# Water Heating Calculator for Time, Energy, and Power

*The calculators on this page compute how long it takes to heat water, how much energy is consumed, and how much heating power is required.*

The calculators supports Celsius/Centigrade, Fahrenheit, Watts (w), Kilowatts (Kw), Btuh, Joule, British thermal unit (Btu), liter, gallon, kg, lb, cubic inch, cubic foot etc.

The calculator supports decimal numbers, use dot or comma.

## Water Heating Time Calculator

This calculator tells you how long it takes to heat water from start to end temperature with a given heating power.

## Water Heating Energy Calculator

This calculator tells how much energy will be consumed to heat the water from the start to end temperature.

## Water Heating Power Calculator

This calculator tells you how much minimum heating power is required to heat the water within a specified amount of time.

This page contains multiple calculators. You can find the individual calculators with additional information, such as, for example, formula, on:

**Dennis**- December 24, 2023

**dsiddens**- September 25, 2023

**Off-Grid Garage Austrlia**- July 17, 2023

**EagleEye**- June 12, 2023

Well played. Neat calculator. And a number of helpful responses.

**antonio**- July 21, 2023

**Dark Cloud**- March 22, 2023

Daytime temps are usually in the mid-30°s, rarely below 25°F for more than 72 hours. Nighttime temps average in the high-teens but can be 0°F on occasion.

TIA

**Robert**- February 4, 2023

**antonio**- February 4, 2023

It may be that much more energy is needed to heat the pool water up by 1 degree than for preventing it from dropping 1 degree (in the same amount of time). Possible explanation: when first heating it up, all the surroundings of the pool also need to be heated up and that consumes energy.

**Robert**- February 4, 2023

Robert

**Robert**- February 4, 2023

I just read your reply to William on April 24th, 2018. That’s exactly what I need to do—and I am doing it as we speak! Since my plot of water temperature vs. time (during heating) is a straight line, I am assuming that this is telling me that most of the heat loss from the tub is due to convection and not to radiation (would you agree?) So I am going to see how much the temperature of the water drops in cooling by about 5 deg F and will then calculate how many Watts would be needed to reverse this cooling effect–as you suggested. Stay tuned…but I doubt if it’ll be enough to explain the apparent loss of heat I am experiencing according to the calculations. It’s a pleasure discussing this with you! Robert

**Robert**- February 4, 2023

**antonio**- July 21, 2022

Update 2023: The calculators above are now improved.

**David**- September 12, 2022

**Richard**- August 14, 2022

**Dave**- July 21, 2022

Thanks for adding metric

**Dennis**- July 6, 2022

**Richard G. Toronto**- June 5, 2022

Thank you!

**BG**- May 16, 2022

Why must the end temperature be less than 212f (boiling)?

Thank you,

Bill

**NEC**- April 22, 2022

My new swimspa is struggling in trying to maintain 37 degrees c. My plan was to use the spa at 37 degrees for the older members of the family, and as the day went on to allow it to cool for younger members to swim in a cooler temperature. Pleasingly, and even in the depths of winter there is very little loss during the day provided the spa is kept covered. Overnight it consumes 21kW of lecky and sometimes will increase the temp from say 35 degrees (loss during the day when used) to 36 or 36.5 – sometimes not even that.

Using any of the three calculations above for heating 7200 litres with the inline factory fitted 3kWh heater gets me no where near what it should.

Desperate for some additional thoughts please, the supplier and manufacturer haven’t any thoughts and are so far passing the buck!

Other owners of similar Swimspa say do not have a problem – I suspect they have more pennies than I do!

TIA

Nigel

**Michael**- April 2, 2022

A great help. I wanted to find the best size water element to heat my 250 litre hot water service to 58 degrees from a 6.6kW PV array. I settled on 1800 Watt.

**The Horny Toad**- March 31, 2022

**BigDave**- January 26, 2022

**Chris**- January 13, 2022

Quick question. I used your formula and my answer differed slightly from the one your online calculator gave me.

I think you are rounding your answers UP to the nearest minute, rather than to the closest minute.

Is that the case??

Rounding up would make sense to do for a number of reasons. I guess most importantly that the actually time to heat the water would be longer than your formula suggests due to heat lost to the surrounding environment.

**antonio**- July 21, 2022

Update 2023: the calculators now support comma or dot.

**Brian K**- January 10, 2022

**antonio**- July 21, 2022

Update 2023: the calculator now supports comma or dot.

**Dan**- January 9, 2022

I wanted to have the modification be simple and as broad reaching as possible.

**john**- November 8, 2021

**Bruce Glover (self)**- November 1, 2021

I am testing a variety domestic thermal solar options to heat out house.This is so we no longer need dead trees to be cut down for fire wood to heat the house and they can remain standing for birds to nest in.

Many of us back yard inventors are not so good with numbers and this calculator makes it possible for me to compare, evacuated tube, parabolic trough, flat plate, and polly pipe, collectors very easily.

By making this available you have made a great contribution to preserving the planet.

Best Wishes Bruce G. Perth Australia

**Fred**- September 6, 2021

Awesome to use.

Thanks

**CH. SATYANARAYANA**- September 6, 2021

**Ken**- August 26, 2021

**Ken**- August 26, 2021

Other sources have said that feeding the heating elements directly with dc heats faster as the power is continuous where as powering it via an inverter means that it only gets 50% of the power due to the sign wave dropping voltage as it crosses over 0° at 60 htz. (full power at either end of the sine wave at 180°)

Is the watts figures for AC., or DC?

**bmw**- July 15, 2021

Thanks

**Gordon**- July 6, 2021

**Paul**- June 30, 2021

**Matthew**- May 23, 2021

**John**- May 4, 2021

I am starting a new project and here are my requirements.

I need to maintain 1200 gallons of water in a tank at 170F

I will be using a closed loop boiler at 400MBH input 96%

With a exterior plate heat exchanger single pass with 2” in/out connection rated at 150PSI 230F.

Boiler exit temp to heat exchanger 190F

Tank water temp.at initial start ambient 70F entering heat exchanger.circulating pump for tank loop 3/4Hp at 35GPM

Boiler has built in circulator maximum length of pipe 2” to tank one way is 30’

Will this work to maintain my tank temperature?

The tank holds water with some cleaning agents for steel plates processing and degresing

John

**Clark**- April 27, 2021

I left school in 1986, and even though physics was one of my top subjects – after 35 years of not doing anything like this – I am a bit rusty 🙂

This is exactly what I was looking for, cheers!

**David Smith**- April 2, 2021

Much appreciation for your efforts in helping others.

**Saleh**- March 30, 2021

May I get the equations that are used to perform the calculations or the references please?

**Kahn**- March 29, 2021

how to calculate the tank capacity and temperature rise.

**Imran**- February 27, 2021

**SpaFreaking**- February 27, 2021

**Dan M.**- February 23, 2021

**antonio**- May 13, 2021

**Emil**- February 15, 2021

**BugReporter**- February 12, 2021

**MAnisha**- February 7, 2021

**antonio**- July 21, 2022

Please see the improved water heating time calculator linked in the article above.

Update 2023: this is the improved calculator now.

**Juan Chamorro**- February 5, 2021

**chuck**- January 22, 2021

90 gallons of 38 deg f water to 82 deg f

**Robbie**- December 28, 2020

**amaroo**- November 29, 2020

**Johnny**- September 19, 2020

**David Snitt**- August 29, 2020

Other than that a very handy tool and thank you for creating and hosting it.

**MasterCATZ**- August 25, 2020

do you have the calculation for heat loss

ie) how long it will take for 1000L to drop from 20 deg to 17 deg if ambiant temperature is 4 deg and or only had 300w heater etc

**S. Aneedl**- July 25, 2020

**David**- June 22, 2020

**Ralph**- June 19, 2020

As another calculator for us math dummies could be total kw x cost of electrical supply

eg 250kw x .26c = $65

At any rate a really good calculator q

**Czar**- June 3, 2020

how long will take to raise the temperature from 20 C to 30 C ?

To achieve this I will use 4 exterior stainless steel tanks of 72 m3 each, heated with 1500 watts glow plugs.

any comments or ideas? … please?

**antonio**- April 27, 2020

Update 2023: the calculator now supports dots or comma.

**Coliny**- April 27, 2020

If i change the power to 1.6kw . The answer is 6 hours 24 minutes.

An I missing something here?

**antonio**- April 16, 2020

The calculators use the specific heat capacity of water of 4186 J/kg/°C (Joules per kilogram per degree Celsius). The heat capacity is largely constant in the temperature range that the calculators work (34-210°F or 1-99°C). It is the way it is taught at school. For practical purposes, it should be precise enough. For rocket science one might want to calcualte it more accurately.

**Heat field**- April 15, 2020

**Anna**- April 5, 2020

**Andreas**- March 23, 2020

The water is recirculated from the system to a storage tank, using a pump 30.000 lit/h.

The tank is heated by steam using heat exchanger at 74 degrees of Celsius.

The water returns from system to tank at 70 degrees of Celsius.

My question is how I can calculate the heat power required to maintain the temperature forward and return at the same level (eg 74 degrees of Celsius).

Thank you

**John**- March 16, 2020

**Ishfaq Ahmad**- January 26, 2020

1 litre of water is 1kg

1 meter cube = 1000 Ltrs

M = 7 meter cube = 7000 Ltrs or 7000 Kg

The heat capacity Cp of water is 4.186kJ/kg-C

ΔT = 80-20 = 60 C

So, the energy required to raise the temperature of 7000 kg of water from 20C to 80C is:

Energy E = m•Cp•ΔT = 7000 x 4.186 x 60 = 1758120 kJ

or = 488 .36 KW

Power = Energy / time

And this is 488.36 Kilo Watts of power (since 1J/s = 1W)

1758120 kilojoule/hour = 488.36666667 kilowatt hour

488 kwh for 1 hour

For 4 hours 488/4 = 122kw

For 8 hours 488/8 = 61kw

**Rafek Tamer**- January 26, 2020

Appreciate the calculators but i was wondering if you could tell me the exact equations used for the “Water Heating Energy Calculator” or give some sort of excel sheet to double check it. I am guessing its q=m*cp*dt but when I do the calcs by hand, i get something off so i would like to know my mistake

**Andrew**- January 21, 2020

A thousand of something is a “kilo” for example a kilogram. In turn the abbreviation, for one thousand, is k.

Bringing these two together, a kilowatt is a “kW”, not as you have, “Kw”. Over an hour it would be “kWh”.

**antonio**- October 10, 2019

I do not think it makes much of a difference in practice.

As for efficiency, an electrical water heater can convert electricity with almost 100% efficiency to heat that can be transferred to the water. The insulation of where the heating device sits and whether any heat can escape into something other than the water (like a wall, concrete etc.) is more important for efficiency.

As for effectiveness, a large enough pipe with a pump can probably in theory transfer more heat per second (meaning power, e.g. watts) to water. When using a simple coil submerged in water, then it will start to heat the water just around it. Since water gets lighter when heated it will start to ascend around the coil and cold water will flow to the coil from below. This way a circulation develops in the water container given the shape of the water container permits it. I think of it this way: the power limit of heating coil simply submerged in water is reached when the water starts to boil around the coil while the water in other places is still much colder. Is that realistic?

Please remember, that the calculators above assume 100% conversion efficiency. For an electrical heater that can be a good assumption, but not for a gas heater.

Also, the calculator is only correct in the theoretical case that while the water is being heated, no heat escapes from the water to the environment (air or walls etc.). For that reason, these values are the minimum and some margin should be added to them. How much depends on the shape of your pool, the quality of the insulation of the pool, the difference of the desired water temperature to the environment and how long you are willing to wait until it is heated.

**Giulio**- October 9, 2019

is this calculation valid only when submerging the heating device in the water, or also when the water is circulating through a heating pipe using a pump?

Wouldn’t it be faster in the latter case, because the water flow would exchange heat more efficiently than just by convection?

I’m trying to figure out what would it take to heat my pool using a heating pipe after the filtration pump.

Thanks

**Stan**- September 11, 2019

This is the best set of calculators on the internet 🙂

Thanks again!

**Iggy**- July 16, 2019

Howmuch joule are needed to boil water.

So from going from liquid to gas.

I already know howmuch +1°C needs, but now still need to know howmuch energie is needed for vaporize, So I can calculate howmuch energie it takes when , I put it in hot liquid metal.

**MtnDoo**- April 22, 2019

**RM**- March 31, 2019

I’m struggling to calculate how many BTUs that is per hour. I’ve come up with two separate answers: 41,000 BTUs/Hour and 298,800 BTUs/Hour. I’m not confident either are correct.

All I know is my wok is very very hot and almost unusable. Can you help me find the answer given the information above?

Reuben

Houston, TX (essentially sea level if that matters)

--Email removed--

**antonio**- January 5, 2019

**Larry Moore**- January 5, 2019

**antonio**- January 5, 2019

it takes the same amount of energy to heat water from 48 degrees to 52 degrees as it takes to heat water from 58 degrees to 62. But when the state of water changes from solid to fluid (e.g. -2°C to + 2°C) or from fluid to gas (e.g. 98°C to 102°C) this does not hold true any more. It would be more complicated to build calculators that can handle that and I have not done so. This is why the calculators complain in these situations.

The calculators cannot handle points or commans, only whole numbers. So they may complain when you input commas or points.

Update 2023: The calculator can now handle dots or commas.

**John McGrath**- November 14, 2018

**Haroma**- November 6, 2018

**Allen**- July 14, 2018

**Geir**- July 5, 2018

Would this formula be correct to use for a heating loop?

I have a heating loop with a average deltaT of 1°C over a period of 22hours and a rated flow of 27,5 l/s.

Using this calculator i get 1381 kWh

is this the formula used?

volume in litres x 4 x temperature rise in degrees centigrade / 3412

**Jonas Ntiako**- May 28, 2018

**Anonymous**- May 15, 2018

**antonio**- April 24, 2018

that is impossible to calculate with the information given. It very much depends on the surrounding temperature, the heat conductivity of your hot tub material, and the shape of the tub.

But I have an idea. If you can test how fast the water cools down in your tub, then you can calculate how much heating power is required to prevent that from happening. Let’s make a calculation example for a tub with 250 us gallons that cools down from 105f to 103f in 2 hours. Using the “Water Heating Power Calculator” above (250 us gallons, start temperature 103f, end temperature 105f, 120min) tells us that a heating power of 611 watts is required. If you use a 611 watts heater in this example, then it will be about enough to reverse this cool-down from 105f to 103f by heating the water back up from 103f to 105f during the same period (realistically, of course, the water does not cool down, but stays at the same temperature).

That’s just an example, I have no idea how fast a tub cools down. Also, you should have some extra power available, just to be sure. And for testing/sampling the cool-down, measure a larger drop in temperature, such as 10f, else the result will be unreliable (reading 1f or 2f changes off a small thermometer may be very inaccurate). And as always, double check with some other source and my help is without any guarantee or similar.

**Willaim**- April 24, 2018

**David**- April 5, 2018

**antonio**- March 18, 2018

I have done the calculations in the calculator above for heating 33’000 US gallons from 68F to 90F in 5h (300min) and got a result of approximately 1’200’000 btuh.

I have done the calculation again manually in an Excel spreadsheet and got the same result.

**New pool, cold water !**- March 17, 2018

I’m looking at a 50,000 btu boiler with a heat exchanger to heat a 33,000 gallon pool to 90 degrees f.

I’ve run then calculation through the formula above, and it’s coming up with a 600,000 btu boiler to raise the water temp (68f) in 5 hours to 90f, which seems to be a really high number?

**Sven**- January 13, 2018

**Frank**- December 30, 2017

**rox**- November 19, 2017

**Brian**- November 2, 2017

**Dave**- March 19, 2017

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Juha- February 26, 2024