How accurate is NREL PVWatts? And did 3 solar contractors under size my system?

[sun grid]

The local utility is now offering a system rebate of $1/W until Dec 31st so...I'm Crunching.

We used 19000KWh last year and my goal would be 100% grid tied power replacement by solar. I've had 3 contractors size our system from11.2 to 12.3KW and when I run these systems trough PVWatts I get only up to ~ 14KWh/year.

Is PVWatts conservative?

Are the contractors conservative?

Thanks!

Victor

[Wy_White_Wolf]

Could be that the systems are designed to offset 100% of your bill. If you have TOD metering that is how the system should be designed.

WWW

[J.P.M.]

Victor:

1.) You have some homework to do. Without a lot of information, it's impossible to tell, but my guess is they probably oversized it.

2.) Good luck, but if you're in the U.S., and CA in particular, you're pretty unlikely to get a grid tie system up/running by 12/31/2015.

3.) Get a copy of "Solar Power Your Home for Dummies" before you go further.

4.) After that, read the PVWatts help/info screens and you will find that PVWatts is a model that estimates long term average PV systemoutput. It does not predict. As such, actual system output will vary by as much as about +/- 10% yr./yr. and probably by more when comparing month/month or shorter periods. That's because the weather, including how sunny it is (the fuel for your PV) varies from yr./yr., etc.

As for conservativeness, among folks who use PVWatts regularly, there seems to be some agreement that it under estimates output by maybe 10% or so. Get familiar with it and you'll know as much as most peddlers know about it. Change the system loss parameter from 14% to ~~ 9-11% and PVWatts will probably give you an output that's closer to reality.

This is not an exact science, but if a better estimate can be had for little/no additional work, where's the problem ? Self help is the most effective.

[bcroe]

We used 19KWh last year and my goal would be 100% grid tied power replacement by solar. I've had 3 contractors size our system from11.2 to 12.3KW and when I run these systems trough PVWatts I get only up to ~ 14KWh/year. Victor

19KWh are only worth a couple bucks. Better fix your numbers. Bruce Roe

[deyounte]

19KWh are only worth a couple bucks. Better fix your numbers. Bruce Roe

I'm sure he meant 19,000 KWh/year. That could be some significant coin by a factor of x1000.

Tim D.
Melbourne, FL

[Drastyn]

Victor, you must be more specifically. Where? Tilt? Orientation? After that we can estimate the production. Then you can compare the “estimation” with real data from real plant with PVcompare or PVoutputs (if you can find one similar) .

[sun grid]

Thanks all, I'm in SW Missouri 64804 with an 90/270 azimuth @ 26° with ~%80 @ 270

I only need to have the application submitted by 12/31 but I don't want to push it. The utility uses net metering with no cash payback so over sizing is wasteful while under sizing loses payback time.

I'm considering either LG NEON 300w or Sunmodule PRO XL 320 panels in a Solaredge system.

And I've made the change to 19000KWh

[J.P.M.]

Thanks all, I'm in SW Missouri 64804 with an 90/270 azimuth @ 26° with ~%80 @ 270

I only need to have the application submitted by 12/31 but I don't want to push it. The utility uses net metering with no cash payback so over sizing is wasteful while under sizing loses payback time.

I'm considering either LG NEON 300w or Sunmodule PRO XL 320 panels in a Solaredge system.

And I've made the change to 19000KWh

PVWatts is your friend. Use an 9%-11% system losses instead of the 14% default, run 1kW systems for each orientation and scale up the sizes to get what you want. looks like 180 az., 26 deg tilt will get ~ 1,483 kWh/yr. per kW and 270 az. will get 1,242 kWh/yr. per kW using 10% system losses. Merry Christmas.

That's for zip 64804.

Add: W/ the 80/20 split (80% @ 270 az., 20% @180 az., 11.2 kw syst. output will be ~ 14,700 kWh/yr. 12.3 kW syst. output ~ 15,800 kWh/yr.

[radareclipse]

Thanks all, I'm in SW Missouri 64804 with an 90/270 azimuth @ 26° with ~%80 @ 270

I only need to have the application submitted by 12/31 but I don't want to push it. The utility uses net metering with no cash payback so over sizing is wasteful while under sizing loses payback time.

I'm considering either LG NEON 300w or Sunmodule PRO XL 320 panels in a Solaredge system.

And I've made the change to 19000KWh

If Empire approves your app this month, confirm as it should give you until the end of June 2016 for total completion and approval. The Sunmodule XL320 and SolarEdge is a good combo.

[Drastyn]

PVWatts is your friend. Use an 9%-11% system losses instead of the 14% default, run 1kW systems for each orientation and scale up the sizes to get what you want. looks like 180 az., 26 deg tilt will get ~ 1,483 kWh/yr. per kW and 270 az. will get 1,242 kWh/yr. per kW using 10% system losses. Merry Christmas.

Add: W/ the 80/20 split (80% @ 270 az., 20% @180 az., 11.2 kw syst. output will be ~ 14,700 kWh/yr. 12.3 kW syst. output ~ 15,800 kWh/yr.

On PVcompare I’ve found a similar plant like your in your zone . It produced 1187 kWh/yr last yr and it is 3yrs old, with Solarworld module and Enphase as inveter. It have a low quantity of shade so it is almost similar to what PVWatt estimated (1242 kWh/yr)