Your electricity bill has two halves that behave completely differently, and almost every decision about reducing it turns on knowing which half you are looking at.
Supply is the energy itself, the commodity. It is the part of the bill that is open to competition, the part a supplier can quote you a different price on, and the only part an on-site generator can displace. Delivery is what the wires company charges to bring it to you, and on a commercial account it carries the demand charge, which is billed on your single highest interval rather than on how much you used.
Why the distinction decides what solar can do
Solar produces energy. Energy is supply. So an array works directly against the supply half of the bill and only incidentally, and unreliably, against the delivery half.
That is not a detail. On many commercial accounts the delivery side, driven by demand, is the larger of the two. A proposal that quotes savings as a percentage of the whole bill, without saying which half it is attacking, is describing a number that cannot be checked. Under supply-only net metering the point sharpens further: exported energy is credited at the supply component specifically, which is the same half again.
The part you can shop, and the part you cannot
Supply is competitive. You can take the utility's default rate or buy from an alternative supplier, and the price differs.
Delivery is not. It is set by the filed tariff for your rate class, and no supplier switch changes it. This is where a common and expensive confusion lives: a business shops its supply rate, sees a modest change on the total bill, and concludes there was nothing to find, when the larger opportunity was sitting untouched in the delivery half the whole time.
Which half is shoppable at all is a question about the state you are in. Illinois and Ohio both opened supply to competition, and the comparison behaves the same way in each: the quoted rate moves the supply half and nothing else. If you are outside ComEd territory, the Ohio version of the exercise is here, including what we have and have not reconciled there.
So which half is worth attacking
That is a question about your own bill, and it has a definite answer that does not require a projection.
Pull your last bill and separate the two. If the supply half dominates, energy reduction and rate shopping are where the money is, and solar is attacking the right target. If the delivery half dominates, the peak that sets your demand charge is the target, and an array is a weak instrument against it. The interval data behind the bill tells you which interval set that peak and what was running when it did.
We do not need to guess at which case you are in. It is already recorded.
What is an electricity supply charge?
The portion of your bill covering the energy itself, as opposed to delivery, which covers moving it over the wires. Supply is competitive; delivery is set by the utility's filed tariff.
Does switching suppliers lower my whole bill?
It changes the supply portion only. If delivery is the larger half of your bill, a supply switch moves the smaller number and the larger one is unaffected.
Does solar reduce delivery charges?
Not directly. Solar displaces energy, which is supply. It reduces a demand charge only in the narrow case where the array is producing at the exact interval that sets your peak, in every billing period.