I wondered how the power consumption of the mains voltage heated bed compares to the MK2 while holding a certain temperature, assuming that the PSU will cause significant losses. However, not having two trusted electricity meters at hand, I tried it with a pair of OWON B35T bluetooth multimeters. The csv-export functionality of the app is flawed with bugs, so well, it did not work. Also, I suspect some of the readings were bad.

I will have to redo this, but however, I was able to capture the readings for full-on and idle, and they're still interesting.

The setup

I wired the MK2 in default configuration, with the heated-bed power rail of the RAMPS 1.4 powered through the PSU shown in the photo below. The other power rail of the RAMPS 1.4, which powers the Arduino and logic is powered by another PSU that is not seen in the photo since it belongs to my 3D printer. This way, I ensured that the logic power consumption does not influence the measurement.

The current that goes into the above PSU and eventually powers the MK2 is shown on the left multimeter, the current that goes through the SSR and eventually powers the mains voltage heated bed is shown on the right multimeter. Since both are attached to a common 230 V~ mains line, I can compare the power consumption, even if (lacking a third multimeter for measuring the mains voltage) I don't know the power consumption exactly.

Idle

The idle power consumption is not surprising, since the PSU has the fan spinning and has to maintain some idle current on all its power rail, it must consume more power than an open switch.

Turns out the PSU draws about about 0.69 A, which results in an idle power consumption of about 16 W. The mains voltage heated bed is very savy here, since it's just off, but the SSR appears to let through a small idle current of 6mA. This idle current is suspiciously high, I expected this to be only a few µA. Another reason to redo the measurement: It's a potentially crappy multimeter, maybe just a bad reading.

Full-on

The full-on current surprised me a bit. Even though we are comparing a supposed 120 W heated bed to a 400 W heated, the current draw of the 400 W heated bed is only 54 % higher than the current draw of the MK2. Which raises the question of how much power is actually lost in the PSU. I will probably have to do some more measurements on this matter, measuring output current and voltage of the PSU as well. Eventually, the MK2 consumes 260 W of electricity through the PSU, while only half of this reaches the bed in form of heat.

Maintaining 100 °C (measurement failed)

As said, this failed. I wanted to graph the data of current consumption of both boards over a period of time while they would maintain 100 °C, but the CSV export of the data failed. I also couldn't get steady readings from the app for the mains voltage heated bed. The first problem is a bug in the csv-export of the OWON B35T smartphone app, and the latter is just because multimeters are not electricity meters and their true-RMS window is just to small to capture enough half waves from the SSR. On the smartphone app you can spot a yellow graph (MK2) and a red graph (mains voltage heated bed). The yellow one is ok, but the red one's just too noisy. I also didn't take a better picture of the graph since I expected the csv export to actually work. I'll try better next time :)