# Power consumption

Discussion in 'Motorhome Accessories' started by OddSocks, May 14, 2014.

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I can never get my head round this.

If I plug a laptop into an inverter, how do I figure out how many amps the inverter will draw?

EG

I understand my laptop charger is 65 Watts
I plug it in to a 150w inverter.
I plug the inverter into the 12volt supply.

How many amps is the inverter using?

2. ### JeanLucFunster

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The usual Watts/Volts = Amps still applies, but the volts are 12.

Therefore:

65 / 12 = 5.4 amps.

Then allow a bit for inverter inefficiency - say it's 85% efficient (but a good one might be 90%, a cheapo only 80%) so divide 5.4 by 0.85 and you get 6.3 amps.

The 150W is the maximum power that the inverter can deliver (ignoring surge capability) so the actual load of 65W applies in the calculation.

Last edited: May 14, 2014
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3. ### J HFunster

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Why not just use a 12v adapter directly into the laptop?

4. ### vwalanFunster

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its easier just to divide the watts by ten . that works as a good average .

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Thank you! Makes sense again now

Because It costs £31 for my laptop as it has an unusual power connector which is not included in any of the universal ones. Then I have to get another for my wifes laptop at about £16 - but I already have the inverter. Just trying to save some cash.

6. ### JeanLucFunster

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Same for me; a MacBook Pro has a 'magsafe' power connector for which there is no Apple 12V charger. There are 3rd party 12V adapters at around £25 but if anything went wrong Apple would not be interested. In any event, the cost of a small PSW inverter at around £100 for a good one, is a lot cheaper than a broken Mac (or even Windows PC).