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moderate energy consumption of 720Wh (60Ah at 12V) per day that we hope to recover
from solar panels. Given the PSH data from above (4 PSH per day in the tropics after ad-
justing for cloud cover), we would need 720Wh / 4h = 180W of power output from our
solar panel(s). Manufacturers provide “Maximum Power Ratings” for their solar panels
but these are based on a maximum voltage higher than the typical battery charging voltage
(about 13V on average). To account for this difference, we have to reduce the published
Maximum Power Rating by about 20% to make it comparable to our calculated power re-
quirement. Conversely, we can divide our calculated power requirement by 0.8 to com-
pare it to the number in the panel specifications: 180W / 0.8 = 225W.
That is a sizable footprint for a relatively modest daily energy consumption. It also as-
sumes that we can mount the entire panel area free of obstructions from rigging and other
above deck equipment - a challenge, especially on smaller boats. By way of comparison:
we could get away with about 150W of panel power during summer in the Chesapeake
Bay, reducing our footprint requirement by one third (based on a typical PSH value of
eight hours before cloud cover adjustments).
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