An MPPT controller will take higher voltages and regulate them down to the lower voltages needed for battery charging
. In the case of two similar panels, you can wire them in series for 24v plus voltage so you can either use smaller feed lines and/or have less line loss to the controller. The controller will automatically go into a float charging
state when the batteries are fully charged so no need to worry about having too much charging capacity. For those who haven't checked your panels output, a nominal 12v panel will generate way more than 12v in bright sunlight. MPPT controllers convert higher voltages, I'd guess closer to 40 volts in optimum conditions, into charging voltages for batteries which is about 14.2 volts give or take a few tenths. Don't know whether you'd need a special MPPT controller to handle more than 24v nominal out put if you wired more than two panels in series. MPPT work with nominal 12v input as well, they just lose some of the efficiency that you are paying dearly for.
Realestate for mounting panels is a problem on most boats. On my boat, I had a welder extend the Pushpit top rail about 2', added another vertical support and cross bracing to mount a 135 watt Kyocera
panel each side. The panels are wired in series for a nominal 24v into an MPPT controller. I thought this would be overkill for my relatively small needs, no refrigeration
, TV, all LED lights
, self steering
vane, etc. When the sun shines they do produce way more juice than I need but the pesky sun don't always shine. Any shading drastically reduces the output of a solar panel. Even a slight haze can reduce the output by 1/2 and an overcast by 80% or more. Forget getting any charge from a full moon. Also, you have to be cognizant of stuff on the boat that will shade the panels. Mounting the panels on a bimini
with a boom overhead will significantly decrease output unless the boom is swung out of the way. Mounting the panels as high and as far aft is the best but still has some issues. On my recent TransPac, I was heading into the afternoon sun the whole way. After about 1400, the mast
began to shade and totally shading the panels by about 1600 which resulted in a significant reduction in total daily output. Also, I was on Port tack with prevailing easterly winds for about half the distance so boat and panels were heeled away from the sun reducing their output.
On my summer slide to Hawaii
had heavy overcast for almost half the trip. After about 10 days, my 220amp house battery bank was getting below 12v and needing charging even with my limited need for juice. Underway, I run a Garmin
3206 plotter, AIS
, and knotmeter
all the time, intermittant use of Ham Radio
, Pactor Modem
and Netbook Computer and LED running and interior
lights when the sun goes down. Overall usage is probably less than 3 amps per hour overall. Still, that's almost 70 amps a day if my calculations are correct. Enough load to drain my battery bank in a few days without charging. My 270 watts of maximum possible solar output really helped but didn't keep up with the daily usage in overcast conditions. Once the sun finally came out, the batteries slowly recharged so I didn't have to run the engine
to charge the batteries on the 15 day passage
. For those with refrigeration
, an autopilot
, and a crew that needs constant visual and aural stimulation, way more generating capacity is going to be needed.
Beware of efficiency claims for panels. FWIU there are two types of panels. The higher efficiency panels are not shade tolerant. Otherwise they are more effected when not pointed directly at the sun in clear sky conditions. Other types of panels are not as ultimately efficient but more tolerant of less than perfect conditions. Overall, the one type is better for the less than perfect conditions on a boat. This is what I've heard so if I'm wrong on this, hopfully others will correct me.
So in your case, put on as large and/or as many panels as you can fit if solar is going to be your only non fossil fuel
charging source. Putting a 200+ watt panel on your stern may be the best way to go. The one you are looking at has a high enough voltage so it should make good use of the greater charging efficiency of an MPPT controller. If you can stow additional panels to supplement your output at anchor
, all the better. On a 27' boat, stowage is at a premium, however.
The best way to become energy self sufficient is to cut the amount of amps you need. We lived aboard and cruised for nearly 4 years without refrigeration and don't have it on my current
boat. We didn't/don't miss it. Amazing how much better food
tastes at room temp, even beer
. Most food
will keep for several days without refrigeration even in the tropics.
Some form of self steering
is mandatory on a cruising boat. An autopilot
will draw a tremendous number of amps in the real world of sailing a boat on the open ocean. You'll not only need to buy the autopilot but all the spares for the inevitable breakdown, the increased battery bank to handle the load, and generating capacity to feed the hungry bastard its electrons. In short the real cost of an autopilot for an offshore
sailed boat is way more than twice the initial cost of the pilot. A self steering
vane works for free and will steer the boat for about 99% of your cruising time. It also is a fairly simple mechanical device that can be fixed with a little chewing gum and duct tape by the average sailor should anything go wrong. Contrast that with an autopilot that requires an EE degree just to read to read the manual. TV's, DVD
players and Muzak piped throughout the boat are trappings of our modern society. We've managed to do fine, thankyou, without that external stimulation. Somehow I'm never been bored on the boat and don't have that niggling desire for more 'stuff' to occupy my time. Of course, that's the need for more stuff that doesn't have to do with sailing the boat.
Sounds like you have a good plan for installing solar on your boat. It's certainly made my boat way more pleasant not having to run the engine