Kill a watt, how accurate?
Wanting to know just how much power the window shaker uses, I bought a Kill a watt. It's been to cold to test the AC but most things I have tested are under the rated watts. The 1875w hair dryer w...
Forum Discussion
Likely just due to voltage drop.
Household appliance ratings are at the nominal voltage rating of 120 volts. The wattage and voltage will be on a label on the appliance. The voltage can drop depending on what/where you are plugged into and the higher the appliance rating, the higher the voltage drop will be. Every appliance I have checked at home and in our TT are marked as 120 volts. Sometimes something with a motor only in it is rated at a lower voltage. Utility supply companies deliver voltage in accordance with ANSI C84.1 at nominal 120 volts in the US and Canada.
And... Wattage output of an appliance (except for AC units) will vary as the square of the voltage. For example an appliance at 5% voltage drop ((114 volts) would lose almost 10% of it's rating. (0.95 x 0.95 = 0.903)
Working backwards from your 1875 watt hair dryer, the 1660 watt reading could be due to a 6% voltage drop which could very well be the reason for the "error" at the higher wattage items you checked. The oil-filled heater would suggest a voltage drop of 4% which would make sense due to it's lower wattage rating and current draw. The 100 watt light bulb would not draw much current compared to 1500+ watt appliances and could easily not affect the reading on the Kill-a-watt.
I would suggest that you compare apples to apples and see what the voltage says between different items you test. What are you plugged into? If at home and you're plugged into an adapter on your RV cord and a long skinny gauge extension cord, you WILL get false readings. The false readings on the higher wattage appliances would support this theory.... Voltage drop in wiring is cumulative - the shore power cord, an extension cord, the branch circuit wiring in your house and even the wiring from your panel out to the pole & transformer on your street. And we all know that voltage can dip a LOT in CGs too.
BTW, why are you using a 100 watt incandescent bulb these days? You ought to be testing with a CFL lamp, lol... :W
Household appliance ratings are at the nominal voltage rating of 120 volts. The wattage and voltage will be on a label on the appliance. The voltage can drop depending on what/where you are plugged into and the higher the appliance rating, the higher the voltage drop will be. Every appliance I have checked at home and in our TT are marked as 120 volts. Sometimes something with a motor only in it is rated at a lower voltage. Utility supply companies deliver voltage in accordance with ANSI C84.1 at nominal 120 volts in the US and Canada.
And... Wattage output of an appliance (except for AC units) will vary as the square of the voltage. For example an appliance at 5% voltage drop ((114 volts) would lose almost 10% of it's rating. (0.95 x 0.95 = 0.903)
Working backwards from your 1875 watt hair dryer, the 1660 watt reading could be due to a 6% voltage drop which could very well be the reason for the "error" at the higher wattage items you checked. The oil-filled heater would suggest a voltage drop of 4% which would make sense due to it's lower wattage rating and current draw. The 100 watt light bulb would not draw much current compared to 1500+ watt appliances and could easily not affect the reading on the Kill-a-watt.
I would suggest that you compare apples to apples and see what the voltage says between different items you test. What are you plugged into? If at home and you're plugged into an adapter on your RV cord and a long skinny gauge extension cord, you WILL get false readings. The false readings on the higher wattage appliances would support this theory.... Voltage drop in wiring is cumulative - the shore power cord, an extension cord, the branch circuit wiring in your house and even the wiring from your panel out to the pole & transformer on your street. And we all know that voltage can dip a LOT in CGs too.
BTW, why are you using a 100 watt incandescent bulb these days? You ought to be testing with a CFL lamp, lol... :W
