How Much Energy Does It Take To Supply Hot Water?

I read an interesting article recently about how to calculate the size of an on-demand hot water heater.  OK, while not exactly what most people would consider light reading, what I thought was interesting was how much energy it takes to generate something we take for granted.  The article discussed how running a single shower would require the incoming water to absorb energy at a rate of about 75,000 BTUs per hour and that if the tankless water heater was 80% efficient, then the heater would need to have a rated input of 94,000 BTUs per hour.  If you want the ability to run two showers simultaneously, then the numbers double so that the tankless water heater would have a rating of 188,000 BTU.  (For reference, the boiler I have for heating my house and domestic hot water can modulate up to 150,000 BTU.)  The other thing that struck me is how poorly traditional water heaters perform.  Running a single shower consumes about 1250 BTU's per minute (75,000/60).  A traditional water heater rated at 45,000 BTU input and running 65% efficiency (which may be generous) actually transfers only 29,250 BTUs/hour to the water (45,000*0.65) which is 488 BTUs per minute (29,250/60).  That's the reason hot water tanks are as big as they are – we need a big reserve of hot water because the heater itself is only capable of heating water 39% as quickly as we need to run a single shower.

So, besides the shear scope of the numbers involved (188,000 BTUs? Wow!), I got to thinking, how much energy will the solar tank save?  If it's cold outside and the collectors can only raise the temperature in the solar tank to, say, room temperature (20°C), how much energy am I saving?  If I'm in the shower, how many additional minutes of shower time will the solar tank add?  To answer these questions, I've come up with a little spreadsheet that allows you to change several parameters:

  • The temperature of the incoming water
  • The temperature the water heater is set to
  • The storage capacity of the water heater
  • The efficiency of the water heater
  • The size of the solar tank
  • The temperature inside the solar tank
  • The temperature of the shower (or whatever your “load” is)
  • The flow rate of the shower

The calculations are located on the “Analysis” tab of the spreadsheet.  As I built the spreadsheet, I had to make several assumptions.  For example, to keep things simple, I assume that the shower takes place early in the morning or late in the evening when the solar collectors are not producing any additional heat.  These assumptions are listed on the “About” tab of the spreadsheet.  I also thought up a few scenarios to try and test the spreadsheet – these are listed on the “Scenarios” tab.  

Let me know if you have any questions of suggestions.


Water Heating Reqs v2_2.xls34.5 KB

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I'd like to use my solar collector with a pre-heat tank, but can't find cheap tanks. An insulated 80USG tank costs >$300.

I can get cheap (free) 42USG barrels, but I can't pressurize them meaning I'd need a heat exchanger. I could make a cheap heat exchanger out of copper pipe, but I'm concerned it would not be efficient enough.

Sorry, I'm not going to be able to offer much help.  My tank is a 650L insulated, non-pressurized tank with two internal heat exchangers (essentially great big coils of copper pipe) - one for pre-heating domestic hot water and one to tie in with the IBC boiler.  The tank comes with the solar pump attached, drain valve and sight glass.  It was part of the package I bought from Taylor Munro Energy Systems so I don't have a seperate price but it was probably pretty expensive.  Maybe $1000?  Since this is not my area of expertise, I was relying on "experts" to come up with a system for me. 

The only thing I can think of is that, years ago, I had looked at buying a large plastic water tank for an uncle with a farm.  At that time, there was a place off the highway on the way to the International Airport that had some.  This was probably 12 or 14 years ago and I can't recall the name of the place but, at the time, their tanks didn't seem to be too expensive.  

A recent study by NRCAN, "Hot Water Use Field Test Results", by M.Thomas, S.Hayden, K. Wittich, D.MacKenzie, 2008, looked at a relatively small sample (40) of houses in the Ottawa area, and concluded, among other observations, "Daily hot water use appears to be in the order of 163 Litres as opposed to the 243.4 Litres used in the current test standards."

Today I measured the flow rate of one of our showers. The head is rated max 2.5 GPM (9.5L/min). The flow rate measured was only 4L/min with a minimum water pressure of 30psi. I have estimated my daily hot water use at around 150L; that is with 2 adults and 4 children in the house. That seems to fit will with the 25L/person*day figure I've seen for PassivHaus.

I just noticed in the spreadsheet that the value in cell C6 is pounds per litre not pounds per US gallon. I believe the correct value would be around 8.35.


My apologies.  I've uploaded version 2 of the spreadsheet that corrects the lookup problem but I see the spreadsheet still has some issues.  I suspect the root cause is because the original version was developed in OpenOffice.  I'll look into this and post a corrected version ASAP.


I've updated the spreadsheet and uploaded version 2.2.  Please let me know if you notice any other problems.


Did you factor in that you are not taking a shower at the full 140 degree f water tank temperature?

Would a typical show use about 20 000 BTU's then?



The posting is based on transferring 1250 BTU/min or 75,000 BTU/hour into the incoming water.  To get those numbers I'm assuming that the temperature of the incoming water has to be increased by 60 degrees F (about 33 degrees C).  Maybe the incoming water is a cold 40 degrees F (4.5 degrees C) and the shower is 100 degrees F (38 degrees C) or maybe the incoming water is warmer but the shower is hotter.  Whatever the scenario, I'm working with a 60 degree F increase between the water coming into the house and the water coming out of the shower head.  I also assume that the flow through the shower head is 2.5 US gallons per minute.  Finally, US gallons weigh in at 8.34 lbs each.  Calculating gives:

2.5 gal/min * 60 degree F temperature change * 8.34 lbs/gal =  1251 BTUs/minute (rounded to 1250)

1250 BTUs/minute * 60 minutes/hour = 75,000 BTUs/hour 

(My apologies - I should have included the calculations in the original post.)

So the answer to your first question is: I based the calculations on the temperature difference between the incoming water and the water coming out of the shower head so yes, I took into account that the shower is not at a full 140 degrees F.

For your second question: Would a typical shower use 20,000 BTUs? 15 and 20 minute showers would be:

15 minutes * 1250 BTU/min = 18,750 BTUs

20 minutes * 1250 BTU/min = 25,000 BTUs

So your number of 20,000 BTUs should about right.  Don't forget that I'm talking about how much heat has to be transferred to the water and not how much natural gas must be burned.  In order to find how much gas needs to be burned we need to divide the BTUs required by the efficiency of the water heater.  If you run with an old tank style heater that's 65% efficient, the 20 minute shower will required the water heater to burn (25,000 BTUs / 65% efficiency =) 38,462 BTUs of gas.  If the water heater is rated at 45,000 BTUs per hour, the water heater will run for (38,462 BTUs / 45,000 BTUs/hour =) 0.8547 hours which is just over 51 minutes.  According to the math, the water heater runs for 51 minutes to cover a 20 minute shower.  This ties back to why our water tanks are so large.

I hope that helps.


Nice piece of analysis this. One of the obvious things, is that efficiency with a solar collector will change not only with the efficiency of the equiptment, but also regionally and seasonally. In my location in the UK for example, the output would not have justified the cost, certainly not this year.

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