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When Using Electricity To Heat Water
The system designer may want to modify the preceding heater recovery and storage tank capacity
information when using electricity to heat water.
This is because electricity for commercial use, including water heating, is often sold on a demand
rate basis. This means, in addition to the energy charge (measured in kWh), there is a charge for
the demand (measured in kW) that a customer imposes upon the electrical service. Your power
company will provide and explain rate information upon request.
kWh= ENERGY USED
(HEATS WATER, COSTS PENNIES)
kW= DEMAND
(DOESN’T HEAT WATER, COSTS DOLLARS)
The presence of a demand rate means the system designer should minimize recovery (heater kw
rating) and maximize storage capacity (heater tank size.) Demand charges can greatly increase
the cost of using electricity to heat water.
Another approach to minimize electric demand is to provide enough hot water storage to allow the
elements to be turned off during periods of peak
electrical usage. This may be done with a
locally obtained time clock or through demand limiting equipment supplied by State or others in
the energy control business. Working with the customer, power company, heater supplier and
electrician can often result in significant power cost savings by providing control over the electrical
demand.
Estimating Water Heating Costs
Occasionally the system designer may want to project energy expense and make fuel cost
comparisons as a part of the system design project.
If so, use this formula and the example as a guide.
Cost = (Gallons per time period) x (8.25) x (temp. rise) x (cost of fuel per sale unit
(Btu content of fuel per sale unit) x (Heater efficiency)
Cost example of heating 50 gallons of water with electricity:
Cost =
(50)x(8.25)x(100)x(.08) Notes:
(3413) x (1)
Cost = 2062.5 8.25 - Weight of gallon of water
3413
8.00¢ per kwh assumed
Cost = 96 cents based on 100% efficiency, plus 1 kW = 3413 Btu/h
demand and fuel adjustment charges
if applicable. Efficiency = 1 (100%)