First off, there's no reason to give them light right after you sow. They can't use it until they germinate.

Secondly, it's really hard to replace direct sunlight. The sun outputs 1000 watts per square meter. So if you had perfectly efficient grow lights, you'd need 1000 watts per square meter (conditional... see below) to match direct sunlight. Except that 1000 watts of grow lights don't give you 1000W of light -- or even close. If it's some type of fluorescent light, you'll probably get 20-ish percent efficiency, so that's *5000* watts per square meter. LEDs, 50-60% efficiency, so 1650-2000W per square meter.

Now, you have a couple advantages over the sun. One, the sun is only up for a limited number of hours per day; you can leave your lights on as long as you want. Two, the sun is almost never head-on, but instead comes at an angle, and is thus less intense relative to the cosine of that angle. Three, you can pick your spectrum. Red and blue are better than green, so the redder and bluer your lights, the more the plants will be able to make use of it. So, to match the sun with full-spectrum fluorescent grow lights on 24/7 and assuming an average angle for the sun at 50% with it out for 11 hours, you'd need about 1650W per square meter to match it. If you tailor your spectrum and/or use LEDs, you can improve on that number.

Basically, to sum up: it's really hard to match the sun.

Oh, yeah -- and I forgot to account for clouds and reflections. But still, you get the idea.