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Old 01-02-2017, 04:20 PM   #16
Ken E.
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City: Bellingham WA
Vessel Name: Hatt Trick
Vessel Model: 45' Hatteras Convertible
Join Date: Nov 2016
Posts: 1,973
Quote:
Originally Posted by djmarchand View Post
Yes, a small shadow can have a big effect on a solar panel's output. There are two ways to partially deal with this problem:

1. Some solar panels divide up the panel into 4 or more elements and isolate them with blocking diodes in the junction box. That way if 1/4 is in a shadow the other 3/4 will continue to put out. But of course this depends on where the shadow is.

2. There is always the argument whether to wire 2, 12V panels in series or parallel. Series (if the MPPT controller is designed for it) reduces wiring size and voltage drop. But if one panel is shaded that causes an internal resistance that blocks both panels. If you wire in parallel then if one panel is shaded, the other will continue to put out full. So the choice is whether shade is a possiblity or not.

Another question is 12V (17V mp) or 24V (34V mp) panels. The 24V panels are usually cheaper per watt, but because they can't be shipped UPS like 100 watt panels, if you just buy one or two they can be more expensive due to MF shipping costs.

12V, 100 watt panels are pretty darn cheap now anyway. Amazon has them for about $100 each, shipped free with Prime membership.

And if you are only going to use 100-200 watts, consider a cheaper PWM controller. You will lose 15% of the potential power with that controller compared to a MPPT but it will be half the cost.

David
Passagemaker Magazine had a lengthy article a while back on boat electrical, not discussing solar but rather ideas on reducing the cost electrical power derived from running engines, either propulsion or genset. Bigger alternators and only running the genset when house voltage dropped below a certain threshold were discussed, as well as bigger and/or more efficient house batteries. It seems like the marginal cost of a watt of energy could be compared between solar and other sources. I was curious if David has run these numbers.
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