Greg, I was actually kind of looking forward to writing some code for this, and I agree with you that I shouldn't reinvent the wheel. Thanks for the advice!
Matthew On Tue, Jul 21, 2020 at 9:17 AM Greg Troxel <g...@lexort.com> wrote: > Matthew <redin...@gmail.com> writes: > > > I'm thinking of installing a solar electric system on my house, and am > in > > the planning phases. One of the things that you use in planning these > > systems is "peak sun hours per day" for your location -- the number of > > average hours each day during which the sun is shining at 100% of > maximum > > for a given location. > > average hours at 100% of theoretical is very different from calculating > KWh/m^2 per day. When the sun is low in the sky, it might only deliver > 250 W/m^2 to horizontal, but still be "at 100%". But I don't see why > people would care about this, rather than light arriving at the panel at > the angle it's really at, perhaps combined with the W/m^2 ==> > W(electricty) transfer function. > > Look at the graph of solar radiation. On blue-sky days, you will see > zero at night, very slowly building to maybe 50 in the morning, and an > abrupt transition up when the sun hits your sensor (when it gets over > the tall pines trees, and yes I see you are NM not MA so that's a joke > but the concept reamins). Then it traces a curve which is sort of > sin(elevatio) until the sun goes behind an obstacle, and then it fades > out. > > On cloudy days, you get less. On partly sunny days, there is sort of > the curve I described, sometimes at it, sometimes less. And, there are > often values above the blue-sky line, which I think is lensing from > cloud edges. > > > You can look up these values for a nearby city (in my case, the closest > is > > Albuquerque, 6.77 peak sun hours per day or 6.77 kWh/m^2/day), but I've > > been running a Davis Vantage Pro 2 Plus at my location (with a watt > meter > > and a UV sensor) for about fifteen months now, collecting the data with > > WeeWX, so I should have pretty reliable data by now. But I don't know > how > > to turn the wattage data (instantaneous watts per meter) into what I > need, > > which is KWh / m^2 / day. Probably it's possible with a lot of math > and/or > > a spreadsheet, but I was wondering if perhaps WeeWX has some built-in > > method. > > I would suggest writing a program and reading the weewx database and > calculating, rather than extending weewx, because you want to calculate > over your historical data, rather than have an online today's energy > report. > > The Davis sensor tries to measure W/m^2, and I believe that's arriving > on a horizonal square meter. So if you take a day, and for each value > (assuming a 5 minute archive interval), multiply the value by 5m/60m (= > 1/12) and add them all, you will get the total energy (in kWh, which is > like Joules, but 3.6E6 bigger). > > Is your sensor level (bubble level)? > > You also need to check calibration of your sensor. I haven't done that > yet for mine, but on a blue-sky day you should be able to calculate the > standard solar irradiance for your latittude (and maybe humidity/temp??) > and compare that to the curve you measure. > > Keep in mind that for solar, your panels will be angled. So the energy > is different, and generally more. > > There are various libraries out there for doing this sort of thing. > Before you write much code, see > https://pvpmc.sandia.gov/applications/pv_lib-toolbox/ > > > This isn't at all critical to have, but it should be more accurate than > > Albuquerque's data (Albuquerque being about 70 straight-line miles north > of > > me). Thanks! > > It's more accurate locally, but it's 1 year vs (probably) an average of > multiple years, and you have the calibration issue to deal with. But I > encourage you to do this anyway! > -- Matthew McCleary KF5VUD - www.kf5vud.org voice 505.239.1044 -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to weewx-user+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/weewx-user/CAECeYYS7VHaUWz6vBRVvpzCELq34sm3rSed8Uv1nPN7J1itVzA%40mail.gmail.com.
