How to decide what type of generator to install: measuring powerrequirements for large appliances

Large Appliances: hard-wired or heavy duty plugs

In my last post I talked about figuring out the power (wattage) required for small appliances that have labels on them. There are other devices in your home that are more difficult to figure out and the best way to determine the power needed is to measure it directly.

Measuring wattage:
You will need to buy a Kill A Watt power meter to do this. The Kill A Watt works simply. You plug the Kill A Watt into the wall and plug your device into it. It displays the watts being used. You can also measure electricity consumed in kilowatt-hours using this meter. Keep in mind that you are interested in the peak wattage which often occurs when a device is active, not the steady state or idle wattage. If you use the lower wattage for your calculation, when the device uses more power (like when the AC compressor turns on) you could draw too much current from your generator thereby damaging it and possibly damaging the appliance also.

This will work for anything in your house that has a standard plug. That means there are appliances that you will not be able to measure.

Some appliances are hard-wired to your house's electrical system. Examples include a wall-mounted oven and the central AC system. Other appliance use a bigger plug with 3 or 4 prongs. We will discuss these two issues separately.

Hard-wired appliances:
Your best bet is to find the sticker on the appliance that has the power information since it is difficult and possibly dangerous to measure the power requirements for large appliances that are hard-wired. There is a risk of electrocution when working with live wires and this is best left to an electrician. You can look at the breakers in your breaker box for some ballpark numbers (at your own risk).

Breaker Box:
[These instructions are for the modern breaker box pictured below. They may not apply to your setup. There is an electrocution risk when coming into contact with electric power even if you are careful. Use caution. There is a risk of injury or death. By reading this, you accept all risk for damages, injury, death, etc.]

You need to access the breaker box for your house and find out which breakers control the appliance that you are interested in. There are usually labels on the breaker box, however, not always. You may have to test which breaker goes to what appliance by turning the breakers on and off until you isolate the right breaker for that appliance. Once you know what breaker you are interested in, figure out if it is a double breaker or a single breaker. Double breakers are 240 volts since they use two 120 volt live wires. Single breakers are 120 volts and use one live wire.

Here is the outside of the breaker box. The door is closed.

 This is the box with the door open. On the back of the door, you can see a table with the numbered breakers and the labels for each breaker. On the right side there is a very big breaker on top with two rows of smaller breakers on each side.

This is a close-up of the top breaker. It controls the entire panel. If your house has only one panel (like mine), it will control your entire house. You can see 200 printed on the switch. That means that this is a 200A panel. If more than 200A (which is 120 x 200 = 24kW of power) goes through the breaker it is supposed to trip (turn off) and cut power to the rest of the breakers on the panel.

 To figure out how much power your appliances use, you can look at the breaker for that appliance. This will give you an over-estimate since the breaker should always be rated for more than the appliance being powered. In this example the top four slots on the right side are labeled "oven." If you look at the breakers in those positions you will see two double breakers taking up the four slots. Each double breaker is labeled 30A. That means that the oven is sitting on a total of 60A of breakers with a maximum wattage of 120V x 60A = 7.2kW.

Measuring power inside the breaker box
So now you know how to figure out what the breaker for your appliance is rated which gives you the maximum wattage for that circuit. If you want actual numbers, you will need to open the box. This is where you could get electrocuted. Only licensed electricians should open a breaker box. I do not recommend it. I will not show pictures since I don't want to be responsible for people injuring or killing themselves trying to figure out wattage for their appliances. One device comes in really handy for doing so. It is a clamp meter. See image below:

A clamp meter is placed around one hot wire (by an electrician) in the circuit being tested and can display the voltage V and amperage A going through that wire. The power going through the wire can be calculated by multiplying V x A. One can not place a clamp meter over a power cord since power cords have at least two wires inside that have electricity traveling through them. The electricity is going in one direction on the hot wire and going in the reverse direction on the neutral wire. Since the flow is in both directions, it cancels itself out and the clamp meter will read 0. 

Measuring power for Large Appliances with non-standard plugs:

NEMA5-15
A standard plug is called NEMA 5-15. The plug and socket looks like this

NEMA 5-15P

NEMA 5-15R

The Kill A Watt works for standard plugs and sockets pictured above. They carry a maximum current of 15A with maximum wattage of 15A x 120V = 1.8kW.


For window air conditioners and other large appliances, different plugs and sockets are used. They carry 20A - 30A in most cases. They are usually 3 pronged like the NEMA 5-15, however, some appliances are 4-pronged.

NEMA 5-20
This is for heavy appliances requiring 20A of power. It is three pronged with one hot wire, a neutral and a ground. One of the prongs is sideways so that it will not fit into a standard 15A outlet. Maximum wattage is 20A x 120V = 2.4kW.

NEMA 5-20

NEMA 5-30
This is also a 3 pronged plug but it has a higher amperage. It is rated for 30A. This is a twist-lock type of plug with projections coming off two of the prongs. Maximum wattage = 120V x 30A = 3.6kW.

NEMA 5-30P

NEMA 14-30
If an appliance uses a 4-pronged plug, it is usually 240 volts. Two of the wires inside the cable will be hot and will carry 120 volts each. There is one neutral wire and also one ground wire. The 14-30 is rated for 30A. Since it has 2 hot wires carrying 30A each at 120V, the total maximum wattage is 2 x 30A x 120V = 7.2kW.

NEMA 14-30P

NEMA 14-30R

NEMA 14-50
This is the highest power you will see, I think it is more of a commercial type connection. It has 2 hot wires with 50A rating. No picture shown.

Measuring Power with Clamp Meter (Electrician only)
To measure any of these heavy duty appliances with the clamp meter, an electrician will have to isolate the wires in the power cord. Since it is not advisable to cut open an appliance power cord, the best option is to buy a short extension cord which can be cut open and temporarily hooked up to test the current and voltage in each of the hot wires. The resultant amperage from each hot wire is added to get the power draw for that appliance. Do not add the amperage from the neutral or ground wires, only from the 1(for 3 pronged) or 2(4 pronged) hot wires in the cord.

For example, I bought a 5-30 extension cord to measure power to an appliance using a 5-30 power cord. Image below:

I carefully sliced open the insulation on the extension cord and found the three wires inside. One of them is hot. Using the clamp meter, the amperage A for that wire can be measured and the wattage calculated. If you accidentally nick one of the wires you can easily electrocute yourself causing injury or death. Only an electrician should attempt this.

Clamp meter isolating the hot wire of a NEMA L5-30 power cord

If no extension cable is available, an electrician can unscrew the outlet from the wall and use the clamp meter on the house wiring that is attached to the plug. Also very dangerous! This is similar to opening up the circuit box while it is live which I do not recommend.