I'm wondering if any of you have a real world guesstimate as to what your panels
are actually putting out. Here is my experience.
We took a 10+ week trip in Florida
with about 3+ weeks in Nov/Dec. and the remainder in Mar/Apr with a lot of sun. The fridge ran on the average about 10 hours a day at 3 1/2 amps and normally there was about 4 hours of computer time at about 2 1/2 amps I figured that on the average we were using about 45 amp/hours a day. Lights were battery
powered so the only real users were the two things above. The amp readings were via a digital amp meter.
...180 watts of solar
at the time on the boat and a good Blue Sky MPPT
controller. During that total time period we ran the gen-set about 5-6 hours, so over 90% of the power came from the batteries. So I'm guessing that we were getting 40 to 45 amp/hrs a day from the solar
into the two 12 volt (new) batteries in parallel.
So in our case if we divided the panel wattage (180) by 4 we would of had a number of 45 which is very close to what I think we were putting in. If this is a trend that matches what others have found then someone looking for the magic panel wattage for their needs could do an energy audit of what their needs were in amp/hrs per day and multiply by 4 and have a fair idea of how large of an array they needed in watts.
I think a lot of people are optimistic on what they are going to get from a panel. A 12 volt panel runs at about 17 volts and the wattage is figured on that. If you aren't running an MPPT
controller you loose a fair amount of the advertised wattage right there since the panel is drawn down to battery
voltage. As has been mentioned the panel output is usually lower since our panels
are operating at a higher temperature than the temp they are rated at. Also who has the panel orientated to the sun at 90 degrees all day or ever in the day. Take all of that into consideration and you aren't ever going to get advertised wattage. Then what about shading on some days or from the boat itself?
My 'own' formula for figuring what I might expect as real output is as follows.
((Panel advertised wattage/17 (typical operating voltage))/2) X 7 (hours of fair sun) = amp/hrs per day
In our case that would work out to ((180/17)/2) X 7 = 37 amp/hr per day
The 37 is a little lower than what I think we were getting but then we did run the gen-set a little also.
One other thing is that as the batteries come up and can't accept as much charge you aren't getting it even if the array can put it out.
If our batteries go below 12 volts I'll run the gen-set and it will usually start with about 22-24 amps to the batteries and withing 15-30 minutes be down to about 10 amps. I'll shut it off and let the solar array finish the job.
So be honest guys what do you think your arrays are putting out not what you see for a few minutes at peak sun,