Is an under sink water heater more efficient than a tank heater?
by Ben
(California, USA)
I'm trying to figure out how much electricity a small, under-sink water heater (with tank) uses. I want to know if it makes sense to get rid of a 50 gallon natural gas water heater and replace it with a small under sink model (just one sink being served here).
Answer from Green Energy Efficient Homes
I suspect that if the only reason you have a gas hot water tank is for hot water at the one small sink, you will save a lot of money and energy by switching to an under-sink model. The only caveats are 1) most of these units have a fairly low throughput; they keep a small tank of water heated and can provide a couple of gallons of hot water, but if you run that out quickly you have to wait for it to replenish, and 2) if it's a small sink that gets used for hot water an awful lot (like washing dishes all day long) it will probably cost you more. If you're only using 5-10 gallons of hot water a day, I would suggest going for the under sink model over the gas hot water heater.
You can find out the wattage of the under sink heater, measure the water temperature coming in, and decide on the water temperature you want out, and do a calculation to determine the energy use for a given amount of water flow. For example, a 2,000 watt heater element will use 2 kilowatt hours over the course of one hour, and will produce 6829 BTU of heat output. You can heat a gallon of water by 1 degree F using roughly 8 BTUs (the exact ratio is that for every 1 BTU you can heat a 0.1219 gallon tank by 1 degree F). So if we assume your water coming in is about 60F and you want to heat it to 120F, or a 60 degree rise, you'll need 8 x 60 BTU to heat a gallon of water, or 480 BTU. So that heater, running for an hour, should be able to produce 6829 / 480 BTU or 14.77 gallons of hot water, for whatever you pay for 2 kilowatt hours. (Actually, it's more like 13.87 gallons - my original calculation assumed 8 BTU per gallon per degree, but it is actually 8.20.)
Does that make an under sink water heater more energy efficient or more affordable? It will depend on a number of things, such as whether this is a tankless under sink heater or has a storage tank; how well insulated the tank is; how well insulated your hot water heater is; how much hot water you use in that sink; how far the hot water tank is from the sink; and how much you pay for electricity and natural gas. Perhaps other factors as well. But I still believe the under sink water heater will probably save you money and energy unless you use an awful lot of hot water in this sink!
For more information on under sink water heaters, see my article
Point of use water heater.
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