Factcheck: Does Turning Off Your Water Heater Save Power?


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I recently came across an online forum where someone asked whether they ought to turn the power off to their electric water heater before leaving on a month-long trip. A self-professed plumber told them that it made no difference, and I found myself feeling distinctly cheesed off… and not for the first time, either.

Turning off power supply saves energy in electric water heaters by reducing power wastage through standby heat loss during periods of disuse. Powering off daily results in significant yearly energy savings, and during prolonged absences, promotes both savings and safety.

Fortunately for the user on the forum, someone helped him implement the powering-off procedure, and he left for his trip feeling reassured, not only about saving some $$$, but about the safety of his house in his absence.

If you’re still unsure about the logic behind powering off your water heater, or you’d simply like to know more about why it’s a good idea, read on.

Electric Water Heater Power Use: Thermostat Facts

You no doubt already know that the power supply to electric water heaters is regulated by a thermostat. Further, you may remember from high school that a thermostat is basically a heat-sensitive switch.

When water temperature rises above a certain point, a water heater thermostat disconnects the circuit from the main power supply. Then, as the water cools, the thermostat completes the circuit and allows power to flow again, to heat water.

From this description, you can already see that if a water heater stays on all the time, the thermostat is going to cycle on and off repeatedly, coming back on each time the water cools below a certain temperature. Of course, the rate at which hot water inside the water heater cools down depends a great deal on the water heater’s insulation – a well-insulated unit keeps water hotter for longer than a poorly insulated one. 

Nonetheless, when hot water is left unused inside the water heater, it’s only logical that some amount of cooling will take place. Eventually, the thermostat is going to allow power to come back on to reheat the water, using up precious (and expensive!) power.

Now, I’ve heard a number of arguments for why it’s better to leave a water heater running all the time. Here are a few of the more common ones (and why they’re not accurate):

Argument 1: Turning power off will allow the water inside the water heater to grow completely cold. It’ll then require much longer to get the water back up to the desired temperature again.

Response: The rate of cooling in a well-insulated water heater is pretty low, meaning that water stays warm even when the unit is powered off. If you think about it, you only require really hot water (e.g., at 120 °F) for activities like bathing and showering, while warm water (e.g., 80 °F – 90 °F)  works well enough for washing hands and toilet use.

Most modern appliances like dishwashers and washing machines have their own mechanisms for heating water, so there’s really no need to have hot water on tap for laundry and dish-washing, either. In fact, laundry detergents today are designed to work efficiently with cold water, so hot water might not even be necessary for laundry!

Bottom line: You should be able to get ample hot water for your major activities like showering by running the water heater once daily in time for peak usage, for example, two to three hours first thing in the morning. The leftover hot water should serve you well for minor needs during the rest of the day.

Argument 2: Since water stays hot inside a well-insulated water heater, there’s really no reason to turn the power off, since the thermostat won’t be cycling on and off repeatedly unless the water grows cold.

Response: How long a water heater takes to reset depends on its temperature setting and the temperature drop of the water. Thermostats typically cycle power back on for temperature drops of 10 — 20%. So, if the water heater temperature is set to 120 °F, it will reset as water cools to 108 —  96 °F.

As you can see, thermostats are designed to bring power back on while the water inside the water heater is still fairly hot. This means that, despite having enough warm water inside the water heater to meet minor needs like washing up and restroom usage, your water heater likely cycles on and off multiple times through the day, hiking your elec bill.

Argument 3: If you power off an older water heater, it might have trouble coming back on, and might even end up completely busted and in need of replacement!

Response: A poorly maintained, old water heater (typically over ten years old) might end up giving you problems if you try to turn its power off and then back on. However, older water heaters that have been poorly maintained are typically also power-guzzlers, and you might be better off replacing such a unit, after all (for more on how to do this, check out this helpful post on power-hungry water heaters).

Argument 4: It’s too much effort to power on and power off a water heater day after day. And if I forget to power it on in time, I’ll have no hot water when I need it!

Response: As you will see from the section below, it’s easy to automate powering on and off in a simple appliance such as a water heater. Not only that, but the tools required for automation would pay for themselves in as little as two months!

The Math behind Powering Off Savings

I’ve heard one or two people say that they don’t want to bother with powering off their water heaters, because they’ve read or heard somewhere that the energy savings would amount to very little. When I sat down to write this article, I decided to do some figuring… exactly how little is ‘very little’?

As a ballpark estimate, I decided to work out how much money could be shaved off the yearly energy bill by regularly powering off an electric water heater. Zeroing in on a standard, 40-gallon water heater, I reckoned how much it costs to run one of these, by using the energy cost calculator on Energy.gov (see the handy table below).

Prices used in the table are the median rates for electricity across states, with the exception of Alaska and Hawaii. In other words, the calculations use the midpoint between the lowest and highest electricity prices – currently, this comes out to 15.6¢ per unit (kilowatt hour or kWh). The savings through powering off provided here are based on the rule-of-thumb that standby heat losses amount to roughly 10%* of the energy bill in an electric water heater.

Electric Water Heater Capacity40 gal.60 gal.80 gal.100 gal.120 gal.
Yearly Energy Bill$475$712$950$1,188$1,425
Annual Powering-Off Savings$48$71$95$119$143

As you can see, the larger the quantity of hot water used, the greater the savings. Even if your hot water use were modest, say, 60 gallons (the national average is 64 gallons, according to Energy.gov), you’d be saving* roughly $70 on energy bills by powering off. Now, this might not seem like a lot of money to some people, but hey, a dollar saved is a dollar earned!

*Note: The numbers presented here are conservative estimates of powering off savings. In reality, people I’ve spoken to who’ve set up powering off systems report saving as much as $20 to $40 per month on their energy bills!

How to Set Up a Water Heater Powering Off System

For starters, let’s look at the scenario where you’re going to be away from home for an extended period. In such a case, it doesn’t matter if you haven’t set up an automated powering off system. You can still simply power off your water heater by turning the double-pole circuit breakers for the water heater circuit to ‘OFF’ – check out how to do this in this handy post.

If you would like to set up a more permanent arrangement, you’re going to need a water heater timer. You can buy one from your local hardware store, or simply order one on Amazon for less than the price of a wireless charger for your smartphone!

WaterHeater-TimerSwitch-PowerRating_lbld.png

Note: Make sure to check that the water heater timer you purchase is compatible with your water heater, by cross-checking your water heater wattage and amperage specifications against those on the timer product label. For example, the power rating label in the image above is from a timer rated for 25 amperes and 6250 watts, which can be safely used with a standard, 40-gallon water heater rated for 4500 watts.

To set up the timer, follow the simple steps outlined below:

  1. First, go to the mains panel and turn the water heater circuit breakers to ‘OFF’. Leave a sign taped over the breakers so they’re not turned back on until you’re ready.
  2. Mark a clearly visible and accessible spot on a wall that is at least 3 feet away from your water heater (NEC code requirements), drill holes and drive in screws to mount the housing of your water heater timer. Be sure to remove the two knock-out panels at the bottom before mounting the timer on the wall.
  3. Use a multi-meter to make sure that the power supply to your water heater is off (see video linked at the bottom of this list to figure out how to do this).
  4. Going to the junction box at the top of your water heater, disconnect the water heater white (or red) and black wires from the incoming power supply wires. See how to do this in this post on ‘Wiring a Water Heater‘.
  5. Draw the 10/2 incoming power cable through one of the holes provided in the bottom of the water heater timer housing, and secure it in place; the cable could be either metal flex or plastic-encased (such as Romex).
  6. Using an adequate length of new, 10/2 cable (either metal flex or plastic), draw one end of the cable through the second hole at the base of the timer housing and secure it in place. Position the other end of the new cable at the water heater junction box.
  7. Connect the water heater power supply wires to the timer by following exactly the printed instructions that came with your water heater timer model.
  8. Make sure that the grounding wire from both incoming and outgoing power cables is connected to the grounding screw inside the timer. (This is typically located either to one side of the power terminals, or at the bottom of the housing.)
  9. Replace the cover panel over the wiring.
  10. Set the timer (see the second video below) by following the instructions that came with the model. Ensure that you set the timer clock first (e.g., 11:00 a.m.), before specifying the time window in which you want the water heater running (e.g., 4:00 p.m. to 8:00 p.m.).

How to Use a Multimeter to Check the Voltage on a Water Heater

How to Set a Water Heater Timer

Note: Different water heater timer models make use of different wiring schematics. For example, some models require you to connect the incoming power supply to terminals (lugs) #1 and #3, and the outgoing, new cable to terminals #2 and #4, whereas some models have eight terminals, of which four need to be hooked up using short jumper wires that come with the timer kit, before you connect the incoming and outgoing power cables.

Tip: To make sure that you don’t have to deal with a cold shower (ughh!), set up the timer so that the water heater comes on at least an hour before your peak daily usage time. For instance, if you tend to shower between 6:00 p.m. and 7:00 p.m., you’d ideally set the timer to power the water heater on at 5:00 p.m..

Now you know for a fact that you can save on your elec bills by installing a timer. What’s even better, you know all you need to in order to install one! 

You could stop reading right here. Or, if you’re interested in other ways to cut back on your water heater elec bills, read on…

Other Ways to Save on Water Heater Power

Considering that a water heater’s typically working constantly, these units undergo a lot of wear and tear. A bit of timely maintenance can go a long way, not only in ensuring that the water heater doesn’t break down, but also in keeping it working efficiently.

So, here are some things you might want to take care of, to keep your water heater’s electricity consumption down:

1. Prevent Accumulation of Sediment (Lime-scale)

Water heaters typically have a lot of lime-scale (or simply ‘scale’) build-up, from grit and minerals in the water that accumulate on the bottom of the water heater tank. Unfortunately, the lower heating element tends to become coated in scale over time, making it ineffective at heating water.

De-scaling the water heater will allow it to work more efficiently. Read more on how to do this in this helpful post <link to post on DIY Water Heater Calcium Removal>. Or, if you suspect that your water heater’s lower heating element may be too far gone to rescue, you could replace it; read how to in this handy post

2. Fix Plumbing Leaks

If hot water’s steadily leaking out your water heater, the unit’s going to have to work hard (and use up $$$) to heat more water! Check the plumbing connections at the top, especially the hot water outlet pipe, for signs of corrosion and moisture.

If the leak is at the top of your water heater, where the hot water outlet pipe leaves, you could try tightening the connectors, or replacing them. Read more on how to in this step-by-step guide. However, a water heater that is leaking at the bottom could mean a badly corroded inner tank, which means that the unit may need replacing. In such a situation, it might be safest to seek a professional plumber’s advice.

3. Ensure Proper Insulation

A major cause of high energy consumption in water heaters is poor insulation. According to the DOE (Department of Energy), poorly insulated water heaters could account for as much as 45% extra in energy bills! 

Providing added insulation for your water heater is a DIY project that you could easily tackle yourself. For details on how to go about it, see this helpful guide.

With these energy-saving hacks, you can now be confident of cutting down significantly on your electric water heater’s energy consumption.

Happy savings!

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