how much power does a electric water heater uses

[xboxman]

i know  the watts  it's printed on a label thats on the side of the water heater which is 4500 watts but how do i tell how many hours it runs per day or week with out setting there with a clamp on amp meter and counting the time it's on or without wiring in a  electric meter to the circuit .

is there any kind of device that will give me this info

all of this leads to a question is a tankless water heater cheaper to run than the tank type ?

[GaryGary]

Hi,

If you can make an estimate of how much hot water you use, you can then estimate how much electric power it takes to make the hot water, and then adjust for standby losses.

If you use 45 gallons of hot water a day, and you heat the water from 50F up to 130F, then the energy to do this is:

Energy = (30 gal)(8.3 lb/gal)(130F - 50F) (1 BTU/lb-F) = 19920 BTU/day, or 5.8 KWH per day

There are some standby losses from the tank -- if you know the Energy Factor for the tank, you can divide by that to get the total power use.  For electric tanks that are not to old, the Energy factor is usually about 0.9.

So, 5.8/0.9 = 6.4 KWH per day.   2336 KWH/yr, or $233 at 10 cents/KWH.

Your 4300 watt element would run 6800 watt-hr/4300 watts = 1.6 hours/day.

Measuring:

If you want to actually measure it, you could use a current transformer and a logger.  I have one from Onset Computer that works well, but its kind of pricy.  

A near no cost way is to hook up one of those electric timers that are used to turn stuff off and on at set times during the day.  If you can hook it up so that the timer only gets power when the element is on, then set the timer dial to 12 midnight on day 0 -- the timer dial will accumulate time only when the element is on.  So, you can read it at the end of a day or two, and see how many hours the element has been on.  The consumption would then be: (hours on/day)(4.3 kw)

A little more on this here:

http://www.builditsolar.com/References/Measurements/measurments.htm#Power

If your tank has two elements, you would either need to turn one off, or have two timers.  

The elements are likely to be 240VAC, so you need a way to get 120VAC out of this to run the timer.  I think hooking the timer from from one element hot to ground would work, but check this with your meter first. Be careful!

Tankless vs tanked:

If both are electric, I don't think the saving is going to be large.  Good tank type electric water heater have Energy Factors up toward 0.95  -- to me, this means the most you could save with a tankless would be 5%.  You can improve the EF on an older heater with an insulation blanket.

If both are gas, there is a pretty good saving, since the EF goes from about 0.62 for tank type up to 0.84 for tankless.

If you go from an tank electric to a tankless gas, it all depends on your gas and electric prices, but in most places you would save a good bit.

Gary

[thirteen]

I replaced a water tank with on ondemand tankless water heater in one of my daughters rental houses. They had had problems with the water tank leaking around one of the elements. So I replaced. Their power bill did drop about $56 per month compared over the last two years. My daughter got a tax credit for the heater and I will be putting one in my retirement home. Go to Google and search for tankless water heaters there are a half dozen to choose from.  

[thirteen]

I installed a naturel gas tankless ondemand water heater I didn't say this before. oops

[Volvo farmer]

To be accurate in your representation, you should also mention how much the gas bill increased. Undoubtedly, your electric bill is going to drop when you replace a electric water heater with a gas one. The real question is how much more efficient is tankless over tanked. Switching from electric to gas in the process confuses things somewhat.