Alright I got sick of not knowing exactly how much he could lift using just the energy from the sun (not including chemical energy that may be provided from food he eats like a normal person).
To make this work, we have to assume that superman absorbs all the energy from the sun that reaches the earth. We know he doesn't because, again, he'd be pitch black if he did. We also have to assume he's naked. That's ok, because he'd look like an empty void, so you'd only get a sense of his junk. We need to assume his conversion to mechanical energy is without waste (human's are only about 15% efficient at converting chemical energy to mechanical) We also have to assume it's sunny, there are no clouds, it's daytime and he's somewhere within about 30 degrees of the equator. Finally, we need to assume that the sun is hitting every part of his nude body evenly, which wouldn't happen, but let's see what it amounts to.
Ok. With those assumptions we will estimate approximately 100 watts of solar energy get to each square foot of superman per hour:
100 w/hour = 86 kcals/hour
20 sq ft of skin = 2000 w/hours
2000 w/hours = 1719 kcal/hours
1719 kcal/hours = 5,304,765 ftlbs/hour
Avg car = 4079 lbs
5,304,765ftlbshrs/4079lbs = 1300fthr
Ergo, superman would, with all of the above assumptions, be able to lift a car using only the sun's energy. In fact, if all he did was lift the car overhead a total distance of about 6 feet, he could bust out about 217 reps per hour. That would make superman, pretty damn strong on a sunny day.
He could lift the H-4 Hercules (the Spruce Goose) 6 feet overhead 3 times in an hour.
He could lift a blue whale or a 3 story American home 6 feet overhead twice in an hour.
If we allowed for 12 hours of such sun per day, he'd need to store energy for an entire month (assuming it works that way) to lift the Golden Gate Bridge once, and then it would be only one foot. He'd need 6 months to lift it as far as the car.
Very strong, no doubt, but only a tiny fraction of the strength that supes is often attributed with. And certainly no match for Goku.