Power consumption of heater when temp reaches set point

When the temp in a tank reaches it's set point indicated by an particular color light on the temp controller, does that mean mean the unit is still using the same wattage? I'm assuming not but thought I'd ask as I'm not an electrical whiz. Since I'm using a 1000 watt heater my hope is once the temp reaches the set point the only power used is for the light indicators.

Edited by az9, 16 January 2012 - 12:19 PM.

Although it may use a bit more than the indicator light requires it is nowhere near the 1000 watts used when heating. It shouldn't use much at all, but it's it's hard to tell sometimes. Still at worst I wouldn't expect more than a couple of watts.

The largest use of energy will be to get the tank up to the set point initially from room temperature, yes. But the heater will need a constant or intermittent amount of energy to keep the water at that set point. Equilibrium rules state that the hotter object (the aquarium) will constantly be giving energy to the cooler object (the room), so in order to keep one hotter, you have to continually add energy to the aquarium. How your heater emits heat, and how much energy it uses and how much money that energy costs you, depend on the type of heater and what controller it has.

If you don't mind, I'm going to reference the first day of my Process Control class and define some terms for you.
Set point: The temperature you want the tank to have. You input this number into the controller. For example, 70 F.
Feedback control: The controller waits until there is a problem to correct it. For example, the temperature is too low, so the heater turns on. Or the temperature is too high, so the controller turns the heater off.
There is some lag time between when the heater turns on and when the temperature starts to increase. There is also some lag time between when the temperature reaches set point and when the heater turns off.
Feedforward control: The controller anticipates that there is going to be a problem in the future based upon the current rate of things, so it tries to compensate. For example, the heater can tell that the water is cooling faster than the heater is emitting heat, so it emits more heat to prevent the overall temperature from changing.

Sometimes your heater is more expensive and has a controller in it. Sometimes your heater is less expensive and lacks a controller. This second type of heater just emits different amounts of heat, and you increase or decrease how much heat it's emitting yourself to try to keep the temperature stable.
Personally, my own heater lacks a controller, because I opted to spend less money initially and got the cheap-o heater. It's got a 'on' mode, and an 'off' mode, and a 'hotter' mode and a 'colder' mode, with ranges of hotter and colder. If your room's stable, you'll quickly find how much you have to turn the hotter/colder dial to keep it at a fairly constant temperature in the tank. Then you keep an eye on it and check it every day to make sure it's still the temperature you want. But it can get 10 degrees too hot or too cold if you're not paying attention. It's not a big deal in my particular setup because I want it as hot as humanly possibly without cooking the fish. You don't really need a controller to keep the heater on 'MAX'. But not everyone's looking for an 85 F tank. If you're aiming for 70 F, you might not like it if the temperature dropped to 60 or rose to 80.

A controller would keep the tank at a more constant temperature, eliminating wild fluctuations. A feedback controller would have some fluctuation in temperature, as the controller needs to read that the tank is colder than the setpoint in order to turn on. Then there would be a period of time where the temperature continued to drop before the heater was hot enough to impact water temperature. A feedforward controller would probably have the smallest fluctuations, because it can modify heat output before a dramatic temperature decrease occurs.

Anyway, long story short, you're going to have to expend energy over time to keep your tank hotter than your room. If you stopped expending energy, the temperature would gradually return to room temp. So the heater will be on at times. But how much that heater is on depends on how good your controller is. The most wasted energy would be with no controller at all, where sometimes the tank gets 10 degrees too hot because you're not paying attention. If the tank gets above setpoint and is too hot, then that extra heat is energy you didn't need to spend. That's why controllers matter. Every degree the water raises above set point is money you didn't need to spend, so tight temperature control saves you money.

Edited by EricaWieser, 16 January 2012 - 01:41 PM.

The only way a thermostat turns the heater off at the setpoint is by interrupting the circuit through the heating element. Mywan is correct - even "a few watts" seems like a very high estimate when the heater is off.

I doubt your heater has a controller as sophisticated as what Erica is talking about, or has a need for it if the tank is indoors.

And Erica, you can accomplish the same thing fairly cheaply by placing more than one heater in the tank. The "hotter/colder" dial you mentioned in a standard aquarium heater is just an adjustable thermostat setpoint, so if you had three heaters set a degree apart, say 69/70/71, then the one with the 71 degree setpoint could be on all the time, the 70 degree heater would cycle as needed to maintain 70 degrees, and you'd have a spare to pick up heavy load if the room temperature suddenly dropped. If room temperature rose unexpectedly, they would just all turn off.

The question is "Is your heater big enough?"
Yes - next question.

What is the rate of heat loss?

Energy consumption depends on the rate of heat loss from your tank.
Heat loss will be from
water changes
water evaporation
conduction
radiation
and convection.
(Even dry air can move a lot of heat if there is vertical height for a downward draft to form.)

for a taste see
http://www.freestudy...fer/convrad.pdf
http://www.engineers...ainer-calcs.htm

Thank you all for your responses especially Erica for your in depth response. Damn girl you make me jealous with your smarts!

Mine does have a controller.

I just realized I could also answer my question with a watt meter, which I bought a while back and forgot I had. If anyone is interested I'll let you know what I find out.

My 1000 watt heater sits in one of two fish tanks which is the 300 gallon one with bluegills vs. the other 200 gallon one for yellow perch. Both tanks share a biofilter, and each have their own 50 gallon clarifiers.

Temp is set for 75 F. which it seems not to have too much trouble staying at a set point in the bluegill tank. The yellow perch tank is 2 or 3 degrees cooler which is fine.

All fish feed like piranhas 3 X per day.