Question

I need to figure out cost of running 4 appliances for 72 hours. I've done the calculations myself, but I would like someone to go over it and check I've done it right. Since openstudy has a character limit for posts, I was unable to write everything here, so here's the link to the remainder of this post: http://justpaste.it/x7c

Answer 1

the math checks out.
not sure about whether the 230V appliances make any difference

Answer 2

The wattage of the appliances which are rated at 230 VAC but are connected to 240 VAC needs to be computed using the actual voltage and resulting current with the higher applied voltage. This can only be done with the measurement of the current (assuming the voltage is actually 240) and multiplying by the actual resulting current.
Other considerations are the type of appliance, for example, a refrigerator operates intermittently controlled by a thermostat and depends on setting, number of times the door is opened etc. As dumbcow pointed out the math checks out. And since the increased power consumption of the 230 volt appliances are probably offset due to there varying use, you are probably pretty close. Another consideration the appliances with motors create a power factor which comes into play when converting Volt-Amp to watts.

Answer 3

I would multiply the current by the proportion 230/240 to get the current at 240V.

Answer 4

At anyrate the power consumed will be the same a 230V and 240V.

Answer 5

Other words you can ignore the increase of current at 240 volts..........I think not
An appliance rated at 230 volts, that draws 3.3 amps represents a load of (using Ohms law) R=E/I=230/3.3=69.7 Ohms. Now place 240 volts across that 69.7 Ohms and calculate the current (I=E/R) I=240/69.7=3.44 Amps. Please note this is more than 3.3 when connected to 230 volts. The power would be 240 X 3.44 or825.6 Watts assuming PF of 1.

Answer 6

Your proportion would work, but you have it upside down, the current increase would be 240/230 ( current would increase)

Answer 7

That's interesting, I hadn't thought of that. Ended up only being about $3 more, but still much more accurate. Thanks.