Wesley's Tool-Box

There are some things to consider and take action when you're installing smart lighting, HomeKit enabled or not, around the house. For the light switches, the biggest concern is the presence of a neutral wire. In most cases, you need this for a smart switch to function, but many switch boxes omit this and make things complicated. To see why things are like this, we need to take a look at the circuit diagram.

For a light to turn on, it has to connect to both ends of a power source. This is generally a single-phase AC power, which can be derived from a 3-phase AC power by using one of the phase wire and a neutral point. The wire connecting to the neutral point becomes the "neutral (N)" and the phase wire, the "live (L)" as seen in the diagram.

With a regular switch, all you need to do is to connect or break the connection between the live wire and the "load" wire leading up to the lamp. Therefore, a switch box only needs to have the live wire and one or more load wires coming out of it. Number of load wires correspond to the number of controllable light fixtures, of course. Neutral wire could also be present, but it wouldn't be connected to anything because there is no need to.


The situation becomes different with a smart light switch. In order for the control module in the device to work, it also needs to connect to both ends of a power source, but at all times and independent of the lighting. As the live wire is already present, we need to add the neutral wire to the device as seen in the circuit diagram. With this configuration, the switch connected to the live wire and the load wire could be controlled either manually or by the control module. This is the reason why most smart light switches require the neutral wire.

The rare exceptions that can forgo the neutral wire have the control module connect between the live and the load wires. The module itself consumes little power and a very low current leaks through the load wire in the "off mode" so as to effectively prevent the lighting from turning on. But this generally requires the lighting load to be sufficiently large. If not, the leaked current may cause the lighting to flicker or cause other problems. In other words, this solution isn't as widely compatible as the switches using a neutral wire.

Home automation devices have to be on standby at all times to respond to commands. This means that the baseline power consumption will increase as more devices are added. So if you're installing them not just for convenience but for more efficient energy use, you have to be conscious of the level of power that they may entail.

Unfortunately, detailed power consumption data are not usually found in the technical specs, likely because they are not deemed important. I had to take individual measurements to get the whole picture, and here are the results for the HomeKit devices I have.

[ Terminologies & Explanations ]

Off: The device is connected to the power but is turned off (i.e. standby mode).
Min.: Device operating under the lowest power mode possible.
Max.: Device operating under the normal or highest power mode possible.
(Minimum and Maximum values are measured with no external apparatus attached.)
Rated: How much the the device is rated to draw electricity at maximum.

1. Or 220V mains. 1,800W for the U.S. mains (120V).
2. For the LED and CFL bulbs. 600W for incandescent and halogen bulbs.
3. The indicator LED light is turned off.
4. For the LED and CFL bulbs. 1,800W for incandescent bulbs.
5. Only one switch is turned on.
6. Steadily decreases to the stable level (9.2W) over the span of 30 minutes.

It became fairly evident that the consumption profile is quite different between manufacturers even though the devices serve essentially the same purpose.

It's been about five months since I installed Seojun Smart Meter at home and took various measures to cut down unnecessary electricity use. Now was a good time to see if the smart meter was recording monthly data accurately, and whether my efforts panned out well. This graph sums up everything that needs to be said.

In terms of accuracy, the smart meter consistently reported slightly lower than the default meter installed in the premise. However, it is more less in line with the advertised margin of error (1%) after the initial month and thus I think it's reliable. This is important because the companion app effectively shows last month's finalized data a full month before it shows up in the bill, and the app also shows the forecast for this month. Now I know that these values can be trusted and I can prepare two months ahead.

Moving onto the consumption trends, my home generally spent around 190 to 200kWh in winter, then fell down to the 170kWh range in spring if the family didn't go out on a vacation. This is already somewhat below average for a 4-person family, but I started making the house even less wasteful starting in February this year by making more efficient use of the appliances and changing the lightings and bulbs to LED. Eventually, it has settled to somewhere between 140 to 150kWh per month now. This is a saving of roughly 30kWh, or more than 15%.

Thanks to this and being much more aware of the consumption in real time, I'm expecting about 30% savings in electricity costs in the summer when the air conditioning is in full operation. Home electricity rates in Korea is pseudo-exponential, so you pay a lot less for seasonal increase if you start out from a lower baseline. Recently, I helped my dad cut down power use at his house by lowering the baseline by roughly 200kWh and I think it'll save him at least US$300 per month during summer. I'm expecting that the money that went into the streamlining will pay for itself in 3 years. Overall, I'm satisfied with the results of the efforts I put in.

As the hunt for even more potential waste of power continued, I brought in the help of thermal imaging technology. By scanning each place with a thermographic camera attached to an iPhone (FLIR One; I'll write about this later) I can find any hot or cold spots that seem to be out of place.

One of such "hot" spots I found was on the side of an air conditioning unit. It wasn't being used, but standby power drawn from the wall outlet was slightly heating up the control circuit and was readily visible via thermal images. I could just pull the plug until summer, but I decided to take a step further.


I replaced the default wall outlet with the one that had an integrated switch. This way, I could cut the power from the outlet with a switch when the air conditioner isn't in use, instead of having to pull the plug. Not only would this be simple to operate, it would avoid the mechanical wear. I should have done this ages ago.

As part of my ongoing data collection, I then tried to measure how much power the air conditioners in the house would consume while trying to cool the house. Sadly, the rooms were not hot enough for them to start cooling the air. I'll have to check them out again when summer comes.