Need to calculate the power from the output %


Thread Starter

Terry Sopkow

I have an omron controller that is setup as on/off control. It is controlling an electric heater. How do I calculate the power (watts) from the output% value that is displayed. Is it simply a percentage of my heater wattage?

Alan Hartwell

For an on/off controller, the instantaneous power is either 100% of the heater wattage when on, or 0% when off, never anywhere in between.
The average power would be the output percent times the heater wattage if secondary effects such as change in heater resistance with
temperature are negligible.

Robert Scott

Yes, as long as the cycling time is long enough for the power to stabilize early in each cycle. I would expect a straight resistance heater
to be quite uniform in this respect - not like a light bulb, for example, which takes more than its rated power during the first few
milliseconds of on-time before the filament has come up to operating temperature.
Yes and no. If you have a 10KW heater load, and the % output (at a given time) is 50%, then 5 KW is being used. However, since controller % output will be shifting over time over a range of values a simple percentage won't do if what you want to end up with is a record of power usage over that span of time.

In that case it would be necessary to integrate power usage. One way to do this is to add up the amount of time the output is turned on, and divide that into the overall sample time. How often this is done depends on what you want to do with the information. A 1 or 5 minute sample period might be desirable if using KW usage to tune a process for least energy usage, but could well be updated every hour, day, or over a the time representing a product run if the object is to track longer term usage trends.

Curt Wuollet

Should be pretty close if the frequency is low enough to ignore the
half cycle uncertainty for zero crossing switching, etc..



Bruce Axtell

First, I am assuming that the controller is truly on/off as you state. Secondly, you didn't state the heater size (KW & V). On most typical on-off controls, the output of the controller itself is generally either a relay contact or triac switch. The controller output usually switches a contactor (for larger heaters). In such a case, it is not good to cycle most contactors faster than 2-3 times per minute. If the heater can be directly switched with SSRs or SCRs, then switching on-off can occur much faster.

In any case, when the heater is on, it's on 100% and when it's off it's at 0%. Therefore, actual power consumed is simply a time-weighted average. If the heater is 10KW and is on 30% of the time, your consumption is 3KW. However, during the ON time, the consumption is still 10KW as far as your utility is concerned.

If your concern is over peak-time utility charges, your utility meter will always see 10KW when the heater is on. To avoid that, you need a different control approach.

That said, (and I'm reading into your message) your question was "how do I calculate the power from....the [controller] output %?" Typical ON-OFF controllers do not normally provide an output percentage. They are either on or off. So I question if your controller is truly on-off.
Time-proportional controllers will give an output percentage, but have different output switching strategies. However, a resistance heater is still just that, and conforms to ohms law. Its power is simply a function of the voltage and resistance (thus current).

Bruce Axtell
Engineered Control Systems
763-712-5477 fax
[email protected]

Anthony Kerstens

No. SSR's (triac) switch at the zero crossing. That means that that the value of your power output is discretized to every half cycle.

You can calculate it _roughly_ as a percent of heater wattage (at the rated voltage, and frequency) with an error of +/- 1 cycle (1/2 at the start, 1/2 at the end).

Anthony Kerstens P.Eng.