The Energy Audit-Ohms Law

As an energy auditor, I am often in homes because of high electrical bill complaints.  Conducting electrical testing and calculating energy costs are part of my job.  There is a formula for figuring these electrical costs.  This formula requires some information about the device that is being tested.  This blog is going to discuss some basic electrical principles, a formula called ohms law, and show some examples of how to figure electrical costs.

Let’s start with some basic electrical definitions.

Voltage – According to Wikipedia, voltage is the difference in electrical potential energy between two points per unit electric charge.  Simply, it is a pressure that pushes charged electrons through an electrical circuit.  Almost all voltage in residential construction in the United States will be 120/240 volts.  Often represented by the letter E for electromotive force.

Amperage – This is the strength of an electrical current, often referred as an amp.  Over current devices such as breakers or fuses and wire sizing are all based on the amperage of a circuit.  100 to 200-amp main electrical service panels are common sizes in residential construction and 15 or 20-amp branch circuits are common for lighting and plug loads.  Current is represented by the letter I.

Resistance – According to Wikipedia, electrical resistance is a measure of the difficulty to pass an electrical current through a conductor.  The concept is similar to friction and is measured in ohms.  Resistance isn’t a very common measurement I use in an energy audit, but occasionally it is needed in calculating the cost of power.  Resistance is represented by the letter R.

Wattage – Wattage is defined as the rate of doing work and is represented by the letter P.  Wattage is the measurement of how electricity is billed and is the most useful electrical information to an energy auditor.  Electricity is billed at a kilowatt hour, or 1000 watts used in an hour.

Now that we have the basic terminology for electricity, lets look at the formulas needed to calculate the cost of power called Ohm’s Law.

Back to Wikipedia, Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.  Where I is the current through the conductors in units of amperes, E is the voltage measured across the conductors is units of volts, and R is the resistance of the conductors in units of ohms.  P symbolizes power, measured in watts.  The formulas below represent the equations that are Ohm’s law:

                                        E/I=R                   E/R=I                   IxR=E
                                        P/I=E                   P/E=I                   IxE=P

All electrical devices and appliances are required to have electrical information printed somewhere on the product.  The photo below shows one of these tags.
The appliance happens to be a boot dryer.  The tag states the dryer operates at 120 volts at 60 Hertz (Hertz is the frequency of power, or how many times per second alternating power completes one cycle, all power in the United States operates at 60 Hertz).  The power consumption of the boot dryer is 55 watts.  Knowing the appliances operates at 120 volts and uses 55 watts, we can use ohms law to calculate how many amps it draws, useful for determining correct circuit breaker and wire sizing, or the resistance of the product.

Working as an energy auditor, I mainly need to know how much the boot dryer costs to operate.  Let’s assume the cost of power is $.10 per kilowatt hour.  55 watts is equal to .055 kw times $.10 equals the power cost.  .055 x .1 = $.0055 or roughly half a penny per hour.

Now let’s use ohms law to calculate power cost using my Kill-O-Watt meter.

My Kill-o-watt meter

The meter is showing the appliance being tested is using 7.05 amps.  This meter will only test 120-volt appliances.  We want to know the cost of the appliance, we need to figure how many watts equals 7.05 amps.  The ohms law formula we are going to use is: I x E = P, and we will assume a $.10 per Kw power cost.

7.05 amps x 120 volts = 846 watts

846 watts = .846 Kw x $.10 = $.0846

If this appliance operated continuously, let’s say it is a dehumidifier, we could calculate how much this appliance is adding to the electric bill.  $.0846 x 24 hours per day x 30 days per month = $60.91.

Another way I gather the information need to calculate the cost of power is with a clamp-on meter.  This type of meter has a clamp that opens and fits around a wire.  The meter senses the amount of power being used by an electrical circuit, helpful if you are concerned with how much power an entire circuit is using or if you are testing a circuit that draws 240 volts and cannot be tested by a Kill-O-Watt meter.  Typically, I conduct this test in the electrical service panel.  I do not recommend this testing procedure be completed by untrained people, I am a licensed electrician with years of experience.  The test requires sticking your hand near live electrical circuits where electrocution is possible.

My clamp-on meter

There you have it, the basics of Ohms Law.  E-mail me if there is a topic you would like to discuss, need further explanation, or disagree with something I’ve said.  I’d like to hear your comments.








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