# [GRASS-user] r.sun2: "Wh" or "kWh"?

Nikos Alexandris nikos.alexandris at felis.uni-freiburg.de
Wed Jun 4 18:24:56 EDT 2008

```On Wed, 2008-06-04 at 22:49 +0200, Marco Pasetti wrote:
> Hi Nikos,
>
> AFAIK r.sun2 calculates the daily global irradiation as [kWh].[m-2].[d-1]
> unfortunately I cannot check it because, at the moment, I don't have GRASS
> installed; but we could simply confirm this by checking the values given by
> r.sun2
> just give me an example of returned data from given location, azimuth and
> tilt (and day, obviously), and I'll tell you if that value makes sense in Wh
> or kWh
>
> Marco

Hi Marco (and thank you Markus for passing the question to Thomas).

We've been working a while on our maps and the numbers were confusing
when we further tried to produce some EvapoTranspiration maps. We
believe the first result (r.sun2) is in Wh.

Our method step-by-step:

1. global_daily_avg

a. generation of daily global_rad(s) in  *** Wh ***

for i in `seq 92 302`; do r.sun2 -s --o  elevin=srtm3_stel
aspin=aspect_stel slopein=slope_stel alb=0.23 lin=5.2 lat=4275945

for i in global_rad_*; do r.mapcalc global_daily_avg=global_daily_avg +
\$i; echo "...adding map \$i"; done

c. getting the average

r.mapcalc global_daily_avg=global_daily_avg/213

r.info -r global_daily_avg
min=1733.646606
max=8620.498047

### If this would be kWh then we would need to multiply by 3600 in the
next step to get kJ. So we did but the result was way too high than what
we expected. ###

d. converting from *** Wh *** to kJ (and integer to ease off/ speed up
calculations)

r.mapcalc global_daily_avg_kJ="int(global_daily_avg * 3,6)"

r.info -r global_daily_avg_kJ
min=6241
max=31033

2. temperature_int #to get rid some problem with d.rast.leg when using
the floating point temperature map

r.mapcalc temperature_int="int(temperature)"

3. formula #this is based on the formula (Rs/2450)*((0.025*T)+0.08)

r.mapcalc
formula="(global_daily_avg_kJ/2450)*((0.025*temperature_int)+0.08)"

r.info -r formula
min=1.110000
max=6.930000

4. Further processing...

```