Variables and style rules

variable	description	key	comment
tz	timezone
lat or latitude	latitude
lon or long or longitude	longitude
dni	direct normal irradiance
dni_et	direct normal irradiance at top of atmosphere (extraterrestrial)
dhi	diffuse horizontal irradiance
ghi	global horizontal irradiance
aoi	angle of incidence
aoi_projection	cos(aoi)
airmass	airmass
airmass_relative or relative_airmass	relative airmass
airmass_relative or relative_airmass	absolute airmass
diffuse_ground or e_diffuse_ground	in plane ground reflected irradiation		wholmgren: should be consistent with sky, diffuse ordering
e_direct or e_beam or direct	direct/beam irradiation in plane		What sounds better for native speakers? wholmgren: I use both, but would lean towards *direct for parallelism with dni.
e_diffuse or diffuse	total diffuse irradiation in plane. sum of ground and sky diffuse.
e or e_global or global	global irradiation in plane. sum of diffuse and beam projection.		wholmgren: or irrad_* or *_irrad
sky_diffuse or e_sky_diffuse	diffuse irradiation in plane without ground reflected irradiation
in_plane_skydiffuse			wholmgren: I think all of pvlib's diffuse sky functions return something that is already in plane, so I don't think we need this separate entry.
surf_tilt or surface_tilt	tilt angle of the surface
surf_az or surf_azimuth or surface_azimuth	azimuth angle of the surface
sun_zen or sun_zenith or solar_zenith	zenith angle of the sun
sun_az or sun_azimuth or solar_azimuth	azimuth angle of the sun
temp_cell or temp_cell_kelvin	temperature of the cell in Kelvin		I think we don't need Kelvin, that is obvious. wholmgren: agreed on no kelvin. docstring should cover it.
volt_mpp or voltage_mpp	module voltage at the maximum power point		likewise: ampere, power. wholmgren: prefer voltage_mpp