Save on Electric Hot Water Heating with Time-of-Use (TOU) billing

To save on your electric power bill when it’s billed by “Time-of-Use” (TOU); you need to shift some of your electrical consumption from expensive daytime to cheaper nighttime – when the hourly rates are much lower.  You can do this with your existing Electric Hot Water Heater – for very little cost and with almost no inconvenience.

Electric Hot Water Tank

First, a little bit about the typical hot water tank.  The hot water is taken from the “top” of the tank, and cold water is directed in at the “bottom” to keep the tank full (hot water rises to the top while cold water sinks down).  There’s a “top” heater element, and a “bottom” heater element.  The top element heats the top half of the tank (about 20 gallons) and passes very little heat to the water resting below it in the bottom half of the tank.  The bottom heater heats the bottom 20 gallons; but because hot water rises, it will heat all the way through and do the whole 40 gallons eventually.  See the diagram at left – thanks to: http://www.simpsonelectric.ca/tips.html

Now, electric hot water heating is generally slow, and it may take 3-4 hours to heat a whole tank all the way from cold to hot.  That’s why you have a 40 gallon tank, so that you can draw off 40 gallons of hot water all at once, without getting any cold.

Now, here’s how you save money with Time-of-Use electric billing.  You use a simple cheap 120 VAC timer (24 hour) to control a relay to connect/disconnect the bottom element (usually about 3000 watts at 240 volts – but you’re using the relay contacts like a timed switch to enable or disable the existing power flow to it).  In your area, you may have different “low rate” time of day’s but the cheapest is always at night, so I’ll just call it “daytime” and “nighttime” and you can set the timer accordingly.

At night, the timer is ON and the hot water tank functions normally, the bottom element comes ON as needed and the whole 40 gallons of water in the tank heats up.  In the morning, the timer disconnects the power from the bottom element – and won’t re-connect it again until next evening.  You have 40 gallons of hot water to use for morning showers.  If you don’t use more than the 20 gallons all day long, then the top element won’t even come on, and all of your power use will be at the low nighttime rate.  If, during the day, you use more than 20 gallons, the top element of the tank will come one and heat up the top 20 gallons and make sure that there’s still hot water for washing the dishes, etc.  This little bit extra power (hot water) will be at the more expensive higher power rate, but after all, while you want to save money, you don’t want to be doing the dishes in cold water.

As night approaches and the electric power rates drop, the bottom element becomes enabled again (by the timer) and it proceeds to re-heat the whole tank of 40 gallons again, ready for the next morning.

Now, what have we saved.  For low hot water use users, all the power used to heat water will be at the lowest hourly price rate.  For medium use users, high cost power is only used to heat after the first 20 gallons, and then only to replace the extra that you use and to maintain a 20 gallon reserve (as compared to 40 gallons).  For heavy hot water users … they are going to use up hot water faster than the top element alone can re-heat it, and they would probably be better off getting a second high-paying job to help them pay for the cost of their electric hot water power usage.

Enjoy …

Mike Merritt

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