Hey, all, I'm trying to check something and I've come up with a number and I'm not sure I've done my math right. What I want to do is figure out how much a device is costing me to operate in electricity charges. I'm not sure I'm doing the wattage calculations correctly (I'm not well-versed with AC calculations and have only light hobbyist experience with DC), and I'm not sure I'm going from the wattage to the kW hours correctly.
I have a device that runs on 120v AC. I put a DMM in line with one of the two conductors in the power cord and I measure 16.56 mA (my DMM has a setting for measuring AC amps and that's what I used). I then used the same DMM to measure AC voltage at the receptacle and I measured 124.8 VAC. I'm not sure if this DMM does true RMS (as I see that some manufacturers advertise that their meters measure "true RMS") and the manual does not say yes or no.
Now, I take the amps times voltage to get watts so: 0.01656 x 124.8 and I get 2.066688 watts. I get kWh by 2.066688 x 24 hours / 1000 (the device is on constantly) and I get roughly 0.0496 kWh per day. Say the billing cycle is for 31 days so that's 0.0496 x 31 = 1.5376 kWh each month and all I have to do is multiply that by my cost per kWh and I should be good (for simplicity, say it costs me 10 cents per kWh it's 15 cents a month).
Did I do all that right or am I mucking up the math or the wattage calculation for AC?
Thanks in advance.
--HC