There is a decision inside most Illinois commercial solar-plus-storage proposals that the customer never sees, is never asked about, and cannot reverse for the next fifteen to twenty years. It is made by the installer, usually by default, usually because it is the cheapest way to build the system.
The decision is how the battery complies with Illinois Shines, and it determines whether your battery is allowed to charge from the grid.
The problem the rule is solving
Illinois Shines pays renewable energy credits on solar production. Once a battery is attached, an obvious accounting problem appears: if the battery can be filled from the grid and then discharged, energy that was never solar could be counted as solar. The program's guidebook closes that hole, and it offers more than one way to close it.
Broadly, the compliance paths fall into two families.
Meter the grid energy out. Put the production metering where it sees the array's output rather than the battery's, or use metering that can distinguish and net the grid-sourced energy. Under these paths the battery may charge from the grid, because the credited production is walled off from it by measurement.
Lock the inverter. Configure the equipment so it physically cannot charge from line power. Nothing needs to be metered separately because the situation cannot arise. Under this path the battery may never charge from the grid, and the guidebook is explicit that the election may not be changed for the duration of the credit term.
The second is cheaper to build. It is a configuration flag rather than an extra meter and an extra commissioning step. So it is very often what gets built.
Why the choice is expensive in ComEd territory specifically
Now put that against how ComEd bills commercial demand.
Billed demand is the highest half hour recorded between nine in the morning and six in the evening on weekdays. There is no off-peak demand component. Energy pulled at two in the morning is invisible to the determinant entirely.
That is close to a gift to a battery. Fill the pack overnight, at no demand cost whatsoever, and discharge it into the afternoon peak. The pack's usable capacity, not the sun, sets how much peak it can cover, and it can do it in December as easily as in June.
Take grid charging away and the battery can only shave with what the array put into it that day. On a winter afternoon, or on a cloudy week, or on a site whose peak comes late in the day after production has fallen off, that is much less. In our own modelling of a synthetic case, the solar-only version preserved only around a quarter of the peak reduction the grid-charged version achieved. That figure is a model result on a constructed load, not a measurement of any real site, and the real ratio depends entirely on how well your array's production lines up with your own peak. But the direction is not in doubt, and the magnitude is large.
So the compliance election is not a paperwork detail. On a ComEd commercial account it can be most of the demand charge saving in the proposal.
What is actually being traded
Nothing here says the inverter lock is wrong. It says it is a trade, and the trade should be visible.
What the lock buys: a simpler, cheaper installation, and a clean compliance story with no additional metering to commission or maintain.
What the lock costs: the ability to charge outside the demand window, which in ComEd territory is where most of the battery's bill savings come from, for the entire credit term.
If the credits are large relative to the demand savings, and the load shape is such that solar-charged capacity covers the peak anyway, the lock is the right call. If the demand charge is the main reason the battery exists, electing the lock quietly removes most of the reason.
Nobody is hiding this. It is in a published program guidebook. It is simply not a question anyone puts in front of the person paying for the system.
The question to ask before you sign
One sentence, and it should be answered in writing:
"Which Illinois Shines battery compliance path does this design elect, and does it permit charging from the grid?"
Follow-ups worth asking in the same breath: what the alternative path would add to the installed cost, and how the proposal's savings figure changes under each. If the savings number does not change between them, the proposal was not modelling grid charging in the first place and you should ask what it was modelling.
And the prior question, which decides whether any of this matters: does a battery pencil at your site at all. A compliance election only matters on a project worth doing, and that is settled by the shape of your peaks, measured from the meter, before any program enters the arithmetic. The ComEd rebate for solar-paired storage is a separate mechanism and does not impose this restriction.
Does Illinois Shines require a battery to charge only from solar?
Not universally. The program provides more than one way to keep grid-charged energy out of credited solar production, and only one of them works by locking the inverter to solar-only charging. That path is commonly elected because it is the cheapest to build.
Can the compliance path be changed later?
The inverter-lock election is fixed for the duration of the credit delivery term, which runs fifteen to twenty years. It is a one-time decision made at design.
Why does grid charging matter so much in ComEd territory?
Because ComEd bills commercial demand only on weekday hours between nine and six, with no off-peak demand component. A battery charged overnight adds nothing to billed demand and can discharge into the peak, which is where most of the bill savings come from.
Who chooses which path my system uses?
In practice the installer does, at design time, and it appears on the battery schematic submitted with the application rather than in the sales proposal. Ask for it explicitly.