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Step 9Power Calculations
At this point you should have a system that outputs a number between 0 and 1023 corresponding to a voltage from 0 to 5v that in turn corresponds to the amount of current measured by the AC clamp. With a little math, we can calculate the AC current from the widget value.
CircuitVoltage = WidgetValue / 1023 x 5
I = (CircuitVoltage - 1) x 100 / Gain x CorrectionFactor
where Gain is the gain of the op-amp and assuming you are using a 10mV/A setting on the AC clamp. It's "CircuitVoltage - 1" because there is a 1v offset provided by the potentiometer.
To convert this to Watts, the math gets a little fuzzy. A number called the " Power Factor " is needed. The power factor is different for different appliances (and even changes for that appliance depending how it's being used.). Some appliances use voltage and current in phase with each other. This is the case with a heater for example. An electric heater would have a high power factor, maybe 90%. Other things, like computers, use power differently and their voltage/current demands are not in phase. In these situations, the power factor is lower, maybe 35% to 50%. Your entire home is a combination of all these devices with differing power factors. In order to calculate power usage in watts from voltage and current, you'll need to make a guess at the power factor. Most people use 60% as a good estimate for a home's average power factor. I used 75% in my calculations. It's important to point out that the power factor of you entire home is not constant but changing all the time. To get power use the following:
Assuming V = 110v (US homes)
P = V x I x Power Factor
CircuitVoltage = WidgetValue / 1023 x 5
I = (CircuitVoltage - 1) x 100 / Gain x CorrectionFactor
where Gain is the gain of the op-amp and assuming you are using a 10mV/A setting on the AC clamp. It's "CircuitVoltage - 1" because there is a 1v offset provided by the potentiometer.
To convert this to Watts, the math gets a little fuzzy. A number called the " Power Factor " is needed. The power factor is different for different appliances (and even changes for that appliance depending how it's being used.). Some appliances use voltage and current in phase with each other. This is the case with a heater for example. An electric heater would have a high power factor, maybe 90%. Other things, like computers, use power differently and their voltage/current demands are not in phase. In these situations, the power factor is lower, maybe 35% to 50%. Your entire home is a combination of all these devices with differing power factors. In order to calculate power usage in watts from voltage and current, you'll need to make a guess at the power factor. Most people use 60% as a good estimate for a home's average power factor. I used 75% in my calculations. It's important to point out that the power factor of you entire home is not constant but changing all the time. To get power use the following:
Assuming V = 110v (US homes)
P = V x I x Power Factor
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Feb 21, 2009. 6:17 PMsydelbow
says:
lol I need you to come to my house and create this again. It's fabulous, I wish I understood most of it. Does it come in handy for you? Have you discovered what takes the most power and what you can unplug? I'm so curious about my own home.
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