Two PV systems

Two equal PV systems, east and west, and a smaller third on the carport. A plug may run on east or west but not on the carport. Everything else is exported, and there is no unmetered base load, so every watt in the house is spoken for. The plug needs 400 W from east and west together, but neither system in particular — either one alone could cover it. That is easy to get wrong: a solver that reasons one source at a time finds nothing it must set aside for the plug on east, nothing on west, and can hand the export watts the plug needed.

What this case shows

• A restriction naming several PV systems is honoured jointly: a load allowed two systems is served from them whenever together they can cover it, even though it needs neither in particular.
• A sink spreads its draw over the systems it may use in proportion to what each has left, so two equal systems each carry half of it.

Each snapshot below is one set of readings on this wiring, drawn as a flow chart — how to read it.

Every watt spoken for

Two PV systems · snapshot 01

East and west make 1000 W each and the carport 200 W. The plug draws 400 W and the other 1800 W is exported. The carport's power can only go to the export, so the export takes all of it, and east and west cover the rest of the export and the whole plug — exactly.

Reading	W	Of gross	Share	Made of
Gross power	2200 W	1		east 1000 W · west 1000 W · carport 200 W
Grid import	0 W	0		nothing drawn from the grid
Grid export	1800 W	9/11		east 800 W · west 800 W · carport 200 W
PV production	2200 W	1		east 1000 W · west 1000 W · carport 200 W
Charging power	0 W	0		no battery charging
Discharging power	0 W	0		no battery discharging
Standby power	0 W	0		no standby draw
Self-consumption	400 W	2/11		east 200 W · west 200 W
Metering imbalanceok	0 W	0		readings reconcile exactly
Home base load	0 W	0		unmetered remainder of consumption

Ribbon widths are watts in every view. Hover, focus or tap a ribbon to see the sensor behind it.

Why this case exists

Group captivity is about a group of sinks that together need a set of sources. This case is its mirror image: one sink that needs a group of sources — east and west together — without needing either one in particular.

That is exactly what a solver reasoning one source at a time cannot see. Ask "what must the plug take from east?" and the answer is nothing, because west could cover it; ask the same of west and the answer is again nothing. So nothing is set aside for the plug on either system, even though it needs 400 W of the pair.

The whole answer fits in one table — watts from each PV system to each sink:

	east	west	carport	total
export	800	800	200	1800
plug	200	200	0	400
total	1000	1000	200

Meanwhile the export does have something set aside: the carport may feed nothing else, so all 200 W of it must go to the export in any valid allocation. The engine used to offer the export a share of all three systems, find the offers added up to more than its 1800 W, and scale every offer down by the same factor — including the 200 W it had to take from the carport. It took 180 W, and the 20 W of carport power it left behind could go nowhere but the plug, which may not use it. The plug was shown drawing 20 W from the carport and a restriction deficit was reported for a configuration that had nothing wrong with it.

Merge east and west into one 2000 W system and none of this happens: the plug then needs all 400 W from that one system, which is set aside for it up front. Whether an installation is metered as one PV system or two should not change the numbers.

The fix is that offers are still scaled down together — which keeps each sink's split in proportion to what its sources have left — but never below what a source must carry in every valid allocation.

This needs no base load to go wrong only because with one, the slack hides the mistake. In the snapshot above every watt is spoken for, so there is nowhere for a misplaced watt to go.