As we look forward to the move into our new house, we’re also looking forward to lower energy bills. Some people have been surprised, then, when we decided to switch from an on-demand (tankless) propane water heater, to an tank-style electric. The former are supposed to me more efficient, the story goes. More on that further down. First, my attempt to calculate how much we are paying for showers here in our trailer.
This is a part measured, part theoretical calculation. Since our water is not metered, we don’t know how much water we use overall or for showers. It is pretty simple to figure how much it cost to heat a quantity of water, so I’ll end up with the cost per minute of shower.
measuring cold tap water
I measured the input water temperature by running the cold water at the kitchen sink into a bowl and measured the temperature until it stabilized. It is astounding how cold our tap water is in the winter and spring here in Haines. You know it is cold when your hands hurt when you wash in cold water for more than a few seconds. Now I know how cold: 3.5 C (38.3F). I don’t have a figure for the water temperature in the summer, so I guessed at 10C (50F), then averaged for use in my calculations.
The next step: I took a shower and measured the flow rate of our shower head, and a comfortably hot shower water temperature, using a bucket to measure the flow and also to hold the water for accurate temperature measurement. I realize a “comfortably hot shower” is rather subjective, but I tried to hit a middle of the road hotness, which in my case was 36.1C (97F). For the flow volume, I adjusted the faucet to a moderate flow. Full on it was about 2 gallons/minute, and at what my conservative back thought to be a minimum flow for a decent shower, 1 gallon/min. So, since I would not expect my family to be quite as stingy as me, I’ve used 1.5 gallons/minute (5.7 liters/minute) for the calculations. I checked our 2nd shower, and its flow rate was similar.
The rest was just some arithmetic and digging up a bunch of numbers (cost of propane, energy content of propane, etc.)
So here’s goes. Cost to heat 1 gallon of water: [I’m going to start off in English units because its pretty easy in this case] 1 gal=8.3 lbs. It takes one BTU to heat 1 lb of water 1 degree F, so 8.3 lbs x 52.9 F temp rise = 423 BTU. This is the energy needed to heat one gallon of water to shower temperature on average (more in winter when the water is colder, less in summer). Now let’s switch to international units: 423 BTU/3412 BTU/KWhr= 0.124 KWhr of energy to heat 1 gallon of water.
Next I calculated the cost of that 0.124 KWhr for propane (cost per gallon of propane divided by KWhr/gal of propane divided by the efficiency of the heater, times the number of KWhr as above) [4.00 $/gal / 26.8 KWhr/gal / 0.70 * 0.124 KWhr = $0.027 per gallon]). Now when I do the same calculation for an electric tank water heater, based on our marginal electric rate or $0.12 per KWhr, I get $0.016 to heat a gallon of water. Let’s turn that into a shower table:
| shower | minutes | cost for Propane | cost for electricity |
| submarine | 3 | 0.12 | 0.08 |
| spartan | 5 | 0.21 | 0.13 |
| leisurely | 10 | 0.41 | 0.26 |
| luxurious | 15 | 0.62 | 0.39 |
| indulgent | 30 | 1.23 | 0.77 |
Summary…The cost for a shower was lower than I suspected based on our propane bill. Again, this is partly theoretical. One thing not accounted for is loss of heat in the water pipes, which in our case make a long journey under the trailer (read poorly insulated). But maybe a bigger portion of our propane is going to other hot water needs and cooking than I assumed.
Now on the electric tank vs gas tankless heater debate… I think the electric should beat the propane hands down due to:
- Propane costs more per unit energy than electricity where we live and probably most places.
- Our propane heater is not very efficient because of the heat going up the flue and the need to heat the make-up air.
- The electric tank heater’s standby losses are welcomed in heating the house probably 80-90+ percent of the year.
Read on if you want more detail to support my assertion.
Efficiency of tankless propane heater...I could not find a stated efficiency of the Bosch 125b. However, based on the stated rating of its replacement, the Bosch 520 PN, given as 78%, I used 70% for our older model. A lot of heat goes up the 6 inch flue pipe…I’ve been on the roof and felt the temperature of the exhaust.
Besides the heat created but not transferred to the water (but instead going up the flue) our Bosch gets its combustion air from the room where it is located, and as that air is drawn into the house it has to be heated (during the heating season). This does not matter in the summer, but in cold weather it can rob you of a lot of energy, indirectly lowering the effective efficiency running the old Bosch. Many newer heaters avoid this problem by having a dedicated combustion air supply.
Also, the pilot light (which burns all the time) uses some propane.
[To take the discussion a bit off track, I think the tankless propane heater is even less efficient when doing the dishes. The off and on nature of dish washing (I’m talking hand washing) seems to me less efficient since, each time the heater fires back up, there is a large woof of flame that I reason can’t be well converted to usable heat, but mostly goes up the flue. Also, with our particular heater, and many others, there is a minimum flow rate needed to keep it working, below which the water turns cold. This makes it hard to conserve by, say, rinsing dishes under a small stream of water vs a larger one.]
Efficiency of electric tank water heater… The main reason tank style water heaters are reputed to be less efficient than tankless type is due to standby losses, ie., heat that continuously escapes the tank whether or not the water is being used. In my opinion this inefficiency is overstated. In many cases the heat lost from the tank is not really wasted…it goes to heat the house. If you live in Florida, yes, it is wasted, and especially so if you are cooling your house. Here in Alaska we can use that “wasted” heat at least 9-10 months of the year. If we did not have this and other such incidental heat sources, we might discover it is too chilly to go without heat most days in the summer here. To account for what little standby loss is really wasted, I used a figure of 90% efficient in the calculations above. My guess is that is low.
Our choice… Other factors went into our decision to get an old school electric tank style heater. One is that our current tankless model is not big enough for our size family (if you are showering and someone else turns on the hot water in the kitchen, your shower will turn cold rather suddenly). A newer, bigger tankless costs a lot more than a tank style, so it would have to be much more efficient to justify it. One more thing…one of the touted advantages of the tankless (on-demand) type is that you never run out of hot water. With three teenagers, that can be looked on as a liability — limited hot water helps keep the bills down.
How about a cup of tea?
If you figure the amount of energy to boil a cup of water (1/16 of a gallon) using a similar tack as above you get 0.025 KWhr which is about $0.003, or a third of a cent per cup. I did not try to differentiate based on energy source. Then I used a plug-in energy meter to measure the electricity used to boil water in an electric tea kettle. The result was a bit higher, about a half a cent per cup. Even when you add the cost of a tea bag, you have a real bargain beverage there!
misc notes:
Our water & sewer are not metered, so I did not figure the flat rate monthly cost into this analysis. Yes, that is part of the overall cost of a shower, but assuming we will have water and sewer service anyway, it is not a factor in deciding to take a shower, or how long of one to take. Similarly with electricity, I do not count the monthly fixed costs of the service, but just the cost of an added KWhr, the marginal cost.
Obviously the water heater is not heating the water exactly to the desired shower temperature, but much hotter. I am assuming that the energy required to get the desired water temperature is the same if you heat the water to that temperature, or heat a smaller amount to a higher temperature and mix it with cold water, which is what is actually happening.
For a similar inquiry, see Knute Brinchmann’s How much does it cost to heat our house?
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