Resolve #140 snlinverter night tare power issue

[ejmiller2]

Fixing #140 by changing the start power (Pso) comparison to use the DC input power (p_dc) instead of the calculated AC power (ac_power).

[wholmgren]

Great. One easy request and one slightly harder request:

Easy: Add a brief note to the "Bug fixes" section of the pvlib/docs/sphinx/source/whatsnew/v0.3.1.txt file and add your name or username to the "Contributors" list.

Slightly harder: Add a new test to pvlib/test/test_pvsystem.py that requires this new behavior in order for it to pass. You basically want a copy of test_snlinverter but with one of the inverters and the input parameters that were giving you the wrong results.

Thanks!

[ejmiller2]

I'll give it a shot a bit later today.

[ejmiller2]

All done (I think).

[wholmgren]

I know I asked for brief, but maybe something more like "Fixes night tare issue in snlinverter. The night tare was erroneously activated when the AC output power, rather than the DC input power, was less than the inverter's self-consumption" or similar. I had to look up Pnt, so I'm sure others would need to as well.

[wholmgren]

@ejmiller2 thank you for fixing this.