Watts required to heat water

i’m looking at maybe making some hot water with my pv panels. i’m trying to get some idea of how much energy i’m going to have to make. i went to a web site and entered the info:
(to my metric friends, sorry).
200 gallons, starting water temp = 55, ending temp = 120 6 hrs to reach temp.
web site is telling me i need 6360 watts total. ??? seems like to little. do they mean 6360 kilowatts per hour for 6 hours (i think so). or do they mean i only need to make a bit more than 1000 watts per hour for 6 hours?

There is a very useful water heater calculator online here:

I assume if you’re working with US gallons then 200 gal = 757 litres.
55F to 120F = 12C to 49C (and wanting to raise the temp by 37C).
a 5.5kW element would achieve this rise in 356 mins, using around 32.6kwh of energy and in just a shade under 6 hours.

What size is the PV array you currently have installed?
What kW rating is the immersion element?
Do you need the water heated up by a certain time?
Have you made provision for legionella prevention? (my rule of thumb is heat the tank to 60C (140F) once a week for one hour). Your local legislation may vary on this.

thank you for the quick rely. i have 10 300 watt panels coming. i haven’t bought anything else yet. i’m wanting to see what its going to take before i put money into it. the water will be used for under floor heating. if this doesn’t work then i’m going to go grid tie. i understand i can’t just plug the solar panels to the heating element, to wasteful. also if i powered a hot water heat pump it would be maybe 3 times more efficient. but, i’m just looking at how much kilowatts is required.

i think you answer my question. i would need an array of 5.5 kw. perfect weather. mppt to extract the max out of the panels. in 6 hours i would have achieved my goal

My experience here in the UK (I work for an electric heating company) is that a 3 to 4kW PV array would be connected via inverter to the home supply and a “PV optimiser” unit such as an Immersun, Free-E or your own home-brew diverter. This would heat up a domestic hot water cylinder (typical UK homes might use a 150 to 300L cylinder - 39 to 79 US Gal) and depending on what other uses of electricty there were, may or may not get the tank to 50/60C in a day, but certainly cut the fuel bills for hot water production.

Using a PV diverter to channel excess energy into a large buffer tank that is then circulated into underfloor heating is certainly possible, but probably more hassle than it is worth, especially as provision needs to be made to heating the underfloor in the hours of darkness too!

you sound just like the person i need to be talking to. i already have a grid tie system in place but my inverter maxes out at 2000 watts. i’m already there with that. what i’m thinking about trying is to store hot water during the day with this new system and my old system heats the house during the day (i have a heat pump). then use the 200 gallons of hot water to help heat the place at night. i have a very small house. maybe i don’t need 200 gallons, thats just a guess. give me a minute and i’ll try to come up with the living space area.

about 400 square feet.

Well just using “back of the beer mat” calculations - if you have 8 hours of good sunshine on a 3kW array, you’ll have 24kwh of energy stored in a buffer tank, assuming it’s a 3kW element and the PV array was not serving anything else. In reality it will vary a bit.

It really depends on how well insulated the building is, if 24kwh of energy will be enough to keep 400 square feet warm overnight, then it would have to be a pretty well insulated building. Is this an RV? If so, I have my doubts that there’ll be enough energy to keep the place warm enough all night.

yes it is a rv, and its not very well insulated. i’m working on that. maybe its not enough to keep the house warm all night maybe just part of the night.

really i only think i’ll get 2000 watts out of the array. i never have much luck getting the “advertised” wattage

only 16 kwh of heating. who knew it took so much power to heat water ?

Ultimately, whilst it will be an interesting project and it will certainly help cut your heating bills, it will probably need to be backed up by the heat pump kicking in at some point into the evening.

If you do try it, let us know how you got on!

will do. thank you


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i was thinking last night and it kind of came to me that the numbers i’m using probably are not correct. the very first time i heat the water it will be about 55 degrees. but after that i would expect that i circulate and recycle the water thru the floors until the water temp is down to 80 degrees. in the morning i would think that the water temp would be something like 75 degrees. thats alot better than 55 degrees.

I’m reading this topic but not replying because it uses strange units. What is a ‘degree’? It’s either °C or °F and if its °F I can’t be bothered.

You haven’r said where you live. Is it in a ‘heating climate’? i.e. somewhere with a significant northern latitude? If so then the main problm is likely to be that you will have an excess of power in summer and not enough to do any significant space heating in winter. Sorry.

He has, it’s in his profile (click on his icon). But we don’t know where in that country he is - it could be next to the Canadian border or next to the Gulf of Mexico.
So I’m guessing it’s °F he talking.

Hi Robert.Yes, my point exactly. His country extends from frozen north to subtropical south and it is important to know where.

And yes, my guess would be the same, but (a) we shouldn’t have to guess ( IassUme) and (b) I’m certainly not going to be bothered to work through the conversions to international units. That’s down to him, in my book.

Calculating the heat input to obtain the requisite temperature rise is easy - knowing or estimating the losses in the process is the hard part.

You weren’t far off the mark. :wink:
His IP profile says Alabama - a Gulf coast state.
He could be inland, and thus up to 300 miles (approximately) from the GoM, but he’s definitely in the South.