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Check My Math please

743 Views 7 Replies 7 Participants Last post by  cbennett
I have just received my electric bill for this month. It has doubled from $62 in December to $122 in January. I changed my light from a 15W from the pet store to an AH supply 36W Bright right at the beginning of January. This is probably the heating in my apartment which isn't working right, but I wanted to check that it isn't the light.

Billing Period: 28 days
Price per KWh: 14.4 cents (average)
Light Hours per day: 12 (it is less than this but lets err on the large side)

Now according to my math this should be

36/1000 = 0.036 KW/h
0.036 * 28 * 12 = 12.096 Kwh
(12.096 * 14.4c) / 100 = $1.74

Seem right?
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check your bill, I bet they came out and didnt estimate this time. I they probably misread by 1000kwh.


Just found out that someone else in my apartments had got one for over $1000. That is more than the rent.
Without even looking at your math I can tell you that a 36 watt bulb left on 24/7 would not raise your electric bill that much. Two things (at least) could have happened: you have a program where you pay about the same every month for electricity, the average monthly use, but at the end of the year they make you catch up by putting all of the amount not yet paid on the one bill (this happened to me). Or, you have at least some of your heat from an electric heater, and the cold weather made it work hard. A 1500 watt heater can quickly drive up the bill.
The 36 watt fixture isn't 100% efficient, so maybe use 42-46 watts for an estimate of power usage (just a guess). Some goes off in the form of heat.

Still a fairly small amount of power usage compared to a $112 bill. A desktop computer turned on 24/7 will suck up more power.
Checks out (with unit analysis)

36W/1000 = 0.036 KW
0.036KW * 12 hr/day * 28 days = 12.096 Kwh
12.096Kwh * $0.144$/Kwh = $1.74
You need to factor in the filter (24hr/day) a heater (?hr/day) etc but the difference between a 15W and 36W lamp is not the sole factor in your increased bill.

I am still looking for a kill-a-watt to measure actual power consumption but I doubt that it is as bad as CSF's approximation. (Ie the advance 4X 32W T8 draws ~8W with no bulb connected)
My meter was mis-read once - raised the bill by about $75. I had them come and re-read it - that adjusted the bill back to where it should be. Take a look at the bill - it should have the meter readings on it if your meter is read every month. Then go check for yourself - yes it is a few days later, but it should still be near the final meter reading on the bill. If it is below the reading on the bill, they read it wrong.
Our january bill is always much higher and this year was especially cold. Your computer uses more electricity than that. Keep in mind they estimate usage every other month and then you make up the difference the next month. Look at the electric bill from last year and see how much you used, probably not that much different.
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