The solar helmet collects energy at a rate of 1 EU/t, which is 20 EU per second, the same as a regular Solar Panel.
When using the SolarHelmet, it would theoretically take 25 minutes to recharge an Electric Jetpack, 50 minutes to recharge a BatPack, and 250 minutes to recharge a LapPack.
A similar item to the Solar Helmet are the Static Boots. These Boots will charge any equipped item in the Chestplate Slot at a rate of 1 EU/5 blocks traveled. Use these items in conjunction for an improved charging rate.
Note: As this is a solar-powered item, this will only work while there is a direct path for sunlight to reach the player.