The wattmeter is inserted between the device to be measured and the power source. It displays the voltage (220 V), the current (A), the power (W or kW), and the consumption (Wh or kWh, as on your electricity bill).
Before starting to take measurements, I print an empty excel board in which I report my measurements (measured device, on, standby, off, remark).
The easiest way to start is to measure a device with steady consumption, such as a bedside lamp. The power meter is plugged into the wall socket and the lamp is connected in series to the power meter. Press the display key (Anzeige, Display) several times to display the power consumption in watts (W), and a value is displayed. For example, for a bedside lamp with a 40W bulb, the wattmeter indicates about 40W. Magical. This direct measurement works for all appliances whose consumption does not vary almost in time: a kettle, a vacuum cleaner, a hairdryer. For appliances that consume more than 1000 W (1 kW), the device displays the value in kW: a hairdryer that consumes 1100 W will be expressed as 1.1 kW. Note that all these devices do not have any standby mode: there are just two values. ON or OFF.
The measurement is just as simple for devices with standby, such as HiFi or TV. We connect the device to the water; we look if it consumes something off. If yes, it will use multiple sockets with the orange switch, to avoid unnecessary consumption. The device is then turned on: the waste is displayed (e.g., 55 W). It is placed in standby mode, and the consumption decreases a little (e.g., 8 W). That’s all.
For devices with variable consumption over time, it’s a bit more complicated. Take the example of a fridge. The compressor that cools the inside of the refrigerator does not work continuously. When you open the fridge door, the interior heats up, and the compressor starts (you hear the noise). At night the fridge remains closed, and the compressor turns only to compensate for heat loss due to inadequate insulation … The power meter will, for example, display 0 W for a moment, then 120 W for a minute, then 0 W, etc.
To know the average consumer, it is necessary to make a prolonged measurement and a mini calculation. For example, for a fridge: I leave the power meter connected for 10 hours (after a reset Wh!). In the end, he tells me 360 Wh. I divide by the number of hours (10), and I get the average consumption in watt, or 36 W. The method is identical for iron, a desktop PC …
The must: measure a laptop.
Measuring a laptop is a bit more complicated because you measure two devices: a computer and a battery charger. For starters, the easiest way is to measure the PC with the battery full: the charger turns off, and the actual power consumption of the PC is measured. A current PC consumes less than 2W off, less than 10W on standby, and less than 40W on. Consumption varies little, so you can use direct measurement, or average one hour of use if you want to be precise.
When the PC is charging, the charger’s power consumption (approximately 30 W) is added to the PC. An off-the-shelf notebook, therefore, consumes around 35 W.
It is also necessary to measure the transformer alone, i.e., without connecting the laptop. The measured value should be less than 1 W. Therefore, always disconnect the transformers or use switches.
The most complicated question is: from how much time of inactivity should I turn off my PC instead of leaving it idle? The answer is: as soon as the computer’s consumption at boot time is lower than the sleep mode. To know the exact duration, one must make an accurate calculation:
- Measure the average consumption at startup. You must reset the power meter (Wh) and use a stopwatch. We start the PC and the Chrono at the same time, and when the office is completely displayed, we stop the timer, and we note the value of the power meter. For example, an iBook starts in 72 seconds (0.02 hours) and consumes 0.6 Wh. On average it consumes 30W (calculation: 0.6 Wh / 0.02 h = 30 W).
- Measure the standby power. An iBook consumes 8 W in standby.
- The time from which the laptop should be shut down is obtained by dividing the consumption at start-up (0.6 Wh) by the idle consumption (8W), i.,e. 0.075 h for the iBook (0.6 Wh / 8 W = 0.075 h). As an hour lasts 60 minutes, 0.075 hours corresponds to 4.5 minutes.
- Conclusion: From an electrical and economic point of view, it would be necessary to extinguish an iBook from 5 minutes of inactivity.