pvwatts_ac returns values less than 0

[wholmgren]

Describe the bug
The pvwatts ac power model is proportional to the quantity eta as defined by

zeta = pdc / pdc0
eta = -0.0162*zeta - 0.0059/zeta + 0.9858

eta is negative for eta < 0.006 or so. While ac power less than 0 is not unphysical, I don't think the intention was for the PVWatts AC model to yield these values. I don't see any explicit reference to this in the PVWatts manual.

To Reproduce
Run pvwatts_ac with a pdc value that is less than 0.006 of pdc0.

Expected behavior
0 should be returned for these values instead.

Screenshots

Versions:

• pvlib.__version__: '0.5.2+35.g0aa4d2e'
• pandas.__version__: n/a
• python: n/a

[cwhanse]

I concur with your recommended change.