Click HERE for our online water heat up time calculation page. Or here.
At ImmersionHeaters.UK, (Call Jamie on 07897 246779) we have 2 types of customer. There are those who tell us what they want as they know their exact requirements.
Then there are others who need guidance and advice as to what they need. One question which comes up time and again is “How many kW do I need to heat up my tank?”
If we can calculate the volume of water and the required temperature rise, we can answer this question.
The following formula is used to calculate the power of heating element needed to heat a specific volume of water by a given temperature rise in 1 hour.
volume in litres x 4 x temperature rise in degrees centigrade / 3412
(4 being a factor and 3412 being a given constant)
for example 100 litres of water, to be heated from 20ºC to 50ºC, giving a temperature rise of 30ºC would give –
100 x 4 x 30 / 3412 = 3.52
meaning that the water would be heated in 1 hour by 3.5kW of applied heat.
Also we can use this information to extrapolate both ways. To heat the same water volume in half the time (30 minutes) would need twice the heating power, ie, 7kW.
Converesely, if we only use half the heating power, 1.75kW, it will take twice as long to heat up to desired temperature, ie, 2 hours.
If we only have a 1kW element available, we will expect a heat up time in excess of 3 hours.
Also we can use this formula as the basis of similar calculations for heating oil. Generally, oil heats up in half the time of water, due to its density. However, oil needs a much lower watts density element than water, as described in the “How to choose an oil heater” article.
elementsofheating 
A good informative article, especially your ‘water heat up time’ calculations page.
thanks for such a concise answer. I’m calculating how much kW I will need for a solar Hot Water system. Such systems get 4 – 5 hours of sun a day, and have to heat around 300litres. A tube is rated around 60W, but only get about half that on poor days.
Using your information it’ll be a snap to calculate.
Thanks for the feedback.
I have no idea why you would want to first multiple by four and then divide by 3412 instead of just dividing by 853.
is the calculation heat up time using immersion heater and catridge heater is same
Yes, in liquid, ie, water.
Excellent explanation thanks
we need heat pump heater calculations. we will be grateful.
Sorry for late reply, I didn’t know these comments were here tbh!
If you let me know exactly what you need, I will find a cleverer friend to do the brainwork, then happily add it to the website, which I am looking to expand.
All suggestions greatfully received.
Hello Sir
Thank You for the valuable article about heating Water
I have the same question to heat up a volume of water of 100 cubic meter indoor swimming pool using an air to water heat pump for winter season
The initial water temperature of water is 10 degrees Celsius, target temperature is 31 degrees
Air temperature is less than 10 degrees
Hi Aram, thanks for getting in touch. Where in the world are you? Cos that is bloody cold! & 31 is kinda high.
Sadly, the grey area is heat loss. My calcs are “Ballpark” as they don’t take into account heatloss, which is all but impossible to calculate.
My only advice is to “over egg the pudding” and if you put in 1.5x or even double what you think you need. Better to be over powered & heating switches off, rather than underpowered & never get to temperature. If you were doing it with my immersion heaters, and we figured you needed 6kW, I’d suggest you fit 2 x 6kW, with second unit covering marging for error, lower temperatures, etc, AND it is a built in spare when your main unit dies, so you can still swim while you source a new unit.
I’d love to see a foto of your pool and put it on my website, G##gle post, etc, as I am a mad open water swimmer myself, rivers, lakes, etc, and I am big on Collaboration, as you can see here – https://immersionheaters.uk/2021/11/08/collaborative-evolution/
Hope to speak to you soon, Sir. Have a bloody marvellous day.
Jamie.
I use another formula, resulting about the same:
To heat 1 Liter of water 1 Kelvin needs 1.16Wh.
Cheers Peter.
how do i contact jamie by email?
immhtr@gmail.com
& I’d love to hear from you, so long as I didn’t break anything!
why 3412?
It’s a “constant”, that is as clever as I get, sadly!
Some cleverer personage worked it out, the rest of us just use it, cos it works.
Thank you very helpful
Thanks for the feedback, Ian.
Hallo. I have a 35 000 kiloliter pool. My heat pump is 7.6kw and the average temperature is 18 degrees. How long will it take to heat the water to 30 degrees? Thanks.
Apologies for my tardy reply, you were lost in a sea of spam!
Have a play here –
https://heatingcalculators.com/liquid-heating-calculators/calculate-the-time-required-to-heat-a-volume-of-water/
…but I make it circa 65 hours.
All the best,
Jamie.