Sizing a solar hot water heater for your household
To size a solar hot water heater, start from how much energy your household uses to heat water, then choose collector area and tank volume to cover a sensible share of it. Oversizing wastes money and overheats the system in summer, so most designs aim at the warm half of the year.
How much energy does your hot water use?
A rough figure comes from physics. Heating one litre of water by one degree Celsius takes about 1.16 watt-hours. Take a household that uses about 200 litres of hot water a day, with cold supply at 8 degrees and a tank set to 50, a rise of 42 degrees. That is 200 x 42 x 1.16, roughly 9,700 watt-hours, or about 9.7 kWh a day. Over a year that is a little over 3,500 kWh.
These numbers are illustrative assumptions. Your own use depends on the number of people, showers and laundry habits, and the incoming water temperature, which is lower in winter. A look at your gas or electricity bill and the water heater label helps you refine it. The wider cost picture is in solar hot water cost in Canada.
How big should the collector array and tank be?
Installers normally relate collector area to daily demand, then choose a tank sized so that one or two days of sun can be stored. A rule of thumb is a storage volume proportional to the collector area, though the right ratio depends on the product, and the supplier's design software should give it. Keep two points in mind.
- Summer surplus. A system sized to cover a full winter load would produce far more heat than it can use in July and could overheat. Most systems cover the summer demand and part of the winter.
- Tank sizing. A solar tank is often larger than a standard heater, because it has to hold the midday heat until evening. See hot water tank sizing.
Ask for the expected annual solar share, and ask what assumptions sit behind it.
What do orientation, tilt and shade change?
Collectors facing roughly south and tilted at about the latitude do well over a year, and a steeper tilt favours winter output. Facing east or west cuts the total, and shade at the wrong time can cut it much more. A collector that is shaded by a chimney every winter afternoon will miss part of its best low-sun hours. The geometry is in solar collector orientation for heating.
Alberta has a sunny reputation, and an overview of sunshine by city is in Alberta sunshine hours. Even in a sunny region, winter output is limited by short days, which is why a backup heater always stays in the system.
How do you pick the system type?
Once the size is set, the type follows the climate. In places with hard freezes, closed-loop systems with antifreeze or drainback are usual, as described in drainback versus glycol systems. Collector choice is covered in evacuated tube versus flat plate collectors. Compare with other ways to heat water in solar thermal versus solar PV.
Drake Landing Solar Community sized its collectors to carry space heating through a seasonal heat store. That meant 800 collectors for 52 homes, far more per household than a domestic hot water system needs. The DLSC solar hot water page explains the scale difference.
Quick answers
How many collectors does a house need for hot water?
It depends on household use, collector type and local sun. An installer should provide a calculation based on your daily hot water demand.
Is a bigger solar hot water system always better?
No. A system sized for winter will have more heat than it can use in summer and may overheat, so most are sized for the warm months.
Do I still need a regular water heater?
Yes, in most Canadian homes. The solar tank preheats the water and the conventional heater covers cold and cloudy spells.