A smart plug can tell you something about an appliance. A year of energy bills can tell you where to start asking questions about the whole home. Begin with the information you already have.
Separate Usage From Price
Record the units consumed as well as the amount paid. A higher bill can reflect more consumption, a different tariff or a longer billing period. Compare like with like before deciding that a particular appliance has become inefficient.
Note major changes in the household: more people at home, unusually hot weather or a new appliance. These observations are not a formal energy model, but they make a conversation with an assessor or installer more useful.
Pick One Question to Investigate
Instead of buying several monitors, choose a question you can answer safely. Perhaps you want to understand an appliance's operating schedule or whether a timer is being used as intended. Follow the equipment instructions and do not open electrical panels or alter wiring yourself.
The Department of Energy's home-assessment material describes how an assessment can identify improvement opportunities and when professional evaluation is useful. US Department of Energy home-energy assessment guide Local buildings, tariffs and climate conditions differ, so apply advice in the context of your home.
Compare Measures on the Same Basis
For any proposed upgrade, ask what information supports the estimated saving. Is it based on measured use, a standard test or an assumption? Request a clear explanation of installation cost, maintenance and the expected conditions of use.
Treat a sales estimate as an estimate. A promised percentage is difficult to evaluate without knowing what is being compared. Keep a simple record so you can check the result after an improvement.
Review Before Expanding
After making a change, allow enough time to observe it under reasonably comparable conditions. Do not attribute every difference in the next bill to the new device.
The aim is to understand the largest practical opportunities, not to monitor every socket. A useful first step may be a maintenance check, a schedule change or a professional assessment rather than another connected product.