What smart devices actually do
Smart home devices do not save energy by themselves. They change when something is on, or give you information so you can change your own behaviour. A smart plug can switch an appliance off on a schedule. A motion sensor can turn a light off when a room is empty. A smart thermostat or a hub can coordinate heating with the times you are home. Any saving comes from the thing being switched off for longer than it would otherwise have been.
That points to a useful question before buying anything: is the thing you want to control actually being left on when it does not need to be? If not, a smart device will have little to save.
Smart plugs
A smart plug sits between a socket and an appliance and lets you switch the appliance on and off using a phone, timer or voice command. Many also measure how much electricity the appliance uses, which can be more useful than the switching. Seeing that a device draws a fair amount of power while idle can change how you treat it.
They are most helpful for things that are left on out of habit, such as a printer, a games console, a set of speakers or a desk lamp. They are less helpful for appliances that should stay powered, such as a fridge, a freezer or a router you rely on. They should not be used with items that must not be switched off abruptly or with heating appliances you cannot supervise, and check the plug’s stated maximum load before using it with anything powerful. Using a plug with a timer is not a safe way to run something unattended if the appliance was not designed for it.
Sensors and hubs
Motion and occupancy sensors can switch lights off in places people tend to leave, such as hallways, utility rooms and garages. They suit rooms where lights are often left on, and are less useful where you sit still for long periods, since a sensor may turn the light off on you. Door and window sensors can tell a heating system when a window is open, which avoids heating the outdoors. Temperature sensors in individual rooms give better data than a single reading in a hall.
A hub connects these devices and lets them work together. It can be useful, but it is also another device that must stay powered and connected. The more elements in the system, the more there is to configure and, sometimes, to fail.
Standby draw and the cost of owning more devices
This is the part that is often overlooked. Smart devices are always on, because they must be listening for a command. Each one has a small continuous power draw, and the amount varies by product. For a single plug it is small, but a home filled with plugs, bulbs, sensors, speakers and a hub accumulates draw that runs all day, every day. This is a close cousin of ordinary standby power, with the difference that you chose to add it.
If the saving from switching something off is smaller than the device’s own draw, the net result is negative. That tends to happen when a smart plug is attached to a low-power item that is rarely left on anyway.
There is also the manufacturing footprint to consider. Every device is made from plastics, metals and electronics, and its production emissions are spent before it saves anything. This is the same logic as in any life cycle assessment: a product can save energy in use and still only be worthwhile if it is kept and used for long enough. Short product lifetimes, discontinued apps and lost cloud support can leave perfectly working devices unusable, which adds to electronic waste. Before buying, check whether a device can work without a cloud account and whether it will keep receiving updates.
Where smart devices tend to help most
Heating is usually a larger share of household energy than small electronics, so controls that improve how heating is scheduled can matter more than any number of smart plugs. A programmable thermostat, or a smart one that learns your schedule, can help if the house is heated when nobody is in. Whether it saves anything depends on how you used heating before, so be cautious about promises. Lighting is another area, though the move to efficient LED lighting has already cut the energy used by lights, leaving less to save.
Simpler alternatives
Many of the same results can be achieved without smart equipment. A basic mechanical timer plug, a power strip with a switch, and the habit of switching things off at the wall all reduce idle use at low cost and with no network connection. Lowering the heating setpoint by hand costs nothing. If you already have these habits, smart devices may add convenience more than savings.
A sensible approach
Start by measuring. If you have a plug with energy monitoring, find the devices with the highest idle draw. Add automation only where there is a clear, repeated waste. Avoid buying a full kit at once, keep the number of always-on devices low, and keep what you buy for as long as possible. If a device makes your life easier and also trims energy use a little, that is a reasonable result, but it should not be expected to transform your bills.