# Percent sun numbers

Posted by Bill Li on August 7, 2008, 5:18 am

Another seemingly basic question which I was hoping someone could verify the

I found a set of tables containing percentage sun per month for different
cities in North America.  Elsewhere(s), I've found tables containing
(presumably average) daily solar radiation per month (horizontal, which I
presume means flux normal to the surface of the Earth) for different
locations.  (the latter tables all seem to roughly agree with one another,
despite different sources, so I mostly trust them)

My question is: do I need to de-rate the daily solar radiation numbers by
the % sunshine, or can I take these numbers to be averaged taking the cloudy
time into account?  Also, presumably the daily solar radiation average is
the mean for the entire day, so peak solar radiation at solar noon on a
sunny day ought to be higher.

I think I can just ignore the % sunshine numbers except to use to come up
with numbers for how many days during the winter I need storage for in
between sunny days, on average. (not that I'd necessarily be able to store
that much heat economically)

Thank you for your help.

Bill

Posted by Martin Riddle on August 9, 2008, 7:48 pm

| Another seemingly basic question which I was hoping someone could verify the
| answer for me...
|
| I found a set of tables containing percentage sun per month for different
| cities in North America.  Elsewhere(s), I've found tables containing
| (presumably average) daily solar radiation per month (horizontal, which I
| presume means flux normal to the surface of the Earth) for different
| locations.  (the latter tables all seem to roughly agree with one another,
| despite different sources, so I mostly trust them)
|
| My question is: do I need to de-rate the daily solar radiation numbers by
| the % sunshine, or can I take these numbers to be averaged taking the cloudy
| time into account?  Also, presumably the daily solar radiation average is
| the mean for the entire day, so peak solar radiation at solar noon on a
| sunny day ought to be higher.
|
| I think I can just ignore the % sunshine numbers except to use to come up
| with numbers for how many days during the winter I need storage for in
| between sunny days, on average. (not that I'd necessarily be able to store
| that much heat economically)
|
| Thank you for your help.
|
| Bill
|
|

I would use the tables as they are. But account for system efficienies.

Cheers

Posted by Ron Rosenfeld on August 9, 2008, 8:09 pm

The source for the tables should tell you how they are constructed
(horizontal plane vs a plane tilted at the latitude angle are two common
methods).  Usually, these are a multiyear (10+) average on a month by month
basis, so would take into account "cloudy days".  They represent the
"effective full sun hours" for the location & time.

There is data available that actually gives you hourly meteorologic data
for a "typical year".  TMY2 data has been averaged over a 30 year period
(1961-1990) and developed by NREL.  (Google TMY2 for more info and
sources).  TMY3 is also available, but I've not explored that data.

That is correct for the data with which I am familiar.
--ron