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.

Let me start with the apps that measure your solar array parameters for you, listed from my favorite to least favorite:

Solar Checker, by inverter manufacturer SMA Solar Technology, was the best of the bunch. It estimates your annual and lifetime energy production and takes a stab at the rate of return on your investment. But it can be awkward to use. For instance, it makes you enter the total array area in square feet rather than the number of panels.

Solar Meter ($3) is good, too, although it lacks the ability to recalculate the annual energy output as you move the phone around. The app lets you specify your array size in terms of the number of modules and peak power output per module. A nice feature is a graph of the seasonal variation in energy production. A financial screen estimates the cost and payback period, and the values were broadly consistently with other analyses I’ve done for my system. The app’s main deficiency is that it doesn’t fill in data for the U.S.—the closest it could get was Kingston, Ontario.

Solar Friend, by panel manufacturer Bosch Solar Energy, forces you to select among three Bosch-branded modules—how tacky. It also assumes that all American utilities charge the same rate for electricity; there is no place to enter local information. It calculates the total lifetime energy output rather than the annual production, which makes it inconvenient to compare directly to your electric bill.

Photovoltaic, by French manufacturer and wholesaler Axus Technologie Solaire, displays a spiffy graph of predicted energy generation by month. Although it denominates your financial savings in euros, the currency is irrelevant; if you enter your electricity rate in dollars, you can interpret the output values as dollars. The app's main failing is that, outside Europe, you need to enter the sun data for your location manually. The relevant preference is confusingly labeled "Solar Indicator (kWh/kWc)." It took me a while to figure out what this meant: kilowatt-hours of annual AC energy production per kilowatts of rated DC power output. For my location, this value is about 1,100. It equals the effective number of hours of full-on sunlight—for me, just over four hours per day times 365 days a year—times the AC-to-DC conversion efficiency, typically about 75%.

Solar Panel Advisor ($1), by San Diego-based engineering firm Microtrend, has a clunky interface, but provides a variety of data that other apps don’t—notably, the instantaneous power output at your location, given the time of day. It's a handy way to see whether your solar array is working as advertised.

Evasol, by a French solar installer of the same name, insisted that I live in the French region of Ardèche and wouldn't let me override or enter my own electricity rate. Its measurements were imprecise: the best it could do was narrow my tilt to the range of 15 to 35 degrees.

iSolBuddy, by researchers at an engineering school in Lyon, and Solar Total, by the eponymous British solar installer, measured the tilt and azimuth, but failed to return location-specific data or let me enter it manually.


Vario Solar Calculator, by a German renewables installer, measured the tilt of my array, and that was all: no azimuth, no local data. Enel Solar Power, by an Italian solar installer, measured the GPS coordinates, but not the tilt or azimuth.

For completeness, let me mention several other apps that provide more limited functionality, again in rough order from my favorite to least favorite:

PV Test (free version), by Italian solar installer Aniketos, doesn't measure your tilt or azimuth—you enter it manually—but, like Solar Panel Advisor, calculates what your instantaneous power output should be, given the time of day.

Solar Panel Installer ($5), by New Jersey Solar Consulting, has a beautiful, step-by-step interface to collect your geometry and budget data. It calculates how many panels you’ll need to produce a certain amount of energy annually. There’s a button on the last screen labeled “Financial Payback,” but, lamely, it doesn’t calculate anything. It just puts up some legalese informing you that your payback will depend on many factors blah blah.

PV Performance Calculator ($3), by a British solar wholesaler, lets you enter your system parameters and a variety of financial data, and it estimates your payback period. I found that the app greatly underestimated my system’s power output.

PV Solar Calculator ($3) let you enter your system size in terms of one parameter—either DC or AC power output, annual energy production, system cost, or surface area—and calculates the other parameters for you. I find the app significantly overestimated the power output of my array, which I attribute to various reasons: its database predicted too much sunlight at my location, it defaults to an AC/DC derating factor of 85% rather than a more realistic 75%, and it didn't adjust for suboptimal panel orientation or tilt. The interface also leaves something to be desired. It forces you to enter the system cost even if the power output is all you want, and it doesn't save your entries, so you lose them as soon as you return to the main screen.

Solar PV Payback calculates how long an array will take to pay itself off based on its size, installation cost, government rebates, and your electricity rate. The app makes you enter the number of hours of sunlight manually—no automatic lookup.

PVm3 calculates how many modules you'll need to produce a given amount of power, given your location.


Solabacus ($1) asks for your location but doesn’t seem to do anything with it. You can enter your electric bill for a 12-month period, but the app doesn’t appear to do anything with this data, either. Its most useful feature is a catalog of solar module specifications.

ElecCalc provides 10 simple utilities of use to electricians, including how many modules you'll need to generate a certain amount of power. But it lacks even the rudimentary data lookup of other apps. You might as well just use the iPhone’s built-in calculator app.

PV Master (free version) began by querying a database for location data. And that's as far as it ever got: the app was never able to reach the database. Based on this performance, I didn't feel inclined to shell out $12 for the full version.

Photovoltaique (free version) started, promisingly, by looking up my location—and ended, disappointingly, by telling me the app wouldn't work outside France.


Clearly, there's an opportunity here for a developer to come in and do a solar app right.

iPhone screen shots by George Musser