Hot water facts

Energy guzzler. Water heating is the second largest energy user in the average home. Only central air conditioning uses more energy.

History. Miami had 60,000 solar water heaters during the 1940s. What happened? Really cheap electricity and utility company “All-Electric Living” promotions. Some utility companies gave away electric water heaters, just to get homeowners to switch from natural gas.

Carbon bigfoot. Everyone is concerned about their carbon “footprint” these days. Well, a home water heater’s carbon footprint is a whopper.

How big, you ask? Using electricity from an electric utility powerplant to heat 80 gallons of water from 72°F to 140°F each day for one year sends almost four tons of carbon dioxide^[1] emissions into Earth’s atmosphere. Yes, really.

How to find your
current electric rate

How many cents per kilowatt-hour are you currently paying? Here’s how to find out:

Locate your most recent electric bill. Your utility company may break the kilowatt-hours (kWh) used into two categories: the first 1,000 kWh and the kWh used above 1,000. If this is the case and your power usage was greater than 1,000 kWh, add the second kWh figure to 1,000 to find your total kWh for the month.

Next, divide the total kWh used into the total amount due for the current billing period; i.e., total including base electric rate, fuel surcharge, utility franchise fees, and taxes. Do not include any past due balances or late fees.

For example, if you used a total of 1,685 kWh and the total amount due for your most recent billing period is $244.33, your current cost for electricity is $244.33 / 1,685 = 14.5 cents per kilowatt-hour.

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Why solar water heating?

Floridians see their electric bills go through the roof during July, August and September and don’t need to be told that air conditioning uses a lot of electricity. On the other hand, while you may use hardly any air conditioning during March, April and November, that big 3,500 to 4,500-watt heating element in your water heater is burning away every day of the year. (Some high-recovery models have two heating elements.) At today’s electric rates (including fuel surcharges and utility taxes) of around 14 to 15 cents per kilowatt hour, water heating can easily cost $800 to $1,200 each year for a three to five person household. In South Florida, a solar water heater can almost completely eliminate this portion of your electric bill.

Solar water heaters offer many extra benefits.

Naturally, switching to solar water heating means going green in a big way. but you might also be surprised to learn that solar water heating gives you benefits that improve upon conventional electric or gas water heaters:

More hot water. Most electric water heaters hold enough hot water for about 20 minutes of continuous use. Then the electric element reheats another tankful. You can run out of hot water if two people are showering at the same time, or if you take a shower after the clothes washer or dishwasher have been running. Solar storage tanks are sized to store an entire 24 hour’s worth of hot water because there is no sunshine at night. But this gives you the added benefit that you are less likely to run out of hot water when there are multiple uses at the same time.
Hotter water. The water delivered by a solar collector panel can be hotter than the thermostat setting on your electric water heater. This means you will have hotter water for the dishwasher and for showers if you you want it. (A mixing valve on the storage tank protects you against scalding.)
Plenty of hot water during emergencies. With a passive solar water heater, or a system with a solar-powered circulating pump, you will have your customary supply of hot water even during electric power outages. This is an important feature even if you have a standby power generator, because standby generators are not typically sized to handle the power consumption (4,500 to 9,000 watts) of a conventional electric water heater.

Pie chart of typical annual Florida residential electric bill

Which home energy uses are the most expensive?

The pie chart above shows where the money goes in a typical Florida family’s annual electric bill. The chart is based upon an electric power consumption study by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. The “typical” Florida household and energy use pattern shown above is based upon these factors:

Florida home built in 1990 with 2,400 square feet of living area
9.5 SEER air conditioner with thermostat set at 81°F during day and 78°F at night
cold weather thermostat setting of 64°F during day and 68°F at night
two adults and two children in household
80 gallons of hot water per day
14 cents per kilowatt-hour electric cost (including fuel surcharge and utility taxes)

Is your hot water use average?

Actually, an active American family of four is probably closer to 100 gallons per average day and just over $1,000 per year at 14 cents per kilowatt-hour. Here is an example that shows how a four-person household can easily use 100 gallons of hot water per day:

three people take an eight-minute shower every day; the fourth showers 12 minutes for a longer shampoo and rinse: (4 showers x 2 gallons per minute x 8 minutes x 80% hot water) + (1 shower x 2 gallons per minute x 4 extra minutes x 80% hot water) = 57 gallons per day
five loads of laundry per week; for example, one load each of whites, darks, towels, sheets, and blue jeans: 5 clotheswasher loads x 32 gallons per load / 7 days = 23 gallons per average day
At least four face and hand washings x 1 gallon per minute x 4 minutes each x 80% hot water = 13 gallons per day
few sit-down meals because everyone is constantly on the run, so the automatic dishwasher is only run twice per week: 2 automatic dishwasher loads x 14 gallons per load / 7 days = 4 gallons per average day
One shave x 1 gallon per minute x 2 minutes = 2 gallons per day
Hand-rinse dishes once a day x 1 gallon per minute = 1 gallon per day

If you happen to have small children, each bath averages about three gallons less hot water than a shower. However, if you happen to have two active teenagers, each taking two showers a day and perhaps creating two or three extra loads of laundry per week, the total can easily jump to over 120 gallons per day. Heating 120 gallons per day from 72°F to 140°F, at 14 cents per kilowatt-hour, costs about $100 per month.

System performance

Because incoming solar radiation—called insolation—is often interrupted by cloudy or rainy weather and fluctuates in intensity with seasonal changes in the sun’s position in the sky, solar water heating systems are usually sized to collect and store an entire 24 hours or more worth of energy to meet hot water demand. A modern solar water heating system storage tank will usually have a backup electric heating element to meet unusual peaks in hot water use or extended periods of very cloudy or rainy weather.

While solar thermal water heating systems are viewed as “pre-heating” systems in many regions of North America, the sunbelt states enjoy solar insolation levels that allow solar potable water heating systems to economically meet 85 to 90 percent of the annual hot water need.

References and Notes

According to the U.S. Energy Information Administration, the average U.S. electric utility powerplant sends about 1.36 pounds of carbon dioxide emissions into the atmosphere for every kilowatt-hour (kWh) of electricity delivered to an end-use. If an average family of four uses 80 gallons of hot water per day, this requires 5,581 kWh per year, assuming a temperature rise of 68 degrees (from 72°F to 140°F) and a 15 percent added energy factor for maintaining the temperature of stored hot water. 5,581 kWh x 1.36 pounds of carbon dioxide per kWh = 7,590 pounds. 7,590 pounds / 2,000 pounds per ton = 3.8 tons of atmospheric carbon dioxide emissions per year.