solar panel output

[walt]

Ive been "collecting" posted solar panel daily output post on the internet. Ie, if you have some watt panel, how many amp*hours per day can you expect from it. The solar output will or course vary a lot depending on weather, location, time of year ect and that is also what is seen in the numbers posted on the internet (highest number is 73% larger than the lowest number)

Ive been using a scaling number "SN" of " amp*hours / rated watt. Then all you need to do is multiply the scaling number by the panel rated watt and you get the daily amp*hours. For example, if the scaling number is 0.333 and you have a 50 watt panel, then the daily amp hours is 50*0.333 = 16.65 amp hours.

Here are some of the SN which people have posted (no personal details given to protect the inocent). I think most of the posters were just repeating what they had read, a couple had actually measured. To clarify again, if someone posted that a 100 watt panel produced 32.5 amp hours per day, the SN would be 32.5/100 = .325.


.406 (used MPPT)
.398
.388
.380
.325
.234

Averge of the bottom five (not MPPT) is .345.

Note that Don Casey uses an estimate of full rated current * 5 hours a day. This gives a SN of .277 to .333 depending on if the panel open circuit voltage is assumed to be 15 or 18 volts. I dont think details were given on if the panel is just facing straight up, sort of pointed but fixed or continually pointed during the day.

Im hoping that someone who has posted solar panel daily output will give details of the set up - ie, panel mounted flat looking straight up, MPPT or no MPPT, location, time of year, ect where they measured it, and how they measured the amp hours.

FYI, the number of .345 amp*hours/rated watt seems high to me if you just put a panel facing straight up on the boat deck. But Ive also never measured this (hopefully next summer I will).

 

[newly anonymous]

I'm skeptical because...

...a scaling number would have to fluctuate with latitude, season and--in your case, Walt--altitude. Add to that the understanding that solar panels vary tremendously by make, model and "generation." Add to that the variance that some of us point our panels straight up while others constantly re-aim their panels at the sun.

I also question the common wisdom that someone is only effectively charging 5hrs per day. I get a lot more work out of my system. Don's system of calculating at 5hrs effectively factors in the same thing you're trying to do with a scaling number, only this one according to time while you're going with a percentage. Can't go both ways at once, I would guess. When I run your calculation by my system, it comes far short of the numbers I actually get.

 

[John Nantz]

Whew! That's a LOT of stuff to track!

Walt - not sure what the end goal is but there are soooo many variables it would be hard to make any sense out of the data.

Take "Newly annonymous" (previous poster), sailing in fog, for example, would be very hard on solar panel output. The fog itself reducing solar energy available, then there is the variable heeling of the boat, water droplets on the panel faces,...

It's bad enough when the boat is siting level in a slip. Throughout the day there are various shadows that track across the panel, like the sail cover on the boom (I don't have in-mast furling), radar stern mast, wind generator mast, piling, powerboat next door, etc.

For my system I use a pair of stern-rail mounted Kyocera 130s which usually charge everything up while at anchor by around 1 or 2 pm. Kind of a no-brainer setup. Also note that sunny days here, unlike California, can be limited.

A wind generator makes a good adjunct to solar panels.

 

[newly anonymous]

Okay, John...

Take "Newly annonymous" (previous poster), sailing in fog, for example, would be very hard...

Allow me to point out, ahem, that it wasn't fog I was sailing in last weekend. Sun on Saturday, drizzle on Sunday. This is what I get for skipping out of church.

But I agree wholeheartedly with your point that the way to go is to augment solar with wind. For half the price of a generator, you can have a system where you'll never need a generator.

 

[Ross]

Solar insolation numbers are well documented and this is a good site for looking at the real numbers for many locations. keep in mind that the solar panel is most efficient when normal to the sun.
http://www.solarpanelsplus.com/solar-insolation-levels/

 

[walt]

Ross, interesting and useful link.

Why worry about all of this - panels are rated in watts and boaters are concerned about daily amp*hours. Im just looking for ballpark peak numbers and the web site could be used to scale for different months and locations. Things vary a lot of course.

From the web site: solar insolation are commonly expressed in kWh/m 2/day (ie, Kwatts per square meter over a days time period). This is the amount of solar energy that strikes a square meter of the earth's surface in a single day.

To convert this to amp hours for a panel is still complex.. You need to know the efficiency and actual collection area of the panel.. And, what happens if the panel is sitting in a fixed postion looking straight up?

 

[walt]

Also interesting from that web site...

The ratio of max power during the summer to min power during the winter for a couple of cities.. (pick the peak in the summer, divide by the min in the winter)

Denver 2.7
Cheyanne Wyoming 4.08
Miami 1.81
Los Angeles 2.8

Some of the areas with the biggest difference - the boat likely on the hard during the winter.

Edit - also interesing. During the summer month June, July, August, the ratio of the max difference between the city with the highest power and the lowest power is only about 1.4 - and the cities range from Montpelier VT to Honolulu HI.

 

[Ross]

The higher latitudes have longer days in May, June and July thus the balance. There are other table that consider the angle of incidence. And still other tables that consider the percentage of cloud cover.
If you measure the surface area of your 5 watt panel in square cm and divide by 10,000 you get the fraction of a square meter for you collector. Then from The Almanac determine the day length and from the bell curve the hourly distribution of solar energy ........................it is winter time you have time to work these calculations.

 

[walt]

Another way to use that web site..

Suppose someone in Los Angles has a 100 watt panel aimed straight up with a conventional charge controller and gets a good average measured 30 amp hours per day in March.

So we know for LA in March, the "SN" = 30/100 = .3 amphours per day / rated watt.

Then, I want to know what a 50 watt panel (in a similar setup) would get in Denver in the month of June.

From the web site, LA in March gets 5.09 kWh/m 2/day and Denver gets 6.71 in June.

So for the 50 watt panel in Denver in June, the daily amp hours is estimated as 50 watts * .3 amphours per day/rated watt * (6.71/5.09) = 19.7 amp hours per day.

Yup, getting a little to complicated to be useful since the calculated number could still be way off of what you actually get.

 

[Ross]

Walt, you're getting there. Just get an almanac for finding the declination so that you can keep the panel normal to the sun. Then figure the atmospheric dust, world wide volcanic activity, snow cover on the panel ( I got snow bound in Rabbit Ears Pass on the 7th of June 1962) and last the little bird that lands on the panel to rest.