Something I Noticed About Electric Consumption

Over the hot spell I noticed that my electrical voltage dropped back to around 115 volts in the day. At night it returned to around 120 volts. Now it may seem like 4.17%. isn't a big deal, but consider this. The average 10,000 air conditioner uses from 8 to 10 amperes. The wattage (that which you pay for in your electric bill) used is between 960 watts and 1,200 watts to power this size air conditioner when voltage is at 120 volts. However at 115 volts the power consumed then becomes between 920 watts and 1,150 watts. (Power=Voltage x Current).

When the utility company reduces power the wattage that you are billed is reduced accordingly. This on the surface seems fair since I am getting less I am therefore paying less. However here's the rub. At 115 volts you are getting only 9,583 BTU's worth of cooling out of the 10,000 BTU's the air conditioner was designed to deliver. OK, so you say what's the problem with that, the AC will run 4.17% longer to get to where it needs to go. NOT SO!

I don't want to get all technical, but the fact of the matter is that electric motors lose efficiency (horsepower) when their voltages are reduced. This isn't in direct proportion to the 4.17% we are discussing. It exceeds that.



Air conditioners lose further efficiency in another way. Again without getting all technical. There is a certain amount of refrigerant allowed to flow between the high side (condensation coil) and the low side (evaporation coil). If this flow is reduced (by less horsepower coming from the compressor motor) there is further reduction in an AC's cooling efficiency in most home units.

As you can see when the power company drops back just 5 volts AC's suffer in two ways. The compressor motor not only produces disportionately less horsepower, but in turn also delivers less refrigerant at lower differential pressures then it does when voltages are at 120 volts (where it was designed to run most efficiently). This results in less cooling even if it runs 4.17% longer.

It may very well be that power companies need to protect their grid with 'brownouts'. But it doesn't hurt their bottom line either if customers are using more electric then they would have otherwise at 120 volts. Summertime also is usually the time they shut down turbines for maintenance. Maybe this helps pay for that, eh?