power and drawing it back later, if the utility allows it.

The guide to solar space heating covers why this mismatch limits what rooftop thermal can do in January.

What did Drake Landing do about it?

The Drake Landing Solar Community in Okotoks used thermal, not PV. 800 flat-plate collectors on garage roofs collected heat all summer and sent it into 144 boreholes about 35 m deep, where the ground held it until winter. By 2012 monitoring reported a 97 percent solar share of space heating, and 100 percent in 2015-16. That works because the project solved the storage problem at neighbourhood scale. The borehole storage page describes the field, and community solar heating versus rooftop solar compares the approach with individual roofs.

Quick answers