There are many professional tools on the market that claim to accurately measure the strength of the sun (i.e. solar irradiance). Solar irradiance sensors, solar survey irradiance meters, pyranometers and pyrheliometers. These devices usually come in two flavors:  expensive and extremely expensive. But necessary for all applications? You be the judge...

Smartphones, on the other hand, are so commonplace nowadays and have so many hardware features, as well as the ability to connect to online services, that they can be used to perform far more tasks than the above devices. Many smartphones have hardware light meters that, with the right software, can be used to determine sun strength and estimate irradiance (even if the technology they use is inferior to the professional devices). Basically, if you can not justify the expense of the proprietary, professional tools, and believe your smartphone or tablet is powerful enough to outdo their feature lists, you might be on to something.

Can you measure sunlight intensity with a smartphone or tablet?

With the Okapi Solar calculator, you can estimate solar irradiance to help determine insolation levels. Solar survey irradiance meters have great feature lists, including items such as an inclinometer, compass and temperature sensing. But wait! Depending on the installed hardware, your phone or tablet can do all these things, and much more, when equipped with the app. The app can give you a full solar survey for your chosen site, or sites, for any date or location. Use GPS or built-in Google map to get your location, run a report on the positions of the sun for any time or future date. Use the compass and pitch detection to see how large objects affect the site. Share this data for in-depth viewing or charting. 

We all know the weather, it's not rocket science! In terms of usable irradiance, it's either full, some, or negligible. If you're looking at spending hundreds of dollars on one of the professional tools, just consider for a moment how necessary it really is (is a pushy PV salesman insisting that you need it?) and is the hardware really, actually, that much better than what is already built into your smartphone? 

The Okapi Solar calculator app, for Android devices, can use the light meter on a smartphone or tablet to estimate sunlight strength.