Quote:
Originally Posted by randen
...The power reduction of 500watts to 350watts relates to a 30% increase in efficiency??...
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Not quite.
The formula is:
Efficiency = (Power Out) / (Power In)
To figure the power in you would need to do a test to see how much energy is required to heat a given amount of water from a beginning temperature to a final (set-point) temperature.
It's a bit complex on a system that is in daily use, but if your parents were going to be away for a few days, you could ask them to unplug the tank before they left. Wait a couple of days, then:
1. measure the starting 'hot water' output temperature of the tank, which should be pretty cold
2. connect the water heater compressor unit to power through a Kill-a-Watt
3. Run until the unit until it reached the set point and measure the hot water temperature
4. look at the total watt-hours used (energy in)
5. divide watt-hours by the run time to get total watts
6. convert the total volume of water to pounds
7. knowing that one pound of water requires one BTU to raise the temperature on degree F.
8. Multiply the total pounds of water by the total temperature rise
9. this gives you (energy out) which you will need to convert to watts
10. divide (energy out) by (energy in) to get efficiency
In my experience, and the experiences I have seen on this forum, your efficiency will not be a meager 30%. Rather it should calculate out to 250% to 375%.
This is where you get your well-earned bragging rights, and it is why you are saving so much money heating your shop.
Best,
-AC