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.

You'll probably still want a few more solar panels and accumulators if you're using laser turrets or expanding your factories. If you're making a remote radar or outpost that isn't expecting any power spikes, this system should be perfect. If a power spike does occur, the accumulators will run out of power before the solar panels can fully support the network, but the network will effectively reset every day at 79 seconds after midnight (when the solar panels can support the network independently), so there should be no long term effects.

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.