I think multi-block solar panels would be a really great idea if balanced.
I'm thinking placing solar panels in a 1 block height cuboid shape should multiply efficiency. A solar panel could have a base efficiency of 20% and every (working) solar panel in the multi-block adds 1% efficiency. If the multi-block isn't cuboid, the multi-block won't form and every solar panel will be stuck at 20% efficiency.
The solar panels will require maintenance in the form of solar modules. If a solar panel is missing a solar module, the solar module is broken, or there is no direct sunlight, it won't produce power and there will be a 2% efficiency reduction. The solar modules won't break over time but anything walking on the solar panel or rain will cause damage. This will make solar panels expensive to maintain in most biomes and players will need to find ways to keep mobs from spawning on the multi-block.
The reductions don't seem like much but when you have a solar grid of 80 solar panels for 100% efficiency, a single mob or griefer could easily slash the power produced and leave you to check each and every solar panel until all the broken solar modules are replaced. Just 10 broken solar modules can reduce power production down to 61%.
Of course the numbers can be tweaked for balance depending on the recipe costs for the components. Hopefully encouraging the use of solar panels en masse as a multi-block will count down on lag (instead of 80 individual solar panels spitting out power, it would just be the multi-block and some simple math).