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May 19, 2018


Before sizing a domestic hot water heater, regardless of energy source (whether electric, gas, solar, oil, etc...) you need to know your annual and monthly domestic hot water (DHW) demand in KWh or BTU.

DHW demand depends on many tangible and intangible factors. Tangible factors (such as Domestic Cold Water inlet temperature, flow, DHW outlet temperature, appliances and sanitary fixtures demand, etc....) can be easily predicted using historical data. The difficulty in DWH annual or/and monthly demand derives from the intangible factors such as human habits and home owners demographics.

Appliances usage frequencies such as dishwasher, washer, dryer, coffee machine, cooking habits are not easily predictable. Home owners demographics are a bit easier to predict: a family of four with babies don't have the same showering frequencies as a family of 4 with 2 adolescents and the list goes on.

 Rules Of Thumb For Sizing Your Solar Domestic Hot Water Heater


Assumptions and Results

Taking into account all or most of the above mentioned tangible and intangible factor goes beyond the scope of this blog (because it simply leaves us with an infinite amount of scenarios).There are many available tools (such as simulation software or even Microsoft Excel) in the market that can do that.

For the purpose of this article, we have made the following assumptions which we believe are applicable for a big portion of the southern part of Canada:

  • Monthly Domestic Cold Water Inlet Temperature (˚C): January-4.54˚C, February-5.25˚C, March-6.67˚C, April-8.8˚C, May-11.95˚C, Jun-13.32˚C, July-14.7˚C, August-15.38˚C, September-12.64˚C, October-9.9˚C, November-7.73˚C and December-5.6˚C.
  • Domestic Hot water Supply Temperature = 60˚C.
  • Average DHW Daily Consumption (GPD) = 12 GPD (Gallon per day).
  • Home with one washing machine having a 6 GPD consumption and one dischwasher having a 7 GPD consumption.

Putting all the above into our simulation software and running the same simulation for a home of three, four and five persons give us the following results:

Number of Person Estimated Average Annual Domestic Hot Water Demand (KWh)
3 4000
4 5000
5 6000

Table 1

If you think the assumptions related to the above tabled results don't apply to your case, don't panick. DHW consumption is hidden in your utility rate energy bill. 

Rules Of Thumb For Sizing Your Solar Domestic Hot Water Heater

In typical average Canadian Home, where annual energy consumption is 200KWh by Square Meter of home area, Annual DHW consumption is 20% of Total Home consumption in KWh (as shown in the above Pie Chart).

If you have electricity and Gas Meter (or any other energy source such as propane, Oil etc..), the above 20% shall apply to the sum of all annual utilities bills.

What Percentage (100%, Less or More) of my Annual DHW demand shall my Solar Water heater cover?

The histogram below shows a typical three persons home domestic hot water demand (in orange) and Solar Water Heater energy output (in blue).

 Rules Of Thumb For Sizing Your Solar Domestic Hot Water Heater

As you can see from the above histogram, Solar Water Heater can't deliver 100% of your DHW in winter. We don't recommend over sizing your Solar Hot Water Heater for it to cover the full DHW winter demand because it leaves you with an excess of DHW in the summer that you'll be forced to waste.

As a rule of thumb solar water heater for domestic purpose only shall be sized for 60 to 70% of annual DHW demand.

The Rule of Thumb

  1. Find your Home Annual DHW demand in KWh from table 1 based on the number of occupants.
  2. Get your December Utilities Bills. Multiply by 0.2 (20%) the sum of your annual consumption (Electricity, Gas, Oil, etc...).
  3. Multiply by 2/3 (67%) the maximum of the above calculated numbers. This should be the output of your Solar Water Heater.
  4. Selecting the right Water Heating kit to match the above depends on your roof configuration (type, inclination, orientation toward the south, etc...), your location (city). Our Technical Article TA-02: How much energy can your hydronic solar system generate??will guide through the final step of solar water heater selection.

We offer free technical assistance for our customers. If you're having difficulty with sizing your water heater (whether for DHW heating or space heating), feel free to contact us on info@hydrosolar.ca or at our Canada Wide Toll Free Number: +1 (888) 686 7652.


N.B: The assumptions taken in our technical documents are of general terms. It's the installer and the designer's responsibilities to make sure that the design and the installation are suitable for the operating conditions (such as shading, snow accumulation, extreme wind velocities, etc…) of the purchased products.


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