Wicked,
If you truly mean "in storage" then a 1.5W panel will do. VW leaves cars in indefinite storage with a 4+15 (maybe 3 watt) panel attached to the battery and these cars will start after at least 8 months.
If you completely disconnect the house bank and attach the panel, you only have to overcome the leakage current in the bank and that isn't very much. You MUST completely disconnect it. Turning off what you can find may not work.
This is all work I did a lot of before the current depression hit. I just restarted and launched a boat that I "put to bed" in 2008. It cranked and started just fine. The main engine bank is now ten years old and was kept alive by a 3 watt panel on top of the tarp.
Matt
