The iPhone seems like the perfect accessory for a solar power enthusiast. Right now, you have to navigate a maze of websites such as PV Watts to calculate how much energy you can expect to produce and how many years a solar array will take to pay itself off. The iPhone could cut to the chase. It even looks like a miniature solar panel. And indeed the App Store is filled with solar apps. What a shame that none of them quite measures up.
I downloaded and tried out 20 solar-related iOS apps—all I could find as of July 15th, apart from those that were restricted to customers of certain utilities, were geared toward professional solar installers, or were merely reference books without any ability to calculate. I don’t have an Android or WM7 phone and would love to hear by Twitter or email from users who have tried out solar apps for those devices.
The coolest apps make use of the iPhone's GPS, compass, and tiltometer. You climb up to your roof and position the iPhone where you want your solar panel to be and these apps calculate how much power it'll generate. None uses the iPhone’s camera or ambient light sensor to measure the intensity of sunlight directly; rather, they calculate it from your latitude, panel geometry, and, in a couple of cases, time of day.
For my panels, the tilt measurements ranged from 10 to 18 degrees and the azimuths from 144 to 186 degrees. For comparison, the N.J. state inspector who certified our array estimated a tilt of 10 degrees and an azimuth of 202 degrees—so the apps are close enough. By and large, they accurately predicted how much energy my array generates in a year.
Two of the best apps, Solar Checker and Photovoltaic, recalculate the annual energy output in real time as you move the phone, making it easy to experiment with different possible configurations. Their readings suggested that the optimal tilt for my location would be 33 degrees, which is close to the 38 degrees estimated by the industry-standard Solar Pathfinder software and the 34 degrees estimated by MACS Labs, a California-based energy consultancy.
Some of the apps include a rudimentary financial calculator. You enter the cost of your system and your utility's electricity rate in order to calculate how long it'll take to break even. I found that these estimates varied hugely. None of the apps can handle all the data that would go into a proper financial analysis. By the way, if you play with this feature, the electricity rate you enter should be the actual rate plus the value of any government incentive. For instance, those of us in N.J. get credits worth about $0.60 per kilowatt-hour.
For reasons known only to the developers, some of the apps work only in Europe. Because the sun doesn't shine elsewhere, I guess. Try to use them in the U.S. and you get an error message. How hard could it be to make an app that works anywhere? The mathematical formulas are universal and free online databases provide solar yield for anyplace in the world.

Good. I tried it