How to measure how much solar we need?

My power needs are constantly changing; heating and air conditioning being the biggest game changers; so I'm trying to plan for both the bare minimum, and absolute maximum power requirements. I'm going with two (portable) 140w panels for four 6v batteries, so I have options... use one or two panels, on a small or large bank.

I determined my energy needs using a kill-a-watt meter, trimetric meter, and some simple calculations based on manufacturers' ratings.