One often over looked aspect, that impacts solar performance, is the temperature of the solar panels. Solar panels are rated at what is referred to as STC or Standard Test Conditions. Depending upon the manufacturer this might mean a panel specifications label rated at 20C (68F) or 25C (77F). The Sunwize panel in the beginning of this article is rated at 30 W & 1.8A. However, this is based on a STC temperature of 25°C at an irradiance 1000 W/m². This means that in order to see 1.8A you will need less than a 77F panel temp and an irradiance level of 1000 W/m².
How long do you think a panel will remain at 77F in full sun? Will you always have 1000 W/m² irradiance levels? Heck no! These ratings are "best case"... How long will it stay at 77F? The answer is not very long, unless it is a cool day.
On boats we get a bit of a break because the panels are mounted horizontally, and the boats, when at anchor, are facing into the wind thus forcing some cooling effect across the panels. On land they are often on roof tops where cooling is minimal compared to a boat course those panels can be aimed and ours can't so any direct comparison of performance becomes very difficult....
Simply put, as the panel or panels heat up the available output performance goes down. Simple stuff. MPPT controllers, for example, count on sufficient voltage to effect a boost when tracking the panels maximum power point. As the panels heat up the MPPT controller simply has less voltage differential to work with, compared to battery terminal voltage. With less available voltage at lets say 115F it can't boost as well as it could at 77F....
#1 Don't expect rated 20C or 25C performance from the panel in mid day sun. Look at your daily cumulative totals and average them out.
#2 Do not place panels directly on the deck where they will have less chance of shedding heat.
#3 Consider a 24V nominal panel if you are using MPPT or test your vessel for series performance over parallel. In theory series & MPPT would always win, but in reality, on a sailboat, it does not always win due to shading.