Real scenario: a fellow had 2x 195w on PWM for 200Ah of AGM and pondered adding more panel if the 390w proved insufficient. IMO, 3x 195w is better than 2x 195w in pretty much every situation (maybe it requires a larger controller), but let’s join the discussion in progress to see how much better and when:
increasing rated panel wattage by 50%
The value of the addition depends on the condition that’s causing the reduced solar harvest and how restricted the harvest is. For example, a 50% increase from 10w is 15w (+5w) but the same relative increase from 200w is 300w (+100w). The panel investment is the same.
Dark skies with rain might reduce harvest to about 10% of your harvest under very good skies. For 390w on PWM let’s say 275w / 10 = 28w. Adding another 190w panel would add 14w for a grand total of 42w. $13.57 per extra watt harvested. D’oh! In this case the panel money might better be applied to a DC-DC charger that can reach the correct Absorption voltage.
Overcast skies might get you 1/3rd of your good sun harvest, so 92w. Adding another 190w of panel brings 46w for a total of 148w. $4.13 per extra watt harvested. . We’re getting somewhere now.
Partly cloudy skies that open up periodically are a case where extra panel really helps; during the cloud breaks the extra panel would add 138w, for a total of 411w. $1.38 per extra watt harvested. Woot!
Low solar altitude (low winter sun and/or higher latitudes) might degrade flat-mounted harvest by 30% to 180w (90w per panel). A third panel propped up facing the sun might collect PWM’s full-pull of 138w, for a combined 318w. Again, $1.38 per extra watt harvested (the flat panels themselves are $2.11/watt harvested due to poor angle).