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.
Delivery vs supply charges, side by side
The two halves are easiest to see on real bills, so here they are on two ComEd accounts we rebuilt line by line and checked against the printed subtotals. One is a home on ComEd's default supply. One is a small business that had already switched to a retail supplier.
| home, May 1 to June 2, 2026, 1,518 kWh | small business, Aug 25 to Sept 23, 2025, 3,873 kWh | |
|---|---|---|
| Supply: the energy, priced per kWh | $161.35 (ComEd default at the posted 10.399 cents, plus transmission and the purchased electricity adjustment) | $289.48 (a retail supplier at 7.474 cents) |
| Delivery: the wires, set by ComEd's filed tariff | $117.44 (customer charge $15.50, metering $3.87, distribution facilities 6.333 cents per kWh, distribution tax) | $643.05 (distribution facilities charge $596.88, set by 40.91 kW times $14.59, plus $46.17 of customer, metering, tax and meter lease lines) |
| Taxes, fees and credits | $2.27 | $63.50 |
| Total | $281.06 | $996.03 |
| Share of the bill that is delivery | 42 percent | 65 percent |
| What a supplier switch could touch | the $161.35 | the $289.48 |
| What solar offsets | the $161.35, at the supply rate | the $289.48, and the demand charge only in the interval that set it |
Read the two delivery rows against each other and the whole distinction is there. The home's delivery half is billed per kilowatt hour; use less, pay less. The business's delivery half is billed on a single thirty minute peak, 40.91 kW, and it did not matter that the account had already found a cheaper supplier. The supply switch did what a switch can do, and the larger half sat untouched.
The same two lines answer the question people actually type: is the delivery charge or the supply charge the one going up, and which one can you do anything about. Supply moves at the posted resets and can be shopped. Delivery moves by rate order and cannot. Whether ComEd is a monopoly is the same split from the regulatory side: one half is, one half is not, and the bill shows you which.
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.