Preparation - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 41

Figure 1: Solar situation between the Chesapeake Bay and the Virgin Islands
If we ignore the impact of clouds for the moment, the parameters that will determine daily
solar power output at a boat's location are the length of the day and the sun's altitude
above the horizon. Solar panels receive maximum light energy when the sun is directly
overhead (altitude = 90°). The light energy received by the panels drops as the sun's posi-
tion in the sky approaches the horizon. This drop is caused by light beams spreading over
a larger area and by increased atmospheric absorption as the altitude of the sun decreases.
The Post Script: Sun Altitude and Solar Power at the end of this chapter illustrates how
these two effects reduce the power output we receive from solar panels.
Figure 1 shows that we actually lose on both counts as we move into the tropics. The
number of daylight hours drops from a peak of almost fifteen hours during summer in the
Chesapeake Bay to an average of about 11.5 hours while we're in the tropics. At the same
time, the gap between our latitude and the sun's declination widens as the sun crosses the
equator on the way to its mid-winter position above 23° south. As a result, the sun will be
significantly lower in the tropics than we are used to from our summers in the Chesapeake
Bay.
To evaluate the combined impact of these two effects on our solar power situation, we use
a quantity that is sometimes referred to as “Peak Solar Hours” (PSH). PSH tell us how
many hours of direct overhead sunlight would provide the same solar energy as the slowly