In order to work this out for yourself it is useful to include the units in your calculation.
Your equation (P=U*I*cos(phi)*sqrt(3)) is going to produce a result which has a unit measure in watts. Watts is defined as Joules/Second. Watts is NOT a measurement of energy, it is a measurement of an energy usage rate. Joules is the primary measurement unit of energy. By measuring the rate over a period of time and then multiplying by that time, we convert the rate, watts, into an energy quantity, Joules. A Joule is a pretty small quantity of energy and so it can produce cubersomly large numbers when metering energy. So by convention we use KWH. Now look at the unit KWH. What does it represent? Lets break it down.
KWH is 1000 Joules/Second * 1 Hours.
The Second unit and the Hours unit are both time units, therefore they will cancel each other out, leaving us with an energy unit. We can rewrite it this way:
1KWH = 1000*Joules/Second * 3600 Seconds.
1KWH = 3,600,000 Joules
Now on to metering. Metering involves totalizing over time. The trick is to find a time slice over which to meter. If the time slice is 1 second then our energy consumption during that time slice can be calculated as Watts * 1/3600 hours. (1 hour has 3600 seconds - But totalizing with 1 second intervals is going to produce a rounding problem - can anyone tell me why?)
If we pick a 3.6 second time interval then energy consumption becomes Watts * .001hours. A 36 second time slice gives us Watts * .01hours.
The trick is in determining the proper time slice. If your load changes frequently, the faster the sampling rate the better the metering.
If you are metering with a stopwatch and a pencil and paper then obviously your sample rate will be somewhat long. There are cases where this is just fine however - such as trying to establish standardized work center rates for allocating manufacturing costs - something which is not recalculated frequently and for which real time results are not necessary.
If you are metering with a PLC, then you can choose a more suitable sampling rate. Metering with a PLC or DAQ system is not a suitable means for billable metering, but for internal use it is often more than sufficient. For the most accruate metering you need to use in instrument designed specifically for metering. These instruments continuously integrate the instantaneous power rate, by both mechanical (like your home power meter) and electronic means.
Try this out: How many KWH does a 100 watt light bulb use if it is on for 1 minute? If you can work that out and get 0.001667 as your answer, then you have what you need to know to solve the bigger problem.
