How much power to heat water tank?

[wopachop]

Wasnt sure where to post this. Curious how much electric power it takes to heat an RV water tank versus how much propane? I started doing some tests on my 10gal Suburban with the water starting at roughly 70F.

Electric: 1.43kwh
Propane: 0.1gal (196grams i hope my conversion is correct)

 

[hwy17]

1.43 x 3400 (btu) / 80lbs water = 60 degrees temp rise.

Back of the hand calculation agrees with your findings.

91,500 btu x .1 gallon x .6 efficiency = 5490 / 70 degree temp rise also close, heater is probably actually slightly less than 60% efficiency.

 

[Aridom82]

Wasnt sure where to post this. Curious how much electric power it takes to heat an RV water tank versus how much propane? I started doing some tests on my 10gal Suburban with the water starting at roughly 70F.

Electric: 1.43kwh
Propane: 0.1gal (196grams i hope my conversion is correct)

On the electric side it depends. If you are using a water heater that uses a resistor or a heat pump to heat the water.
With a resistive load and if you calculate an increase in temperature from 70f to 140f at 100% efficiency it takes 1,76 kwh; 70f to 130f is 1,5 kwh.
If the water heater uses a heat pump you would have to divide that by the cop. My Ariston Lydos Hybrid has a cop of around 2 for example.

Propane has a energy density of 13kwh/kg. So at 100% efficiency the same 1,76kwh would be around 135 grams, but a propane water heater is normally around 90% efficiency.

 

[Warpspeed]

Here is a handy on line water heating calculator:

https://bloglocation.com/art/water-heating-calculator-for-time-energy-power

 

[wattmatters]

Here's a calculator for the physics:

Water Heating Calculator

Use the water heating calculator to determine how much time and energy you'll need to increase the temperature of the water.

www.omnicalculator.com

10gal Suburban with the water starting at roughly 70F.

Electric: 1.43kwh

Propane: 0.1gal (196grams i hope my conversion is correct)

That correct.

1 kg propane = 14.0 kWh, so 196 g = 2.74 kWh

So efficiency of 1.43 / 2.74 = 52%

 

[wattmatters]

On the electric side it depends. If you are using a water heater that uses a resistor or a heat pump to heat the water.

Do RVs use heat pump water heaters?

 

[Aridom82]

Do RVs use heat pump water heaters?

Not by design but there are water heaters that use heat pumps now. I mentioned my Ariston Lydos Hybrid. Depending on how big is your RV or how much water do you need you could consider using one of them.

 

[wattmatters]

Not by design but there are water heaters that use heat pumps now.

I know there are heat pump water heaters, we have govt incentives for installing them here when replacing gas or regular electric. I was more wondering about suitability for RVs.

 

[Aridom82]

I know there are heat pump water heaters, we have govt incentives for installing them here when replacing gas or regular electric. I was more wondering about suitability for RVs.

I am not a RV guy but my lydos water heater could work i guess if the RV is big enough or you need a lot of hot water.

 

[justgary]

Differences in efficiency seen here may all be due to lack of significant digits in the propane weight given. Three digits given on the electric measurement is good, but only one digit given on the propane. Converting 0.1# propane to grams should be 200 grams due to rounding. 0.100# of propane is 196 grams.

I am making this point because I picture the OP weighing the propane with a bathroom scale and then recording the difference. In each case, the estimates used several more digits than the OP gave. It could be 50% efficient or 60% efficient, but probably not 52% or 55% because you only got one digit to play with.

So the question is, how accurate is the propane measurement? Is it possible to directly weigh in grams?

 

[wopachop]

I think it was 0.107 gallons. Almost posted the more accurate number.

 

[wattmatters]

Differences in efficiency seen here may all be due to lack of significant digits in the propane weight given. Three digits given on the electric measurement is good, but only one digit given on the propane. Converting 0.1# propane to grams should be 200 grams due to rounding. 0.100# of propane is 196 grams.

I am making this point because I picture the OP weighing the propane with a bathroom scale and then recording the difference. In each case, the estimates used several more digits than the OP gave. It could be 50% efficient or 60% efficient, but probably not 52% or 55% because you only got one digit to play with.

So the question is, how accurate is the propane measurement? Is it possible to directly weigh in grams?

Even the energy yield from 1kg of propane varies depending on the actual mix used.