# how much heat i need to reach constant temperature

Is there an equation I can use to calculate the heat required (as a function of time) to sustain water at some specif temperature?

I already know how much heat I need to reach the desired temparature using the formula Q=m*cp *(T2-T1). My question is how much heat do I need to keep 30,000lts of water at 70degress celcius for 30min once i have reached such temperature.

Any help would be greatly appreciated

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You need exactly the same power as is lost from that 30 m³. –  Georg Dec 1 '11 at 0:03
This is highly dependent on what type of Object you are heating your water in. To be more specific, where could the heat escape to? Example, heating in a pot would give you multiple heat-loss opportunities: Heat-Transfer via Air and Radiation (Top of the pot) and via the pot itself. –  Michael Dec 1 '11 at 0:15

The amount of energy you need to maintain the temperature of water depends on how fast the heat dissipate in your system.

So in your case, if you use a tank to keep 30,000 lts of water, you should take the heat conduction from water to tank into consideration. If you assume the heat conducted from water to tank in a certain amount of time is $H$, then you need to use a heater that has a power of

$P_{heater} = \frac{H}{t}$

to keep the water warm.

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