The nominal voltage is usually printed on the back of the panel and may vary for different panels.
The LT1074 has a darlington output which drops a fair bit of voltage. Note that the efficiency isn't even 90% in the datasheet. You will be better to go for a controller chip with external FETs & synchronous rectifier drive. If you can get the whole thing integrated, that may be even simpler; obviously you won't be able to tweak as many components & you're limited by the IC choices.