12v supply

I'm sure this has been asked loads, but a quick search didn't break any clear information up. I have an old 2009 mac mini that is now surplus to requirements. So I am thinking of using it as a media PC in my touring caravan. Fit it with a dual USB tuner and USB 3G data dongle and it is good to go. The issue is that I will mostly have a 12v battery feed, and would rather not go through the 240v inverter as much as possible. I don't mind splicing wires, but could do with some advice on how to get the 12v from the battery feed to the 18v is it that the mini takes as an input?

ivnj said:

What's wrong with an inverter?? They work great and no splicing. And some even have a usb port to charge your cellphone.

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using a 240v inverter to from 12v upto 240v then back down to 18v wastes a lot of power. When I am relying on a battery to power my lights and water pump I need to use as little power for the mac as possible to make it viable.