I am debating the effort to save on electricity costs with my electric water heater. Solar water panel heater, hybrid water heater, etc. Before I go down that path, I want to track the cost of the existing setup by measuring the amount of electricity used by the tank over the next several months. How can I do that? I read I can have a sub meter/panel I, but then what?

I have a 70 gallon electric hot water heater, 5 years old. The sticker says avg annual cost is about $550. The heater sits in an enclosed closet adjacent to the outside of the house, and it's shaded. Being in Arizona, closet temp is about 100F or higher in the summer and about 75F in the winter.

I have a hot water loop and circulator that runs 7am to 1am.

1 Answer 1


If you can afford the metering equipment then purchase and install that. Gives you the direct total energy use...

If not then you have to consider a different direction...

So, go the easy route: measure the hot water use over a week: measure shower times and take a suitable average then with the flow rate calculate how much hot water was necessary (gallons or litres... up to you - convert or use which you like). Measure all activities that use hot water and multiply by how many times per week.

Now you have the total hot water used and so you can use Q = MCp (T2 - T1) and get the total energy which leads you to the cost if you know how much you pay per unit. You can take Cp to be 4.18 kJ/kg K for the accuracy you will be working with.

Also include an efficiency figure - the tank is not perfectly insulated etc...

The average hot water use is around 40 to 60 L/person/day at 60 deg C - a European rule of thumb - may well be different for other locations some places have larger bath tubs than others... Or take longer showers...

  • 1
    USA average usage is closer to 100 gal per person per day, based on Virginia location Mar 30, 2019 at 17:40
  • @PhilFreedenberg Thanks - that helps put results into the correct ball-park.
    – Solar Mike
    Mar 30, 2019 at 17:42

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