Oh dear... My brilliant post was actually pretty darn stupid.

I had managed to grab a completely erroneous figure for the surface area of the earth - I didn't check my units, and used square miles instead of square meters.

You're absolutely right about light falling on the surface of the earth proportionally, and thus you're absolutely right about that being the correct way to math this all out.

First off, let's correct my own math, accounting for the new, proper usage of units. Keep in mind that all of this is wrong, due to light falling proportionally.

Wattage supplied to the earth by the sun: 1368 * ((5.101 * 10^14) / 2) watts = 348,908.4 terawatts

99.9% efficiency spillover: 68,000,000 / ((5.101 * 10^14)/2) * 0.001 = 266 watts.

Time to boil cube of water: N/A

99.999999% spillover: N/A

Ok! Now let's do the math while taking into consideration Klein's point about the proportionality of light falling on earth:

Wattage supplied to the earth by the sun: 1368 * (3.14159265 * 6378100^2) watts = 174,831.071 terawatts

99.9% efficiency spillover: 68000000 / (3.14159265 * 6378100^2)*0.001 = 532 watts

Wow, what a difference! 532 extra watts, while over 1/3rd as bright as the noonday sun, definitely won't blind you instantly, and definitely wouldn't boil you or any water nearby! Your question about what it would look like is entirely valid, and I can't give you an answer better than "bright."

It would probably still kill us all, eventually - Assuming it runs day and night, it'd add the equivalent of 1064 extra watts of daylight to every square meter on earth. My knowledge of climate suggests to me that nearly doubling the amount of heat supplied to the earth would be "bad" - Basically defeating the whole purpose of installing the giant light death beam in the first place.

Another fun thought experiment would be to imagine what would happen if you pointed this "engine" at another, earth sized planet. 68000000 / (3.14159265 * 6378100^2)*0.999 = 531,547 watts per square meter. Assuming half of it is infrared, that's enough energy to boil everything on the surface of the planet in less than a half hour, even assuming an average starting temperature of 0C/32F. (Again using some mathematics from my game) While I suppose it makes an acceptable engine, it makes a truly terrifying weapon.

Anyway, I guess this all just goes to show you - Don't instantly believe the confident guy, particularly when he starts spouting math! Check his claims out and see if they hold water.

Cheers!