how many solar panels?

[John]

My wife and I have been thinking of installing solar panels on our boat (a C36). I see some for sale on Craig's List. Seventy-five watt Siemens panels for $250 each. My impression is that this is a pretty good price. Do you agree, assuming that they are not damaged? Also, we'd be using it for maybe up to a week stay up in the (SF Bay) Delta, for instance, running the fridge, and a few other things. According to the energy budget I worked out (based on Stu's formula), I'd be using some 28 amp hours on a day sail and 257 ah overnighting and about 375 ah if we do long distance sailing (which would assume use of radar, among other things). I figure a 75 watt solar panel provides some 7 amps theoretically, meaning, what, some 6 amps in reality? So, would one 75 watt solar panel probably be sufficient? Any other comments?

 

[Dennis W.]

John, There are a lot of variables involved but a rough gereralization is that you can expect about 6 amps on a bright sunny summer day at local noon assuming no shade on the panel. Before and after the output wil be less. On average you can figure on about 25 to 30 AH per day from a 75 watt panel

 

[sailingdog]

That's a pretty good price. A 75-watt panel provides closer to five amps or a bit more, and on average, will provide about 25-28 amp hours per day per panel.

If you're using 28 amp-hours on a daysail and not motoring at all, one panel will probably not be sufficient to keep your batteries topped off if you sailed every day.

If you're using 375 amp-hours per day on a long-distance cruise, you'd probably want at least four panels. That would allow you to postpone having to run the engine to re-charge the batteries by a considerable amount, but not make the boat self-sufficient in terms of electrical usage.

Just curious as to how you came up with the various amp-hour numbers you're using? How does a boat that uses 28 amp-hours on day-sail, end up using 257 amp-hours overnighting??

 

[John]

overnight

Just curious as to how you came up with the various amp-hour numbers you're using? How does a boat that uses 28 amp-hours on day-sail, end up using 257 amp-hours overnighting??[/quote]

The big difference is the use of the refrigerator. I figure I'd use it 24 hours a day, sailing overnight. For a day sail, I simply bring some ice. I also rarely use radar for a day sail; I figure some 48 ah for radar on an overnighter.Then there's the use of lights, etc. But I think the fridge is the biggie.

I forgot to mention that I have a house bank made up of two 200 ah. AGM's, if that makes a difference.

Thanks for the quick replies. There's something good to be said for winter - quicker responses on this list!

 

[newly anonymous]

for the delta

I run a separate freezer and fridge. To make things work in the Delta I need two 80-watt solar panels and a 400-watt wind turbine. I would be able to cut out one of the solar panels if I turn off the freezer and make an ice run three times a week. (One way or the other, having ice for TnT's is essential for the delta.)

If you're talking about an all-night sail, your best friend is the wind turbine. In my current setup with autopilot, radar and two large chart plotters--a Raymarine e120 and e80--and not having yet installed LED running lights, I would need a second turbine to keep my charge up on downwind legs, but what I currently have is adequate for reaching.

 

[Ross]

More insulation on those reefers will help a whole lot. My Ice box has 4 inches of extruded foam and keeps cold for a week on 40 pounds of ice.

 

[sailingdog]

Just out of curiousity, can you post the numbers you're using for your electrical budget. You're running a 400 Amp-hour battery bank, and if you're out for a week, your daily electrical budget is well over 50% of the house bank size. Your numbers for your daily budget sound awfully high. Also, can you say what equipment you're basing your electrical budget on??

 

[Maine Sail]

The big difference is the use of the refrigerator. I figure I'd use it 24 hours a day, sailing overnight. For a day sail, I simply bring some ice. I also rarely use radar for a day sail; I figure some 48 ah for radar on an overnighter.Then there's the use of lights, etc. But I think the fridge is the biggie.

A basic Alder Barbor fridge on a C-36 will draw roughly 4.5 amps per hour. Once the ice box is chilled the cycle time, how much time the compressor actually runs, will be as low as 50% and as high as 75% depending upon the insulation. If your boat is kept dock side it is best to leave the fridge on so that the ice box is always chilled and you start your sail at closer to 50% cycle time. At 4.5 amps per hour a fridge at 100% cycle time will burn 108 amp hours in a 24 hour period. Now cut that in half and you're at 54 amp hours in a 12 hour period.. I know some 60+ footers that don't use 375 ah per day??

I forgot to mention that I have a house bank made up of two 200 ah. AGM's, if that makes a difference.

If your house bank is 400ah it won't last very long with multiple 375 ah discharges.. The max any bank should ever be discharged is 50%! So you really have only 200 ah's available. If you figure that when under sail you'll probably never getback beyond 90% you really only have 180 useable amp hours before a recharge to about 90% of capacity.

The BEST advice you'll ever get regarding your electrical system is to install a Xantrex Link 10 or or Xantrex XBM battery monitor FIRST. This will give you a far better understanding of your own system than hundreds of hours of reading can. I don't advise spending any money on charging or bank upgrades until you first install a battery monitor. It will be the best money you spend as it may save you thousands....

 

[John]

Yes, I already installed a battery monitor. You may remember that I had posted some questions on this in the past. Here is my electrics budget (I hope I can copy this properly):

Y-Knot Energy Budget









Date: see below









Orig: 1-Aug-98












DAYSAIL
OVERNIGHT
AT SEA


EQUIPMENT AMPS QUAN. HRS/D LOAD QUAN. HRS/D LOAD QUAN. HRS/D LOAD
Anchor Light 0.5 1
0.0 1 6 3.0 1
0
Anchor Windlass 50.0 1
0.0
0.25 12.5
0 0
Autopilot 2.0 1 5 10.0 1 5 10.0 1 24 48
Bilge Blower 2.5 1 0 0.0 1 0 0.0 1
0
Bilge Pump 5.0 1 0.1 0.5 1 0.1 0.5 1 0.1 0.5
Cabin fan 0.5 0
0.0 0 0 0.0 2 0.5 0.5
Cabin Light 1.5 4 0 0.0 4 3 18.0 1 4 6
CD Player/stereo 1.0 1 1 1.0 1 3 3.0 1 0 0
Depth Sounder 0.4 1 6 2.4 1 7 2.8 1 24 9.6
Fresh water pump 5.0 1 0.5 2.5 1 1 5.0 1 1 5
GPS 0.5 1 6 3.0 1 6 3.0 1 24 12
masthead light 1.5 1 0 0.0 1 8 12.0 1 10 15
radar 6.0 1 0 0.0 1 2 12.0 1 8 48
Refrigerator (cycle) 6.0 1 0 0.0 1 24 144.0 1 24 144
Running Lights 3.0 1 0 0.0 1 3 9.0 1 8 24
Stereo 1.0 1 0 0.0 1 0 0.0 1 12 12
VHF receive 1.5 1 6 9.0 1 10 15.0 1 24 36
VHF transmit 30.0 1 0 0.0 1 0.25 7.5 1 0.5 15
Wind Speed 0.1 0 0 0.0 0 0 0.0 0 24 0
Daily Amp Hours Used

28.4

257.3

375.6

Total Amps 118

 

[John]

I see that didn't work out. I'll post it in a different format in a little while.

 

[MitchM]

are you running halogen lamps in your cabin ?

saving amps: we use an ipod with internal AA powered speaker for music, and replaced the cabin lamps with oil lanterns from weems and plath to save overnight amp draws. we put a huge block of ice in the icebox and only turn on the refrig when we have to. we put a piece of transparent clear plastic carpet over the whole icebox to save on energy lose when people peruse to find the cold beer. that way the amp draw from the autopilot and fridge don't draw us past the point of no return. lastly we also carry a small 1000W honda portable gasoline powered generator which will run 7 hours on a gallon of gas. we can set up the honda on deck and power up the batteries if we have to.

fair winds,

MitchM on the ahrd on frozen Lake Erie..

 

[newly anonymous]

40 pounds of ice!

More insulation on those reefers will help a whole lot. My Ice box has 4 inches of extruded foam and keeps cold for a week on 40 pounds of ice.

Good grief, Ross! If I tried to squeeze 40 pounds of ice into my reefer, I'd have no room for beer.

My systems are well insulated, thank you very much. But I'm not trying to keep a box cold, as you've suggested. I'm trying to make ice. Doing that in the California Delta, a much warmer environment than Perryville, MD, takes a bit of power.

 

[Ross]

Good grief, Ross! If I tried to squeeze 40 pounds of ice into my reefer, I'd have no room for beer.

My systems are well insulated, thank you very much. But I'm not trying to keep a box cold, as you've suggested. I'm trying to make ice. Doing that in the California Delta, a much warmer environment than Perryville, MD, takes a bit of power.

I consider July and August and the latter part of June Too hot for man or beast on a boat on the Chesapeake Bay.
Just for a check last year I unloaded my ice box. I freeze water in gallon jugs for ice and then for drinking water. I pulled out eight gallons of water or ice, six rubbermade containers of food, a quart of milk and a dozen beverage bottles and three quart size plastic bags of meat.