Do I need to tell UK Power Networks about solar panels in Hertford?

Yes, the network operator is told either way. East Herts District Council names UK Power Networks as the company that runs the local electricity network. A system of up to and including 16 A per phase is notified under G98 within 28 days after commissioning. That is 3.68 kW on a single-phase supply or 11.04 kW on three-phase. Above that, G99 applies and the application must be made before installation. MCS says adding a battery to solar is likely to exceed 16 A per phase, so a solar and battery system will often go through G99. Ask each installer you compare who submits the paperwork and how long they have allowed for it.