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).
a boondocking retirement 
I just bolted another panel on my short bus today so the timing of this blog couldn’t be better. I hope it will prove to a worthwhile addition for my energy system. For me it was a case of use what you have got already rather than a cost per watt added analysis.
I brought a huge, 435 watt, panel with me and it’s been locked up inside blocking my fridge and my tool chest. So I attached it to the side of the bus today. There’s a 190 watt panel on the roof already and I’m going to be doing some roof work this Winter so the side of the bus was the logical place to attach it. Also I’ve noticed how poorly the flat panel performs in Winter. And I boondock mostly in Winter and Spring .So I’m curious how well this vertical panel performs with careful alignment of the vehicle. Then I’ll get cost per watt added.
It’s a 48 volt panel so I had to get a Mppt controller. I paid $300 for the panel and $300 for the controller. That’s a chunk of change. But to me it’s worth it if the batteries are happier. For a married man it’s “happy wife happy life”. For a solo boondocker it’s “happy batteries happy life”. And now I can try out the fridge without a big panel in the way. When I redo the roof I’ll have to consider how many panels to place up there. I might sell this 190 watt panel and try to find another 435 watt matching panel. Decisions decisions..,. But these are the type I enjoy making.
How’s Quartzite?
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> For a solo boondocker it’s “happy batteries happy life”.
Exactly so.
A south-facing vertical panel could make a major contribution any time the sun is lower, and would make decent power the rest of the time. Urban environments or lakes where there is reflected light could be especially interesting.
I’m making a much smaller addition to my own; I wrote it up yesterday but the WordPress block editor wouldn’t let me cut/paste to another post. Grrrr….
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