How to factor roof angles and inverter efficiency in sizing a solar PV system?

Question

I want to install a number of Mission Solar 420 W PV panels (pdf) on my roof. By estimating an average of 5 solar hours per day over the course of the year, I believe I need 9 kW of panels. So by a nominal calculation of 420 W x 22 panels = 9240 W, or 9.24kW

However, that is only using the nominal wattage. How do I reduce that number based on roof angles and inverter efficiency to determine how many panels I actually need?

If my inverter is 95% efficient, then my usable power with 22 panels is reduced to 8778 W, so I need to add another panel to get back above 9000 W.

As for roof angles, do the average solar hours that people use for a quick calculation (5 hours) include a basic estimate of the inefficiency of standard roof angles and assume roof location can get decent coverage? Or do those calculations need to be done separately?

Answer 1

PVWatts

PVWatts provides exactly what you're looking for.

It's a solar calculator maintained by the National Renewable Energy Lab of the U.S. Department of Energy. To use the tool:

1. Enter the location of the proposed solar system. PVWatts will show you nearby weather stations for the simulation, and uses the most relevant solar irradiation data.
2. Enter the system parameters. You can enter either basic info (size, orientation, angle) or provide more detail (inverter efficiency, system losses, etc). You can even draw an outline of your system on the satellite imagery of the proposed location.

The results include monthly solar radiation and AC power production values. If you included your utility rate, it will provide the value of this power as well. You can also download hourly data which includes much more detailed information on irradiance and factors such as temperature and wind speed which affect the output.

How to input your specifics

Roof angles. Entered as tilt. If there are different facets of your roof with different angles you could model each as a separate system and add it up at the end.

Inverter efficiency. Under the "Advanced Parameters" menu you can specify the efficiency of your proposed inverter if you know it, or stick with the default of 96%.

Note that there are multiple other sources of loss in a PV system as well -- PVWatts assumes total system losses of 14.08% based on the latest research. This covers multiple factors, each of which you can tweak:

The help menus accessible in the interface provide a lot of good info, or for even more detail and the research that backs up the tool, you can look at the technical manual (pdf).