ASK THE EXPERTS: Submetering for a Guesthouse

Beginner
Monitoring the whole house with TED.

I would like to install an inexpensive meter to record the electricity usage in a detached guesthouse, so I can equitably divide the bill between the main house and guesthouse tenant. The guesthouse has four circuits that are supplied from the main house.

Carl C. • Columbia, South Carolina

There are many options for submetering individual circuits or groups of circuits. They generally fall into two categories—either directly measuring the energy by having it flow through the meter, or by using a current transformer (CT), a ring that surrounds a wire to measure the current without splicing into the wire.

Individual plug meters are an example of the first type. For larger loads or for metering whole buildings, kWh meters, just like your utility uses, which fit in meter sockets, are available refurbished for very reasonable prices—often for less than $50 for a meter and socket. The drawback is that they require wiring skills to insert into the circuit—usually done by an electrician, which can increase the overall cost significantly. They can also measure only a single line—if you have multiple circuits to measure, you need to run them all from a subpanel, and then meter the supply to the subpanel. It sounds like a utility-style kWh meter would meet your needs.

CT-based power meters vary widely in quality, price, and capabilities. Some can only measure one circuit, and some can measure dozens of them. Some are very accurate and some may be off by up to 20% at times. Many of them have online or web-based viewers, but some just have a display on the meter. They are usually easier to wire than utility-style kWh meters, though you still may want an electrician to do it because of hazardous voltages present when the electric panel is opened to install them.

A web search for “home energy use meter” reveals several options. You’ll find meters that can measure only one or two circuits, or the house as a whole (by measuring the input circuit), which cost about $100. However, these may not measure voltage or power factor, which can introduce an error of 10% to 20% at times.

In the $200+ range, more accurate ones like TED (The Energy Detective), are available. Most of these measure only the whole house. Sense Labs is now offering their Sense home monitor that can identify common loads and differentiate them on the display—without having individual CTs on them.

Searching for “100-amp AC power meter” gives many choices for a $20 single-load monitor, useful for measuring a large load, such as an air conditioner or water heater that’s too large to measure with a plug-based meter. These can measure either 120 VAC or 240 VAC circuits.

If you are a looking for a high-quality web-based meter that can be revenue-grade certified, the Egauge is my favorite. It can monitor up to 12 circuits, so it’s ideal when you are trying monitor individual appliances, or individual rental units. The drawback for this is the price—starting at $600, before installation.

Zeke Yewdall • Mile Hi Solar

Comments (1)

Todd Cory_2's picture
Todd Cory_2 (not verified)

just install a private kWh meter upstream of the guest house's electrical load center. small and simple is best.

$20.00 will buy one here:

http://visionmetering.3dcartstores....

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