"There are two basic types of energy monitors: Those for a single appliance and those that measure your entire home’s energy use. The best way to figure out which option works best for you is to decide what level of energy monitoring you want to achieve, how much you want to spend, and how much you want to save.
Single appliance monitors cost about $25 to $75 and calculate the usage of a single device, which is plugged directly into the monitor. Most monitors will tell you how much energy is used during a given period while displaying the current usage. Measuring usage over a 24-hour period or longer is the best way to determine your appliance’s efficiency as many of them cycle on and off throughout the day.
These monitors are useful for keeping track of how your appliances are holding up. You can measure your refrigerator’s energy usage at the beginning and end of a season to see if it its efficiency has changed. If it’s draining more power than it did in the past, it may need a tune up.
Whole house monitors are a bit more expensive at about $100 to $200, but they are a useful way to get a wider look at how much energy you are using and how much that usage is costing you. You can enter your utility rates for peak and off-peak usage into most monitors to calculate just how much you can save by, for example, setting a timer to start your laundry or run the dishwasher during the day or in the middle of the night.House monitors work best for measuring usage that is distributed throughout the entire house, such as central heating or cooling. It also helps you determine the savings of lifestyle changes that affect more than one appliance."