the pw-70a is probably most conveinient for what you're doing. Run wires from your battery to your regulator (ITPS i would assume) and run wires to a DC Plug, like
this (i don't know if that's the right size, but it's a pic) and then plug it into your pw70a. That way, you just uplug the DC jack, bring it inside, and plug it into
this. (there are cheaper places to get something like that, like
www.idotpc.com or something).
Edit: That is probably the cheapest option for a DC-DC power solution, coming in at around $110. (ITPS, PW-7a, and wall adaptor)
Or you could go the inverter route, and probably spend $30 on an inverter, and $30 on a standard PSU. From a technical aspect, that's more ineffiecient than using a DC-DC PSU. But people who use inverters around here swear by them. So that's $60, but it's a lot more clunky than using the PW-70a and a Regulator which means the case is bigger, making it harder to move around as you wish to do.
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