[see
Forum Thread this is taken from for more]
How many panels do I need?
Panels are rated in watts at a very specific set of lab conditions you'll probably never see. Without getting into really complicate equations, folks use an
insolation map. This will give you the amount of "usable" sunlight hours per day. So, you can multiply the panel wattage by the number of usable hours and then multiply by .8 for other losses to get roughly the average amount of watt-hours your panel will generate by day (this also depends heavily on the panel's tilt angle, the time of year, etc.).
SAM is a program that can give you seasonal data with various tilt angles.
Example: From your bills or power budget you decide you need to generate 35 kWh each day. From the
insolation map you see you get 5 hours of usable sunlight. 35,000 Wh/d / 5 h/d = 7000W. Due to conversion and other losses you'll need to divide by .80 for 8750W. You pick the type of panels your want e.g., 340W LG Neon2, since they're 340W panels you need 8750/340=25.7 panels, round up to 26 panels. Note that the actual power per day generated will vary throughout the season and weather conditions.
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Another thing to consider:
[see
Forum Thread this is taken from for more]
Efficiency is only relative to the physical size on a solar panel; that is a 200W panel produces 200W at STC regardless of it's efficiency or size. So, how do you compare solar panels other than price?
First it seems there's a a few break/make factors:
Salt
You can detect salt from the pacific ocean in South Carolina. Fortunately for most of us you only need to worry about it if you're within a mile of a salt coastline, live somewhere they're still salting the roads, a desert area, a salt pan, well... you get the idea. For this, look for the IEC 61701 rating if it applies to you.
Ammonia
IEC 62716 (Ammonia corrosion testing) may be important to you as modules that are installed onto livestock farms and greenhouses are subjected to particular environmental conditions (ammonia from fertilizer) that may be very hostile to regular solar panels.
Wind
You can Calculate Pounds/sqft force from wind speed by: PSF = 0.00249 MPH^2 and 1 pascal (Pa) = 0.021 pounds per sq. foot (psf)
So, at first I thought that from a manufacture's panel datasheet I could assume a wind rating of 2400 Pa is rated for 145 mph winds, 5400 Pa is rated at 208 MPH.
But not so, tilt and the attachment points of the panel are uber important. See Basics of Wind Loading.
Fire
Roofs have "classes": A (e.g., concrete tile, fairly fire proof), B, C, and unrated (e.g., cedar shake roof). Originally all panels were class C, and so they came up with a subdividing class I, II, III. Then it changed again to types 1 through 17. This is all really confusing to me. Still is, but try SolarABC's explanation. |
Once you've eliminated panels using the above factors above, it's time to crunch the economics. I think the general way to do this is to calculate all the watts the array could generate over it's warranty life; one year at a time, then multiply by the cost of power accounting for inflation in that year, sum them all, then divide by the cost of the array. Or do what I did, ask
SAM.
Effect of Temperature Coefficient (Pmax)
Standard test conditions are at 25 degrees C, and the datasheet will have a value for Temperature Coefficient (Pmax). I've seen them vary from -0.25 to -0.43 % / °C . Basically this is the percent power lost from the panel for each degree above 25, or gained below 25. NREL and some panel datasheets have NOCT data, which provides the number at 20 degrees C with a 1 m/s breeze. NREL may also have hourly temperature data for your location for the last few years.
During operational hours, Panel temperature is higher than ambient temperature: Tcell = Tair + (NOCT-20)/80*s; where S = insolation in mW/cm2.
So how big a difference does this make? You could use the meteorological data from NREL and calculate it all... but I was too lazy. Instead I created a hypothetical ~8 kw system in SAM and using 11 cents per kWh calculated the costs of the lost power lost over it's lifetime.
Code:
Pmax Annual Output delta from -.25 Lifetime Output loss $/life
-0.25 11,088
-0.30 9,498 1590 39,750 4,372
-0.35 8,635 2453 61,325 $6,745
-0.40 7,737 3351 83,775 $9,215
Does that mean you should go for the lowest pMax?
Nope! It'll depend on your local day-time temperatures; for a cold climate that would reverse. But, if you live where it's usually above 25 C from 9am to 5pm on the roof, then a low Pmax might be for you.
Effect of Panel Degradation
Most panels offer a 1st year loss followed by linear decline. For example, Biku panels will generate 80% of their power at 30 years. Sunpower will generate 94% of its rated power in 25 years. Panasonic HIT panels also have good life characteristics.
Unfortunately SAM doesn't seem to factor that in automatically, you have to manually input the numbers if the
Lifetime screen. It also won't account for the big first year loss. So, let's say we use a 7.25kW that provides about the power needed. I thought this would have a bigger impact, but it turned out to only be less than ~$60/yr.
Code:
loss/yr% Total $saved@ 2.5% inflation Notes
0 52,000 Hypothetical
.3586 49,542 Panasonic HIT
.6 48,108 Q CELLS 325 WATT 85% @ yr 25
Finally there are some other factors that may or may not apply to you. For example, if you live in harsh conditions double-glass (goes by a variety of names) typically have 3x better resistance to environmental factors.
Where to get Solar Panel Data
The
California GO database is the best listing of consistent and complete information I've come across
