Watts So Confusing?


Most motors are designed to a set amount of work, usually rated in either watts or horsepower (746 watts per HP).

Watt's law states that watts = volts x amps. If a particular motor needs to do 1 horsepower of work at 120 volts, it will draw about 6.22 amps. And yes, in an inductive load like a motor, it's not quite as simple as V x A = P, but we are keeping it simple here.

A motor designed to do the same amount of work (1 HP) at 240v will draw half the amps (3.11).

That does not make the second motor “more efficient” because the power company charges by the kilowatt, NOT by the amp.

If you take a load that is designed for a particular voltage and DROP the voltage, it will also decrease the wattage according to Watt's law (watts = volts x amps). The amperage will also decrease, according to Ohm's law (so long as the resistance remains the same).

Let's say you take a 5kW heat strip rated as 5kW at 240v, and you connect it to 120v instead.

It would then only produce 1.25 kW and draw 1/4 of the amps. While we may call it a “5-kilowatt heater,” it is actually just a fixed resistor designed to do 5 kilowatts per hour of work in the form of heat at 240 volts. Cut the volts in half, and you also cut the amps in half. You decrease the amount of work done down to 1/4 because watts = volts x amps.

—Bryan

P.S. – If you want to learn more about horsepower, watts, and other motor-related power terms, check out this HVAC School article written by Steven Mazzoni.

 

Related Tech Tips

Refrigeration Without Refrigerant
We just wrote about rejecting heat to the atmosphere via radiant cooling. That’s one example of cooling without refrigerants, but there are quite a few others out there. In this article, we’ll look at some other cooling methods that don’t use refrigerants. Vortex tubes Vortex tubes swirl gas in a chamber, separating it into hot […]
Read more
The 3 Fan Laws and Fan Curve Charts
This article was written by Tim De Stasio with Southern Comfort Consulting and Service. It was originally published on his blog, which you can access by clicking THIS link, but he permitted us to republish it on HVAC School as well. Thanks, Tim! It is critical for an HVAC technician to understand airflow and how […]
Read more
A REALLY loud compressor
Yesterday, I walked up on one of our managers who was talking to a junior tech diagnosing an intermittent controls issue on a pool heat pump. In the background, you could hear an EXTREMELY loud compressor. The junior tech had just been moving some wires around, and the next thing he knew, there was a […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from