The rule changed in 2025 and most payback math has not caught up. Under Illinois net metering as it now stands, new commercial distributed generation earns supply-only credit: the energy you push back across the meter is valued at the supply component of the rate, not at the full retail rate you pay when you buy it. Systems interconnected before the change are grandfathered. New ones are not.
That single change moves where the value of a commercial array comes from, and it moves it in a direction that makes the shape of your load matter more than the size of your roof.
What supply-only credit actually changes
A kilowatt-hour your building consumes the moment it is produced never crosses the meter. It displaces a kilowatt-hour you would otherwise have bought, at the full price you would otherwise have paid, supply and delivery together. That value is untouched by the rule change.
A kilowatt-hour you export is different. It now returns the supply component only. So the same panel produces two very different economic outcomes depending on whether your building was drawing power at that moment.
Self-consumption is therefore the whole game, and self-consumption is a property of your load shape, not of the array. Two identical systems on two identical roofs can land on opposite sides of a decision because one building runs a weekday afternoon load and the other does not.
The part no export ever touches
Exports do not reduce your demand charge. Neither, in most cases, does the array itself in any reliable way.
Demand is billed on the single highest interval in the period. Solar reduces that interval only if the array happens to be producing hard at the exact moment your building peaks, and only if it does so in every billing period, including the cloudy day that sets the month. Published work on standalone commercial PV puts the median demand-charge reduction in the single digits, far below the energy savings on the same system.
This is the specific place installer payback math tends to break. A proposal that credits solar with cutting the demand portion of the bill is claiming something the tariff does not do, and on a commercial account the demand portion is often the larger half of the delivery side.
So what decides it
Three things, in order, and none of them is the panel price.
How much of your production you consume on site, which is a fact about your own interval data. What you pay for supply, since that is what an exported kilowatt-hour is now worth. And which incentives you can actually capture, which is usually what turns a marginal project into a closing one.
The first of those is measurable before anyone climbs on a roof. It is in the meter data your utility already holds, and it is the number a proposal is least likely to have looked at.
Did Illinois end net metering?
No. It changed what an exported kilowatt-hour is credited at. New commercial systems receive the supply component rather than the full retail rate, and systems interconnected before the 2025 change are grandfathered under the prior terms.
Does solar reduce my demand charge?
Only to the extent the array is producing at the exact interval that sets your peak, in every billing period. Published results for standalone commercial PV show a small median reduction. Any proposal assuming solar reliably cuts demand charges is making a claim the tariff does not support.
Is self-consumption really worth more than export?
Yes, and that is the direct consequence of supply-only credit. Power consumed on site displaces the full price you would have paid. Power exported returns the supply component only.
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