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Water heating calculation


DeepBass9
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Can someone please explain how to calculate how much energy it takes for heating water? If I have a 2kW geyser element for instance, how long will I have to run it to heat water say 30 degrees warmer? I'm basically trying to work out how long I will have to run my generator to heat the geyser up to SWAMBO's temperature tolerance limits..

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27 minutes ago, DeepBass9 said:

Can someone please explain how to calculate how much energy it takes for heating water? If I have a 2kW geyser element for instance, how long will I have to run it to heat water say 30 degrees warmer? I'm basically trying to work out how long I will have to run my generator to heat the geyser up to SWAMBO's temperature tolerance limits..

Been using this site for +- times

http://processheatingservices.com/water-heating-time-calculator/

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1.16wh to heat one liter of water by one degree centigrade. To work it out from basic principles, the specific heat of water is one calorie per gram. One gram is one ml, so 1000 calories to heat one liter by one degree. Ask Google to convert 1000 calories to watt hours.

Sent from my GT-I9195 using Tapatalk

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  • 9 months later...
  • 5 months later...

slightly mathematically challenged  :-) am i right that if my solar geyser with 200l is at 40C and i want the water to be 55C according to http://processheatingservices.com/water-heating-time-calculator/  it will cost me 3.517kw - a rise of 15C for 200l [about zar5.60 at present eishkom rates].

if so, then using a bit of grid or batteries to give the geyser a little 'push' early evening will result in plenty warm water for household purposes, although it will require moving shower-times from the morning if you are not an 'yster man' fanatic :-)

anyone's 'learned opinion'?

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It takes 1 calorie to heat one gram of water by one degree centigrade, so it takes 1000 calories to heat a kilogram. This bit is easy to remember.

A kilogram of water is of course one liter, and a calorie 4.184 Joules, so it takes 4184 joules to heat one liter by one degree. This bit is harder to remember.

A joule in turn is one watt second. So 4184 watt seconds, divide by the 3600 seconds in an hour, gives you 1.16wh per liter per degree centigrade. Simplest is just to remember the 1.16Wh number and forget the rest :-)

Note that this is an average and as you get closer to boiling point it breaks down, you need a lot more energy at the top end. For electrical geysers where you pretty much stop around 60, it's good enough.

A 200 liter tank, taken from 40 to 55 (delta  = 15), will need 1.16*200*15 ~= 3.5kwh.

In Cape Town that will cost between R5 and R7. So it sounds about right.

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1 hour ago, plonkster said:

It takes 1 calorie to heat one gram of water by one degree centigrade, so it takes 1000 calories to heat a kilogram. This bit is easy to remember.

A kilogram of water is of course one liter, and a calorie 4.184 Joules, so it takes 4184 joules to heat one liter by one degree. This bit is harder to remember.

A joule in turn is one watt second. So 4184 watt seconds, divide by the 3600 seconds in an hour, gives you 1.16wh per liter per degree centigrade. Simplest is just to remember the 1.16Wh number and forget the rest :-)

Note that this is an average and as you get closer to boiling point it breaks down, you need a lot more energy at the top end. For electrical geysers where you pretty much stop around 60, it's good enough.

A 200 liter tank, taken from 40 to 55 (delta  = 15), will need 1.16*200*15 ~= 3.5kwh.

In Cape Town that will cost between R5 and R7. So it sounds about right.

So a bath in Cape Town will cost you about R7 ;) Or about R210 for the month. Makes you wonder, if you could cut your bill by 40% with a solar geyser, at say R20,000, is it really worth it? 8 years pay-back, without taking inflation or maintenance into account?

Just some food for thought 

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1 minute ago, SilverNodashi said:

years pay-back, without taking inflation or maintenance into account?

When my geyser burst, which was around the time you still got rebates, I installed the solar geyser using the money from the insurance plus the rebate.My solar geyser cost me just over 2k out of pocket and it's long paid back.

If I had to do it today... who knows, might not have done it.

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3 minutes ago, plonkster said:

When my geyser burst, which was around the time you still got rebates, I installed the solar geyser using the money from the insurance plus the rebate.My solar geyser cost me just over 2k out of pocket and it's long paid back.

If I had to do it today... who knows, might not have done it.

that's a nice score!

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1 hour ago, plonkster said:

In Cape Town that will cost between R5 and R7. So it sounds about right.

thanks plonkster, you and the team at phs seem to concur!

 

11 minutes ago, SilverNodashi said:

So a bath in Cape Town will cost you about R7

water that expensive over there? a 200l bath ;-)

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Just now, gabriel said:

water that expensive over there? a 200l bath ;-)

He's probably working with different numbers, or that was my assumption. Usually I work with ambient = 15 degrees, target = 55, so delta is 40. Between me and the kids we use around 80 liters (the wife has her own shower which does not enter the gray-water setup, so I don't know how much that one is). Now that 80 liters will cost 80*40*1.16 ~=  3.7kwh, which will cost around R7. So Silver isn't far off :-)

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13 hours ago, SilverNodashi said:

... if you could cut your bill by 40% with a solar geyser, at say R20,000, is it really worth it? 8 years pay-back, without taking inflation or maintenance into account?

My journey. First I had solar EV tubes installed with said rebate at the time. Old geyser was left in place. I knew it was going to go, but not today. Saved a bunch of money.

Later, when the geyser burst, as expected, insurance claim money was used, added about R2k, and got a proper solar geyser, installed it outside.

To really save on solar geysers, use them when it is optimal and use as little of Eskom as possible.

Our bill dropped by +-R1000 pm. Cannot allocate all to the geyser for during that period we started getting wiser ito what electricity was used where and why, and why so much water.

That 40% saving they claimed, it was a marketing buzzword I say. It depends on what your electricity bill was made up of. If it was primarily to heat water, then yes, but if you run say a bunch of computers 24/7, use the oven a lot, heaters then that 40% "claimed" saving gets skewed.

After geyser was installed, no more baths (plural - so we save probably 540 pm) and shower heads (free from by CoCT) was installed to save more water and therefor more hot water.

So my take today on it, thanks Silver for the numbers(!), solar geysers ideally should go with lifestyle change to lower your impact.

EDITED for clarification: By reducing your hot water needs first and then use your Axpert's spare solar power to heat the geyser, is most likely the best possible answer to really save (even buy more panels) versus installing a R20k solar water heating system just for hot water.

 

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24 minutes ago, The Terrible Triplett said:

solar geysers ideally should go with lifestyle change

on the nail! no change in lifestyle would be analogous to an obese person trying to loose weight by drinking tab with a high calorie low roughage plate of food - or brandy and tab ;-)

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54 minutes ago, The Terrible Triplett said:

 connecting geyser to a Axpert, it is most likely a waste to install a R20k system for water heating.

So not true. Needed to heat my water with Eskom only 4 times this year rest all done with my PV.... :lol:

Plus running my dishwasher of PV now isn't that nice


Edit
Realy would like to know how many days on avg does solar geysers not work at all?

Edited by viper_za
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9 minutes ago, viper_za said:

Realy would like to know how many days on avg does solar geysers not work at all?

From September to May it works perfectly. At least from November to April I don't spend a cent on water heating (that's about half the year), and then in spring and late autumn I still get quite a bit out of it. It's only the three coldest months when one might say "it does not work at all". And that's in the Western cape...

My advice these days: Install a HeatBox(tm). It's basically a solar geyser designed specifically to prefeed an electrical geyser. It costs about 10k. It's a good compromise if you don't want to spend 30k on an upmarket solar water heater, and it will make more hot water by noon than your PV panels can manage by the weekend... :-)

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@gabriel
I have a bit of trickery getting mine to work like this.
Timer for winter is set to heat from 9:00 to 15:00 and in summer from 8:00 to 16:00 but it's not as easy as that.
I have mine connected to a relay that is controlled by the readings from my BMV and Inverter.
On the BMV side it monitors the battery watts and voltages and inverter it checks to see what stage of charging I'm in to kind of predict excess power I should have available depending on voltage needed to maintain that stage.
This seems to be working perfect in detecting clouds and not starting the geyser when they are still blocking the sun. Don't want to heat my water with my batts.
This is still a work in progress in trying to find a better way to control when I hit the float stage as voltage change and reaching that voltage on a full battery does fool it even with clouds.

PS
Did not want to invest in some fancy light monitor

Edit

Normally water is heated at around 13:00 to the max the controller allows

Edited by viper_za
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13 minutes ago, plonkster said:

My advice these days: Install a HeatBox(tm).

Been playing with the idea to add a prefeed geyser 50 liters with maybe a 500w or also a 1kw element to turn on once the main geyser is at max temp.
Should also help out with some more hot water but the work required to install another geyser and redo a little bit of plumbing is keeping me away from this :D

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1 hour ago, viper_za said:

So not true.

Ag tog!

I said use a Axpert (obviously with panels and not Eskom) to heat the geyser.

For to spend R20k on a solar geyser, if the above is done, is a waste?

What is not true Viper? 

EDIT: Obviously the Axpert is used for more than just the geyser, like dishwashers et al, just to be precise before I get "bitten" again. :P

EDIT2: I fixed the sentence above also @viper_za

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