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I want to buy this. However, I am not sure whether 240W power supply that I choose will efficiently power a 12 W device. Is the 240 W power supply constant or can it change depending on the power required by the device. So, does that mean if i buy 240W to power a 12W would be a waste of electrical energy?

Thank you

Maximus Su
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  • Why buy such an oversized power supply? – StarCat Aug 16 '21 at 09:06
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    "*So, does that mean if i buy 240W to power a 12W would be a waste of electrical energy?*" If that was the case then unplugging your 90 W laptop power supply from the laptop would cause it to dissipate 90 W in the power supply as there is no current being drawn. That would cause your laptop PSU to get almost as hot as an old 100 W incandescent lamp. Does it? – Transistor Aug 16 '21 at 09:07
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    @Transistor Your point is perfectly valid, but note also that laptop power supplies are required to have a very small parasitic power draw (at least under EU regulations), so they'll pull fractions of a Watt when idle. Compared to that, some of these cheapo no-name modular PSUs can easily draw 10+ Watts doing nothing. – TooTea Aug 16 '21 at 09:30

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Rated at 240 W, 20 A, it looks like this is a 12 V output power supply.

If you connect a 12 W 12 V device to it, the device will draw 1 A from the power supply.

The power supply will draw 12 W from the mains to power the device, and in addition will draw a few watts more to run itself, and supply its losses. You may have graphs or tables of the efficiency of the power supply at various loads, in which case you can estimate accurately what the extra power drawn will be. You can rest assured that it will be a lot, lot less than 240 watts.

It rather depends on the designer of the power supply, and what specification he was designing to. The specification may have required a no-load power consumption of less than one watt, to meet certain green requirements. Absent that, the designer might have reasoned that in a 240 W power supply, nobody would care if the no-load consumption was 10 watts. You'll need to read the data sheet, or do a measurement.

Neil_UK
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  • Thank you for your answer. So, this is like the transformer question that you answered yesterday. Feel like building my own transformer. – Maximus Su Aug 16 '21 at 08:52
  • And why the prices spike up for higher current with the same amount of voltage? You can just adjust the number of coils right and thats so cheap? – Maximus Su Aug 16 '21 at 08:53
  • @MaximusSu more current with the same amount of voltage means more power. Power is the thing you pay for *pro rata* in power supplies, more power means larger transformer, larger semiconductors, more cooling, though fixed costs will be amortised into a larger device. There's little point to buying a 240 W supply to power a 12 W load, other than future-proofing your investment. – Neil_UK Aug 16 '21 at 08:57
  • @MaximusSu All the parts inside the power supply have to be bigger, or the high current will make them explode. That's why power supplies with more current are bigger and cost more. – user253751 Aug 16 '21 at 12:25