Solar for water heating

Solar thermal collectors are flat plates or evacuated tubes that absorb sunlight and transfer heat to a fluid, which is pumped through a heat exchanger into a storage tank. On a sunny day they can supply most of a household’s hot water; on a cloudy stretch a backup electric or gas element takes over. The equipment is a plumbing and roofing job: roof-mounted collectors, insulated lines through the attic, a pump, an expansion tank, controls, and a large tank in the utility room.

A heat-pump water heater looks like an ordinary tank with a small compressor on top. It pulls heat from the surrounding air and moves it into the water, using roughly a third of the electricity of a resistance element. Powered by rooftop PV, it heats water with sunshine indirectly, and the panels that feed it are ordinary panels that also feed everything else. There is nothing water-related on the roof.

Colorado’s hard freezes shape thermal design. Open-loop systems that run potable water through the collectors are out; the standard here is a closed loop with a glycol antifreeze mix, or a drainback system that empties the collectors whenever the pump stops. Both work, both add complexity — a glycol loop needs periodic fluid testing and replacement, a drainback system needs correctly sloped lines and a reservoir — and both have more moving parts and more places to leak than a PV array, which has none.

Roof penetrations for insulated water lines, collector weight, and hail exposure on glass-covered collectors are all things a thermal contractor handles routinely, but they are not things a solar electrician should touch. If you want thermal, hire a plumbing contractor who installs and services it. We are glad to install PV alongside a thermal system for the rest of the house; we just do not put water on the roof.

A conventional electric resistance water heater for a family of three or four uses roughly 3,000–5,000 kWh a year, depending on hot-water habits and incoming water temperature — and Colorado groundwater runs cold, which pushes toward the high end. A heat-pump water heater serving the same household uses roughly 1,000–1,800 kWh, because the compressor moves two to three units of heat for each unit of electricity. The gap, 2,000–3,000 kWh a year, is bigger than most people’s total lighting load.

That gap is why we push the heat-pump unit before the panels. Covering a resistance heater with solar takes six to ten panels; covering a heat-pump heater takes one to three. If you heat water with gas today, switching to a heat-pump water heater adds only 1,000–1,800 kWh to the electric side, which is a small solar increment for getting one more appliance off gas. The go-all-electric reason page covers the sequencing for the whole house.

At Colorado’s roughly 1,300–1,600 kWh per installed kilowatt per year, a heat-pump water heater’s 1,000–1,800 kWh is 0.7–1.4 kW of panels: two or three at 400–440 W, one if your hot-water use is light. That is about 40–60 square feet of roof and roughly $2,000–$4,500 at our $2.80–$3.25 per watt pricing, folded into the larger system. Water heating is a steady year-round load, so unlike AC or a pool it depends on winter production and net-metering carryover; in co-op territory with annual true-ups, size a bit conservatively.

Placement matters more than people expect. A heat-pump water heater cools and dehumidifies the room it sits in and needs a decent volume of air around it; a Colorado basement utility room usually works well, a small closet does not, and a garage that drops below freezing can push the unit onto its backup resistance element in winter. It also produces condensate that needs a drain. A plumber handles all of that; we handle the panels and, if needed, the electrical capacity — a 240-volt, 30-amp circuit is typical.

A tank of hot water is a thermal battery. Under Xcel’s residential time-of-use plan, weekday power from 5 to 9 pm runs about 2.7 times the off-peak rate, and most heat-pump water heaters can be scheduled: heat the tank to a higher set point in the early afternoon on your own production, then hold through the evening without running the compressor. Many units offer a mode that does exactly this with a couple of taps in the app. Morning showers pull from the tank; the unit reheats mid-morning as the panels come up.

This is the cheapest peak-shifting most solar homes can do, and it needs no battery. A resistance heater on a timer can do a cruder version. If you already own a home battery, the water heater is one of the loads it covers in an outage — a heat-pump unit draws only 400–600 W in heat-pump mode, so hot showers through a shutoff are realistic, which is not something a gas tankless with electronic ignition can promise.