I performed an assessment of my wattage requirements by merely connecting an amp-meter between the positive post of my primary battery and the positive battery cable. Then it's just a matter of turning on the equipment you would normally be using. No guesswork involved. I then purchased the solar system I needed to keep that battery charged with the typical current draw. The most difficult part of the job was determining how to mount the panels so they would be out of the way, easily adjustable and open to direct sunlight through most of the daylight hours.