The only input you need is the standard consumption of an electrical network (Watts) when not charging accumulators, henceforth referred to as P.
For a perfectly optimal network, you need:
23.8*P / MW solar panels (or 23800*P / GW)
20*P / MW accumulators (or 20000*P / GW)
This will generate and store exactly the amount of power needed by an electrical network for every day. The accumulators will completely empty at exactly the time that the solar panels are able to run the network on their own. For the math, check below.
Edit: Thanks to Khyron for catching some of my mistakes. This should all be correct now and usable for most electrical networks, though I'd recommend using the equation I posted later on this page if you really want to be perfectly accurate.