Sanity check on power used to heat hotwater

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prob

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Master Geek

#319879 10-Jun-2025 14:21

Trying to rein in power use now that we have solar.

Our 180 litre hot water cylinder (3kW) uses about 10kWh a day.

There are just two of us at home, so a couple of showers a day and the occasional hot wash for clothing.

That seems excessive. Anyone else able to comment?

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#3382479 10-Jun-2025 14:23

Seems inline with what I would expect.

2-3kWh per day is what I saw on just the hot water and alarm system and a couple of sensor lights when the rental was empty. I see about a 2 hour recovery on the tank after a shower, so that is another 6kWh.

Richard rich.ms

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#3382482 10-Jun-2025 14:29

If its just 10KWh for the just the HWC would be very high unless you are draining it a few times per day.

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pdh

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Ultimate Geek

#3382494 10-Jun-2025 15:07

OK - let's do some math...

Domestic hot water in NZ used to be limited to 55 C - we're now all much safer (sarc) at 50 C.
Today, my incoming cold water is 16 C.

So, assuming you're at the new limit, that's a rise of 34 C - which needs 40 Wh (Watt hours) of power per litre.

Single shower heads in NZ range from 25 (very old school) to 6 (newest & most parsimonious). Let's assume 15 as a fairly likely & happy medium. So that's 15x40 or 600 Wh per minute under a hot shower.

Adjust as needed to suit your real activity.

So - to burn 10 KWh (10,000 Wh) of electricity in a day - you'd need to run the shower for about 17 minutes (10,000/600).

That's quite a bit for two showers a day - but only you can tell ;-)

pdh

339 posts

Ultimate Geek

#3382497 10-Jun-2025 15:15

If that seems impossible - you might:

(a) have an overflow problem. That used to be more common on old low-pressure HW systems - where the pipe on the roof would tilt or break off and HW would spill continuously into the gutters. Heat wasted.

(b) have a very cold & draughty environment for your HW tank - and/or poor insulation around it. That would cause you to lose/waste significant heat (and thus electricity) just in keeping the tank warm.

timmmay

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#3382504 10-Jun-2025 15:22

We use about 10kw per day for hot water right now. That's 5 showers per day, dish washing, misc use like hand washing, no laundry use.

OldGeek

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#3382518 10-Jun-2025 15:55

Just out of interest - ho do you measure power consumption for a hot-water cylinder?

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timmmay

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#3382526 10-Jun-2025 16:21

OldGeek:

Just out of interest - ho do you measure power consumption for a hot-water cylinder?

I use a Shelly EM Pro hooked up to Home Assistant.

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#3382527 10-Jun-2025 16:21

OldGeek:

Just out of interest - ho do you measure power consumption for a hot-water cylinder?

I have a Shelly EM with a CT around the feed to the cylinder.

bfginger

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#3382533 10-Jun-2025 16:52

OldGeek:

Just out of interest - ho do you measure power consumption for a hot-water cylinder?

If it's on ripple control your electric meter may display kW usage separately for controlled and non controlled. E.g. there'd be a total kW number plus a controlled figure and a non controlled figure.

WWHB

35 posts

Geek

#3382577 10-Jun-2025 18:58

pdh:

OK - let's do some math...

Domestic hot water in NZ used to be limited to 55 C - we're now all much safer (sarc) at 50 C.
Today, my incoming cold water is 16 C.

So, assuming you're at the new limit, that's a rise of 34 C - which needs 40 Wh (Watt hours) of power per litre.

Single shower heads in NZ range from 25 (very old school) to 6 (newest & most parsimonious). Let's assume 15 as a fairly likely & happy medium. So that's 15x40 or 600 Wh per minute under a hot shower.

Adjust as needed to suit your real activity.

So - to burn 10 KWh (10,000 Wh) of electricity in a day - you'd need to run the shower for about 17 minutes (10,000/600).

That's quite a bit for two showers a day - but only you can tell ;-)

Thats not how it works, a standard electric hot water cylinder needs to be heated to 60 C. There is a tempering valve installed to bring the temperature down to 50 C. That’s an extra 10 C rise needed.
There is a risk of legionella if your hot water cylinder thermostat is set to 50 C. The risk may be seen as low but I know of 2 cases locally to me where a homeowner turned his thermostat down to try and save on the power bill. The cylinder in this case was set at 52 C

https://www.stuff.co.nz/national/health/70285141/legionnaires-disease-from-hot-water-cylinder-results-in-amputations

Nate001

638 posts

Ultimate Geek

#3382583 10-Jun-2025 19:20

Last week, 2 people, average 3 showers/day, averaged 4.5kWh/day for hot water. Measured using Shelly ProEm on the incoming mains to HWC. So yes 10kWh a day sounds excessive.

pdh

339 posts

Ultimate Geek

#3382616 10-Jun-2025 21:15

>Thats not how it works, ... That’s an extra 10 C rise needed.

Umm, well yes, but no....

You can safely ignore that complexity - to keep the math simple.
Yes you may heat the tank to 60 + C - but then you add litres of cold water into the 60+C stream to temper it down to a safe 50 C.
So you produce additional litres of warm water - and all litres appear to have only risen to 50 C. Some went up to 60 in the tank and came back down - some never made it into the tank at all.

Work an example out for yourself - some water going up to 60, coming back down to 50 with additional cold litres - and you'll see. It's effectively the same as just heating the litres you're using to 50.
Or 48, or 52, or whatever your preferred shower temp is.

The only extra electricity usage caused by fear of Legionnaires - is waste heat leaking from a hotter-than-necessary HW tank. You'll lose more heat from a 60 C tank than a 50 C tank - all other things being equal. If that's an issue - wrap the tank in more insulation; or put the tank in the linen cupboard to use the heat.

pdh

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Ultimate Geek

#3382617 10-Jun-2025 21:25

From purely anecdotal data - anyone with teenage daughters can safely double their hot water usage...

Shower variables - how hot, how long and how intense - seem to me to offer about a factor of 6 between least and most energy used for a 'standard' shower. In other words, a short tepid shower under a 6 Lpm head vs a long hot one with a gusher... could use 6x more energy. So it's awfully hard to do an apples to apples comparison between households.

Scott3

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#3382624 10-Jun-2025 22:06

pdh:

OK - let's do some math...

Domestic hot water in NZ used to be limited to 55 C - we're now all much safer (sarc) at 50 C.
Today, my incoming cold water is 16 C.

So, assuming you're at the new limit, that's a rise of 34 C - which needs 40 Wh (Watt hours) of power per litre.

Single shower heads in NZ range from 25 (very old school) to 6 (newest & most parsimonious). Let's assume 15 as a fairly likely & happy medium. So that's 15x40 or 600 Wh per minute under a hot shower.

Adjust as needed to suit your real activity.

So - to burn 10 KWh (10,000 Wh) of electricity in a day - you'd need to run the shower for about 17 minutes (10,000/600).

That's quite a bit for two showers a day - but only you can tell ;-)

I'll have a try.

Standby losses of a 180L tank = ~1.7 kWh per day.

https://sustainableengineering.co.nz/portfolio-item/dhw-tanks-w-k-from-kwh-day/

Leaves ~8.3 kWh to heat water.

Assume incoming temp 16 deg C, and 37 Deg shower, delta T = 21 deg C.

Specific heat capacity of water is 0.00116 kWh/(kg.K)

8.3/(21*0.00116) = 341 kg (or liters) of water.

Lets assume the hot was uses the same temperature as the shower (it is likely hotter, but this makes the math's easier) , and uses 100L of hot.

That leaves 241L for the shower

Shower flow rate 15L/min.

241/15 = 16mins of shower time. (including any time waiting for the shower to get hot)

We would easily exceed that in our household, but this is in a plausible range.

All the stuff about temp limits is a red hearing. Cylinder still needs to be heated to 60 deg C for legionella prevention, then mixed down in temp to prevent burns, and mixed down again to ideal shower temp. All the mixing doesn't impact the energy, so best to just ignore it and work with the final tap temperatures.

Scott3

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#3382628 10-Jun-2025 22:23

pdh:

From purely anecdotal data - anyone with teenage daughters can safely double their hot water usage...

Shower variables - how hot, how long and how intense - seem to me to offer about a factor of 6 between least and most energy used for a 'standard' shower. In other words, a short tepid shower under a 6 Lpm head vs a long hot one with a gusher... could use 6x more energy. So it's awfully hard to do an apples to apples comparison between households.

The last point is very valid.

6L/min and 24L/minute are both plausible flow rates for showers, and the latter will consumer 4x as much energy.

Likewise, 4 min and 16min are both plausible shower times. again 4x the energy. If we a short low flow shower, with a long, high flow shower, 16x the energy consumed...

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