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This is my first question on here, its quite easy and I am sure I have half the answer, maybe. I was thinking about how much power was being used by these wall chargers etc, you read the label it may say 1A 240V that equates to 240 Watts? Can it be that high or is there another equation to use? Don't be shy to show me math.

Thanks

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If you read "Input 240V 1A" then yes, it's supplying 240 watts (maximum).

More likely, you're reading "Input 240V; Output 5V 1A" (or similar). This means that it's outputting 5 watts and (unless it's a linear regulator, which is unlikely) it's drawing just a little bit more than 5W. Even then, the 1A is the maximum that can be supplied by it. If the load is only drawing 1/10 of a watt, the power being supplied is 1/2 watt and the input draw is a little bit more.

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Quite often there is a huge discrepancy between the label rating on a power supply and the power it actually uses, on the input side, while its output ratings are likely to be reasonably accurate.

Picking one power supply at random I see: INPUT : 100-240V(AC) 50-60HZ 1.5A OUTPUT : 19V(DC) 3.95A

It's quite small and runs warm (but not hot) possibly dissipating 5-10W as heat. So it supplies just under 20*4 = 80W and probably never consumes more than 90W sustained. That would mean 0.9A at 100V or 0.375A at 240V.

The input ratings are deliberately conservative, because their purpose is not to estimate your electricity consumption, but to ensure safety by encouraging you to budget your current to choose the correct fuse, breaker, and not overload the mains circuit.

The fuse rating has to cope with the inrush current, which may exceed 1.5A but for such a short time that the fuse will not blow. This explains why one current rating is given despite the huge range of input voltages. If you have fused cables, a 1A fuse will probably not last, but a 3A fuse (the next standard value in the UK) will.

If you add all the rated loads on an extension cable on a 13A plug, they should come to less than 13A. If you add all the loads on 13A plugs on a 30A circuit, they should come to less than 30A. Given the expected current for a circuit, you can choose the appropriate wire gauge, and so on.

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That is usually the maximum input current under the maximum load.

$$Input Power = Output Power + Efficiency Loss$$

The 1A 240V should encompass both the power passed through to the load and the losses involved in converting the voltage.

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  • \$\begingroup\$ Surely the efficiency loss is not an additive constant as implied by your equation. Shouldn't it be proportional to the output power, so a multiplier? \$\endgroup\$ – Roger Rowland Nov 4 '15 at 4:00
  • \$\begingroup\$ Well, the efficiency can change depending on load. But the power consumption due to waste + power consumption due to output always adds up to the total power. \$\endgroup\$ – Daniel Nov 4 '15 at 4:39
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Here are two additional factors to consider:

For AC power, Power = Voltage X Current X Power Factor. The concept of power factor was developed to account for the fact that the phase relationship between the voltage waveform and the current waveform effects power. Power factor is a number between zero and one. Power factor is also used to account for the effect of waveform distortion. All electronic power conversion devices tend to have a distorted input current waveform. Devices designed to prevent that are more expensive. I measured the power factor for a small power supply using a Kill A Watt (TM) meter and found it to be 0.48. That means that the actual power used by the power supply is less that half of the power calculated as V X A.

The various electrical safety standards dictate how the information marked on products is determined. It is likely that the output current of a power supply is the current that can be drawn without causing the voltage to decrease too much or without the device getting so warm that it's useful life is shorter than expected. The input current is likely the maximum current that the will be drawn if the output current is just below the point that the power supply fails or shuts itself off. The power supply will likely put out at least a little more than the rated output current before failing or shutting off.

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