Really?

http://www.energy.wsu.edu/documents/building/res/ht_pmp_water_htrs.pdf

The differential I'm referring to is temperature of the water being heated. Cool water around the condenser will increase efficiency and hot water around the condenser will lower it. As the water in the tank warms efficiency will become less. I think the range of water temperature wil be much larger than the air temperature range.

Agreed. However, in the presentation of efficiency by the manufacturers, they refer to a fixed differential and don't appear to consider any colder water at the beginning of the cycle.............unless that's the detail that allows such high efficiency...............

There is nothing in there that compares the efficiency to the differential. They make some references to possible efficiencies that can be obtained.

Do you think energystar.gov is BS'n me to?

Those articles only compare electricity usage to heat water. Oil is much cheaper than either a conventional electric heater or a heat pump heater

Do you understand the concept of "differential" and "efficiency"..........or are you just posting relevant google articles?

It appears that you've confused yourself. There is no "differential" when we speak of gas, oil, or manure. These fuels are consumed in the process of making heat and the efficiency of the conversion is less than 1:1.

In the case of heat pumps, run by electricity, the efficiency of the device is based upon the differential between the temperature of the two fluids...........in this case, one is the ambient air, and the other is the hot water.

And, no, I don't deserve a warning for pointing out the obvious.;)

You could do what I did and put in solar hot water. We put in a 120 gallon tank with 3 panels for the main house and an 80 gallon tank with 2 panels for the cottage. Our electric bill has gone down by half.:D The company we purchased from guaranteed a 30% reduction. I never thought that the hot water was costing us that much.:eek:

Around here using an electric water heater is only slightly more expensive than gas. Perhaps 10-15%.....downside is that they can't keep up with demand as well. Our electric is only .06 - .10 cents per kwh on a sliding scale (goes up depending on how much you have used....and maxes out at .10 cents) :D A 50 gallon natural gas water heater set to a high set temp can provide pretty much nonstop hot water to 2 bathrooms/showers without any loss of temperature.

I'd consider a tankless water heater someday when I have a house, but it would have to show worthwhile savings. Where I live right now is a small condo complex with 20 units.....hot water comes from a beastly 100-120ish gallon commercial (gas) water heater. Looks to have about 1.5-2" pipe going in and out of it. :eek: There is a separate ancient beast boiler that runs the hydronic heating.

Electric water heaters have no insulation in the bottom and fairly poor insulation overall. I always construct a well insulated enclosure on all six sides. The standby losses are too substantual to ignore otherwise.

This does not move the efficiency up to the instant type by any stretch. Yet does help quite a bit. I try for about R30 overall above what is on the water heater from the manufacturer.

Electricity is 11-12 cents per killowatt hour here.
It is 5.5 to 6 cents during low demand hours and all weekend.

That is if you install an electric heating storage unit for three thousand dollars and sign up. It of course only stores heat in the ceramic blocks during off hours. Larger electric water heater on a timer to try heating most hot water on low cost time helps as well.

We don't have sliding rate scale here in my area, but we still try to run the laundry and dishwasher after 8PM. Trying to get ready for when we will get a break for off-peak usage.;)