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I've been keeping it off since I moved in, but it would be nice to take some hot or warm showers.

I'm not sure how much electricity it uses, but if someone tells me what to look for, I'll check the labels and update the question.

How much would keeping my 40 gallon water heater on 24/7?

Would it be better to turn it on only before showers?

And how long before showers should I turn it on?

Product info:

  • Model No: M240T6DS-1NCWW
  • Serial No: FM12729626
  • Voltage: 240 / 208 1-Phase AC only 50 / 60 HZ
  • Upper-Lower Element/Maximum: 4500 / 3500 Watts based on 60 HZ
  • Capacity: 40 gallons / 151.4 liters at 150 psi
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  • Does it supply hot water to the kitchen, dishwasher, washing machine also? Do you have the make and model number. Commented Jul 11, 2023 at 15:15
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    I think there are too many variables (cost of electricity, efficiency of heater, daily hot water use) to determine an accurate answer. Is your budget so stretched that you can't try it for a few months and see the impact? Or are you just looking for the most efficient usage?
    – D Stanley
    Commented Jul 11, 2023 at 15:25
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    Note that hot water heaters are usually always kept on except when not used for long periods like vacations (and even then, it's more common to just reduce the temperature). I'm wondering if you think you should turn it off, trying to minimize electric usage, or if it's a budget constraint.
    – D Stanley
    Commented Jul 11, 2023 at 15:29
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    40 gallons tank water heater is more likely gas than electricity
    – littleadv
    Commented Jul 11, 2023 at 16:55
  • @mhoran_psprep --- I have no hot water in my kitchen, so I assume it supplies hot water to the kitchen also. Commented Jul 11, 2023 at 18:21

3 Answers 3

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$35/month is a ballpark figure based on 4,500W (*assuming that the electricity rate is 13¢ per KWH and the 40-gallon water heater runs for 2 hours per day)

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  • So at only 2 hours a day every day... it's $35/month? That's a lot! That's $420/month if I keep it on the entire day? That's $5000/year! Commented Jul 11, 2023 at 23:54
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    No that's if it's Heating for a total of 2 hours per day - modern water tanks are very efficient, so it's not always having to heat water.
    – D Stanley
    Commented Jul 12, 2023 at 1:33
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    @IAmConfused With all due respect, you're overthinking this. I'd be willing to bet that it costs you more to run your fridge than your water heater... I'd just run it for a full billing cycle and see if you notice a difference - you probably won't.
    – 0xFEE1DEAD
    Commented Jul 12, 2023 at 1:44
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    @IAmConfused the “2 hours per day” is via thermostat control. That’s what thermostats are for: to only heat the water (or cool the air) when it hits a certain temperature. Since the tank is insulated, the heating element will only run about two hours per day.
    – RonJohn
    Commented Jul 12, 2023 at 4:59
  • @IAmConfused that is the $35/month.
    – RonJohn
    Commented Jul 12, 2023 at 5:00
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For the past few decades, US law has required water heaters and other major appliances to ship with a big yellow "Energyguide" label which shows estimated annual energy cost. If somebody didn't take it off, this is the best place to start.

If it's old or your power is very expensive (e.g. California), the energy prices may be different from what you're paying now, so look for the kWh figure and multiply it by your current rate from your power bill.

If the label is missing, you may try to contact the manufacturer to see if they have this data accessible.

Energyguide Label

(US Government image)

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    Note that this estimate almost certainly assumes that it is always on, not just on when you need hot water.
    – D Stanley
    Commented Jul 11, 2023 at 18:50
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How water heaters are designed to always be on. Otherwise, they would have better ways to turn them off than cutting off power at the source. If you turn it off for a period of time, the heater must use more electricity to heat the water, possibly more electricity that it would have taken to keep the water at temp, depending on usage. 40 gallons of water will take quite a bit of time to heat up, so you will need to turn it on quite a while before you actually need it, depending on how long it has been off.

Plus the energy spent to heat the water will be disproportionate with the usage - if you need 10 gallons of water for a shower, the heater still has to heat all 40 gallons.

You may also be shortening the life of the heater by turning it off often.

So it's impossible to say deterministically how much electricity you'd save by turning it on or off. My suggestion would be to actually keep it on all the time for a few months and see how much more electricity you use compared to your current method. My guess is you're not saving as much electricity as you think you are.

Another option if you're really concerned about electricity usage is to purchase an on demand heater that only heats water when you ask for it.

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  • "possibly more electricity that it would have taken to keep the water at temp" There may be cycling losses with gas heaters and heat pump heaters in particular, but not with resistance electric, where essentially 100% of the energy goes into the water.
    – user71659
    Commented Jul 11, 2023 at 19:02
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    That's speculation; I remember hearing that it can take more energy to heat a tank from room temp to 120 than by keeping it at 120, but that may not be true. in any case I still doubt that the energy savings is significant, but that's still speculation. Which is why I suggest to actually try it and measure the difference.
    – D Stanley
    Commented Jul 11, 2023 at 19:18
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    "purchase an on demand heater". Tankless water require a lot of plumbing work to reroute the pipes. And the condo might be rented.
    – RonJohn
    Commented Jul 11, 2023 at 19:48
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    @DStanley that might be true. It's been a while but I remember a Youtube video by Technology Connections who did a deep dive on water heaters. How they heat water and maintain heat efficiently is very interesting.
    – Stan H
    Commented Jul 11, 2023 at 20:24
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    Electric on-demand heaters may actually use much more electricity to heat up the water to the desired temperature than tank since they do it on the flowing water and have to bring up the temperature quickly.
    – littleadv
    Commented Jul 11, 2023 at 21:50

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