There is another option I would like to throw out for solar balancing. I don't suggest this for the sake of realism, however, real life can be a source of inspiration for interesting game mechanics.
Why are renewable energy sources not used for all our power generating needs in real life? There are a number of reasons, including economic viability, often requiring exotic materials, efficiency, and power grid load balancing. Two that could be applied well to Minecraft are variability of supply and location-based renewable resources.
What if solars output a variable amount of energy, with a long time based variance, simulating seasonal and weather effects? Suppose that instead of outputing 512 EU/tick every day, an advanced solar would output anywhere between 500,000 and 0 EU/tick. The average EU/tick would remain constant, if you averaged over a long time period (say 30 Minecraft days), but the actual power output would vary from day to day. This would make having alot of of storage capacity vital. You might think "Great! This solar is outputting a ludicrous quantity of energy! I've filled up my MFSU in 10 seconds flat!" only to realise that now your storage is full, you are wasting every excess EU being produced, and that MFSU is going to have to last you a long time when the "sunny" day ends. As an additional component of depth, the variance could be linked to the type of biome a solar is placed in. Naturally, we would expect a desert to be a great site for a solar plant, while a tundra would be far less useful.
This effectively means that the actual utility you get out of solars is dependant on your storage capabilities. The more you rely on solar, the bigger your storage needs to be in order to make the most out of the array, and avoid wasting excess EUs. It ties in nicely with the storage options available, in that some basic solar panels might be efficiently catered for with only a dozen bat-boxes, while more advanced arrays may require a lot of MFSUs to even out supply. It gives a unique challenge to solars, without touching their cost or average energy production. This system also has the advantage of not requiring the development of new blocks, and being pretty easy to implement in the existing mods. My knowledge of Java is pretty limited, but it seems like this would be pretty easy to test without requiring more complex mechanics.