How Smart Light Switches Can Use Power Even When Off

Smart light switches seem a bit of a contradiction when you first think about them, because an electrical switch has two states: on or off. But yet, any smart device must always be ‘on’ in order for it to be smartly controlled. So how does a smart light switch stay ‘aware’ (and ultimately powered on) even when switched off?

Smart switches with a neutral wire readily power themselves from the hot and neutral line, whereas switches without a neutral take a trickle of power from the supply to the bulb – slightly lighting them in the process.

How smart switches generally work

Marketing image of a two-switch Lutron Caseta smart switch
Marketing image of a two-switch
Lutron Caseta smart switch.

Smart light switches are a useful invention and some people prefer them to smart bulbs, because they allow you to turn lights on and off in a room in a fairly natural way.

Smart switches are wired into your house’s electrics, replacing a room’s existing light switch. Therefore they can ultimately only control the light bulbs that the switch previously controlled (unlike smart bulbs which can be independently managed as long as power is flowing to them).

Due to this, smart switches are great for many ‘utility’ or ‘task’ based rooms, such as kitchens, utility rooms and bathrooms because you’re likely to just want the lights on or off in these rooms. Smart switches are perfect for this, especially since some models have motion sensors built into them which automatically turn the lights on as you walk into these rooms.

Once they’re wired into your house, smart switches draw electrical power (more on this later), and then use a communication protocol to ‘speak to’ a central hub, allowing you to use a smartphone app or a smart speaker to turn the switch (and hence lights) on and off.

This ‘communication protocol’ is usually WiFi, meaning that the smart switch connects to your home’s network just like your TV, phone and other devices do. The smart switch can then be communicated with (securely) over the internet.

Alternatively, smart switches can sometimes use different communication protocols such as ZigBee and Z-wave, which use a bit less electrical power and can be more reliable than WiFi.

Either way, the main thing to know is that a smart switch has to draw some power (with or without a neutral wire) so that it can be smartly managed via your app or a smart speaker. The two following sections look at this in more detail.

Smart switches with a neutral wire

The majority of the smart switches on the market require a neutral wire, and this is because any American houses built from the 1980s onwards should have a neutral (white) cable running into light switch back boxes.

In this way, smart switches are wired in between neutral and supply-hot and thus will draw power in the usual way.

For example, if you have a dimmer switch with one black, red, white, blue and green cable, the resulting cabling for a Lutron Caseta dimmer would look like this:

Lutron Caseta wiring diagram with a neutral wire
Lutron Caseta wiring diagram with a neutral wire

A more technical wiring diagram is below:

Lutro Caseta wiring diagram with a neutral
Lutro Caseta wiring diagram with a neutral

As can be seen, the smart switch’s own electrics can readily draw power as needed from the supply-hot line and the neutral line.

In this configuration, the smart switch will control the power to the light via the load-hot (red) wire. Only when the switch is turned on (either physically, or via the smart functionality) will the circuit be complete and the power will flow to the light.

Otherwise the electrical circuit is broken, and the light doesn’t come on.

Smart switches without a neutral

The above wiring diagram is fairly typical of modern American houses, and much of Europe (whose switches and sockets have had neutrals for a while)

But whenever a neutral wire doesn’t exist, a simple 2-wire circuit is used to power a light. Hot and ground go into the switch, and the hot – called the supply-hot – is a permanent live feed from the house’s mains power.

Lutron Caseta wiring without neutral
Lutron Caseta wiring without neutral

The switch then runs another hot cable – called the load hot or switch leg wire – to the light fitting. The switch completes the circuit: when it’s on, the circuit is complete and the load-hot supplies live power to the bulb – turning it on.

When the switch is off, the circuit is broken and no power flows. This works fine overall, but it causes problems for smart switches which have to have their own (small) power supplied to it in order for the smart functionality to work.

Since there’s no neutral to draw power off, instead the smart switch is installed in series with the light bulb:

Lutron Caseta wiring without neutral, showing the switch in series on the hot line
Lutron Caseta wiring without neutral, showing the switch in series on the hot line

The difficulty here is that the load line isn’t designed to power the smart switch too, and so the smart switch needs to ‘steal’ (or leak) some power from the bulb.

This is done by making use of the dimming feature of the switch/bulb, and it draws down a small trickle of power through the live load cable. This is a bit of a ‘clever hack’, and whilst it’s safe, it has a few main drawbacks:

  • Depending on how much load can be drawn to power the smart parts of the switch, a load resistor is sometimes needed to help enable the current leakage.
  • Since this makes use of dimming features to work, both the smart switch and the fitted bulb must usually support dimming. You’ll usually find that smart switches which support dimming are more expensive, too – in other words, a smart switch without a neutral is probably going to be more expensive than one which doesn’t require a neutral.
  • It’s rare that LED bulbs (both normal ones, and smart ones) can be used in smart switches that don’t need a neutral. This is because the trickle of power through the load can cause issues with LED bulbs, and often such switches only work with outdated incandescent bulbs.

In short, smart switches that don’t require a neutral fill an important part of the market (i.e. the majority of pre-1980s American homes!), but they also rely on some necessary ‘hacks’ to power their smart functionality, and this comes with some limitations.

Increased energy use from smart switches

If you’re wondering how much extra power smart light switches consume (since they’re always on), you probably don’t have to worry much!

If the smart switch uses ZigBee, Z-wave or a radio based approach (such as Lutron’s non-smart RadioRA 2 technology), you’re probably looking at a power draw of around 0.2-0.5W max:

  • RadioRA 2 uses 0.2W of power.
  • Whereas a Aeotec Z-wave switch was measured at 0.4W idle.

Having said that, the most common smart switch communication protocol nowadays is WiFi, and this is more power intensive. But don’t worry – even with WiFi, the smart switch will only use around 1-2 watts of energy – a similar amount to a smart plug (this page mentions that a WeMo smart plug uses 1.5W of energy, for example).

In short, a smart switch will use more energy than a standard (dumb) switch, but no more than 2W of energy at idle for a WiFi switch and less than 0.5W at idle for a ZigBee/Z-wave switch.

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