Physical dimensions, rather than system output, is the best way to measure your needs. The geometric shape of the system layout, and if the array is split into separate pieces, are the true determinants. This, of course, varies greatly from installation to installation. Therefore, we recommended that you calculate a precise number before ordering, as we've seen a single roll be sufficient for an 8+ kW system and yet not enough for a 4 kW. To get this figure, simply calculate a linear foot measure of the entire system perimeter.