A second source changes the panel
An ordinary panel has one source: the incoming supply. Switch off the main breaker and everything downstream is dead. Add a PV inverter, a generator or a battery UPS and that stops being true — power can now arrive from inside the house. Every protection decision downstream has to account for that.
Grid-tied PV inverters
A grid-connected inverter is not an appliance you plug in. It needs an agreement with your grid operator, a certified design and metering that handles export. In the panel it gets its own dedicated circuit with a breaker sized for the inverter's AC output, a lockable means of isolation, and clear labelling so nobody assumes the panel is dead when the main breaker is off.
Inverters can also leak smooth DC current on the AC side. Smooth DC blinds an ordinary type AC or type A RCD — it magnetises the core so real leakage is no longer detected. Depending on the inverter's own internal protection, a type B RCD may be required on that circuit, and if any group RCD is type B the incoming one must be type B too (selective RCDs).
Generators and back-feeding
A generator must never be able to feed the grid. If it does, it energises the utility line outside your house through the transformer, at full voltage, on a line that repair crews believe is dead. People have been killed this way.
The fix is mechanical, not procedural: a changeover switch or an interlock that makes grid and generator physically exclusive, so no sequence of mistakes can close both. A plug-and-socket "suicide cord" is not a substitute. The generator also needs its own earthing arrangement, which is not always the same as the grid's.
Backup circuits and what the app does
- Only part of the home is normally backed up: lighting, the fridge, the boiler pump, the router.
- Backed-up circuits are wired as a separate section fed from the changeover switch, not mixed with the rest.
- A UPS section still needs overcurrent and leakage protection on its output — the battery does not provide it.
PlanMyPanel asks whether you have PV, a generator or a UPS, but it does not design that part. When you tick any of them the app says so explicitly and produces a plan for the grid side only. The changeover scheme, the export agreement and the earthing of a second source are exactly the work described in when to call an electrician.