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I am looking at installing some LED lighting which requires a 12v DC 10 amp power supply. The question which I cant seem to find an answer for is this...

I live in the UK which uses a nominal 240 v AC mains power, so what would the power draw be on the power supply? ie. what would the input amperage be at 240 v AC ?

The reason I need to know, is that I don't want to overload the domestic lighting circuit in the house but I can't find any definitive answer to the above. I understand that there's a direct relationship between volts, amps and watts but is it transferable from AC to DC?

Am I right in thinking that the power supply would supply around 120 watts (12v x 10A) so that would infer that the supply would be 120w / 240v, around 0.5A plus a bit for losses?

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  • \$\begingroup\$ It's not that easy. Watts is a DC unit, but you want to perform a calculation on the AC-side. See powerstream.com/VA-Watts.htm - what you need is VA. \$\endgroup\$
    – user17592
    Commented Dec 7, 2014 at 0:36

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Generally, as a first approxomation, for any power supply you can asume that the power in (drawn from your 240V supply) will be equal to the power out to your load, plus some allowance for inefficiency, so your calculation is correct.

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    \$\begingroup\$ Use care with the loose statement of "some allowance for inefficiency". Power supplies can consume considerable power depending on the actual efficiency. Older style power supplies that had big heavy transformers were lucky if they achieved an efficiency over 65%. Modern types of switching power supplies can be found with efficiencies of 80 to 95% depending on the amount of load and how much you pay for the thing. \$\endgroup\$
    – Michael Karas
    Commented Dec 7, 2014 at 1:24

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