I agree the display part is probably challenging, the exact location likely isn't recorded and the density could make it an issue to show on the map; regarding collecting it, I'd be surprised if there wasn't some public-ish record of it somewhere, we do all have to register with the DNO etc as micro-generators so it'd be interesting to know if someone makes an API with that information available.

Aside from that, my real wonder was whether residential solar is factored into national renewables or if it's simply too little to be worth it.