10K Array Output

ColoradoSolar

Not sure if this question belongs here.

If I have a 10k (STC) array how much can I expect to get out of the array in real world conditions?
Also how much can I expect once the DC is converted to AC (after losses)?

Thanks

dennis461

In Colorado? Aimed south or north? tilted up or down....
Get my point, a lot depends on location orientation, tilt angle, shading, etc.
Did you try using this web calculator?
http://pvwatts.nrel.gov/

ColoradoSolar

Yes in Colorado, aimed south, tilted 30 degrees. Completely off-grid.
I have used pvwatts.

What I was mainly looking for is something like "for a properly aligned array you can expect roughly 80% of STC on a sunny day"

Photowhit

ColoradoSolar said:

Yes in Colorado, aimed south, tilted 30 degrees. Completely off-grid.
I have used pvwatts.

What I was mainly looking for is something like "for a properly aligned array you can expect roughly 80% of STC on a sunny day"

What panel do you have? You can likely find or calculate their Normal Operating Cell Temperature value (NOCT) it generally runs about 75% of their panel rating. Then you can multiply that times the Solar isolation (number of hours your area averages during each month)

Please understand that this is NOT what you can use off grid! Off Grid systems must have waste to maintain good battery health, they must be brought to full charge often, a couple times a week. Also in calculating your needs it varies by season. We have a thread going now where someone is trying to heat with solar, a very daunting task. If your needs are greater in the winter time, with less sun, you may have to have a huge waste in the summer. If on the other hand you heat with gas or wood and need air conditioning in the summer, you may find your needs come at a time with more hours of sunlight.

It varies by location and needs, but a rules of thumb is that;
Off grid systems call for an array production of 3x what a grid tied system would require for the same use.

BB.

Take the hours of sun per day (long term average) for PVWatts (by month) and use this equation:

Array Wattage * 0.52 AC off grid system eff * hours of sun per day = Watt*Hours per day

And as Photowhit says--The above equation is about the average "best" daily power output you can get. Some days you will get more, some days less. And if you run ~65%-75% of that number as "base loads" (same loads every day) is probably what I would suggest you plan for.

For sunny days, then you can run the washing machine, shop tools, extra well pumping, running the vacuum cleaner, etc. and when you have long strings of clouds/bad weather, you will have to cut back.

The equation and base load stuff is a bit "conservative"--And it seems to work well as a starting point.

-Bill

Estragon

In Co you may do a bit better if at high alt.

Horsefly

When I was designing mine (still waiting for spring to put it together), I used 0.5 instead of the Bill's 0.52 efficiency derating. Mine was based on muiltiplying together all the efficiency hits I could think of. The fact that it came so close to Bill's standard number makes me feel better.

To the OP: I'm also in CO, and up at nearly 9000 ft elevation. Like @Estragon said, there may be an improvement in production for us due to the high altitude, but for me I'm not counting on it. It will be a bonus if it is better.

It probably doesn't matter too much, but 30 degree tilt would be optimum for someplace in northern Mexico or in San Antonio, Texas. It will also make your winter production even smaller.  I'm using 38 degrees, and maybe tilting a bit more in winter.

ColoradoSolar

Let me give a bit more background. We bought a house in Colorado (7800ft) that has an existing solar system which is too small so we will be upgrading in the spring. I have been researching solar for a couple of years and we have now been living in the house for about 5 months, so I understand about the need to extra solar capacity and using opportunity loads on sunny days. We have 2 pellets stoves for heat and according to my calculations would not have comfortably have enough battery capacity to run them overnight so I installed a Honda EU1000 with propane conversion to handle the overnight loads.

The main thing with the new system is I don't want to size it too small. I have a spreadsheet with my estimated daily usage and many calculations for solar and battery capacity. I have run PVWatts many times and with my proposed array I would have plenty of power. The thing I wanted to understand with the question I asked is how much of STC can I expect out of a panel in real world conditions, ignoring batteries, loads, etc...

ColoradoSolar

My current calculations use something around 0.664 from array to AC and my calculations are lower than what PVWatts calculates.

I'm using 30 degrees because that seems to be the max tilt I can get on the ground mounts I am looking at. PVWatts shows about a 1% difference between 30 and 38 tilt. I know for winter a higher tilt would be better but in my location we get some strong winds and I would rather have the array lower down and more firmly mounted than a pole mount.

Horsefly

What you've seen posted above is the overall efficiency hit of 50% from the panels all the way to your loads.  If you really are wanting only the efficiency loss of the panels, 80% is a pretty good number. At 7800 ft you will probably do better than that, but there isn't any way to know for sure.

Edit to add: I used 75% for the efficiency of the panels in my design, but I was trying to be especially conservative.

Estragon

As you probably know, a big factor in end to end efficiency is your inverter. Big inverters drving small loads can really drag overall efficiency down. Getting a handle on your stove's load and designing accordingly could make a big difference.