In The Business Of Designing & Manufacturing Energy Consumption Monitors !
How To Select An Energy Monitor
Don't Get Duped!
Certain Cost of Electricity Meters or Monitors are Reporting Inaccurate Information:
These meters or monitors only read the current flow of the service panels. These devices make an assumption of the voltage and power factor and falsely display "KWh" or cost of electricity. The measure of kilowatt-hour is based on true power which is the product of Current (I) x Volt (V) x Power Factor (PF).
How False is the reported electricity cost or KWh?
The amount of error depends on the type load being monitored and how much your line voltage deviates from the assumed voltage, (usually 120V or 240V ). The table below shows the expected error for various loads. This calculated data is based on power factor data typical of various appliances.
I can only speculate here, using some common sense. Measuring current and assuming a power value based on current is very easy and cheap to do. If this was a valid way of doing things, our Energy Consumption Monitors would cost much less. So for obvious reasons, I would guess that they want to make some quick money and not really interested in helping the environment.
How can I determine which type of Monitor or Meter reports such mis-leading information?
If the meter or monitor does not access the line voltage, either by direct connection to a panel circuit (not to be confused with a sensor clamped around a panel wire) or accessed via some type of device or transformer plugged into an outlet, you can be assured that you will get false information such as that of monitor type B example in the chart below.
Most of the time, the ad for these type of monitors will "dance" around the proper terminology to make it appear that these are reporting cost based on true power or KWh. In some cases, when it comes to internet re-sellers, I have gone as far as emailing some them to point this out and received a response saying they were aware, but not making any changes.
Usually you will find in their ad (usually small print), a statement stating that this monitor/meter is not to be compared to your electric meter or energy bill.
The web sites dedicated in selling some of these "false-meters", usually will never bring up the term "true power", "real power" or "true RMS power".
Finally, if you are not sure, ask them directly if it displays true power. Also ask them if you can return it if it doesn't display "true power".
How False is the reported electricity cost?
The amount of error depends on the type loads being monitored and how much your line voltage deviates from the assumed voltage, (which is 120V for one particular company)
What impact can using this type of inaccurate Monitor or Meter have?
The error will typically indicate that you are using more energy than you actually are. Many of our customers use our energy consumption monitor to audit the energy/efficiency of various loads and determine if the pay-back from the electricity savings justify replacing that equipment. Unfortunately, using data from these "false meters" could cause some people to discard their old refrigerator, freezer or air conditioner for no reason, since the reported energy is greater than the actual.
Lets assume there is a problem with the house electricity meter in favour of the electric company and you are getting charged more than you should. You decide to compare the KWh from your "false meter" with that of the electric company. Based on your (false) findings, it appears that you are paying less than you should for power, when in fact the opposite is happening.
You may make some daily routine changes in an attempt to reduce your energy consumption and because of certain power-factor shifts these changes appear to make no improvements when you indeed are.
Aren't you just putting down your competition to try to increase your sales
Although any company wants to increase their sales, we are doing this mainly as a public service. In fact, our main competitor does NOT fall into this category as their meters measure "true power" as ours do. Although we can make a list of such companies, we believe in being ethically responsible by not directly putting down any competing company, but making people aware.
Conclusion:
If I asked you if 22ºC (72ºF) temperature is comfortable, you may reply "yes". Now I urge you to fill your bathtub up with 22ºC (72ºF) water, submerge yourself and respond again to that question. Exit the bathtub in a 22ºC (72ºF) room and answer again. The point I am making is that you need more than just temperature to determine comfort. The same applies to energy. You need more than "current" to determine energy. Power factor plays a big part such a humidity in the case of comfort.
Expected Error when using The Wrong Type of Energy Monitor
Based on the "Typical Power Factors" of appliance chart from a government study found on this site(p.23):
http://www.energy.ca.gov/reports/2002-01-11_600-00-039/600-00-039_NOAPPENDICES.PDF
The table below shows how much the KWh value error is expected for those Energy Monitors displaying the "cost of electricity" using current measurement only and assuming a predetermined (usually 120V) voltage for their calculation.
The "false meters" will also suffer additional error due to:
The ratio of assumed voltage to actual voltage. (since many of these devices assume 120V and the actual residential voltage is generally less than 120V, this error will increase the error further by the ratio of 120V/actual Volt)
The current transformer accuracy.
The current transformer linearity, typically poor for low cost types.