My grandmother labeled leftovers with a strip of masking tape and the hour they went into the refrigerator.
The hour seemed fussy until you opened two identical glass dishes three days later. Time had changed one of them. The contents looked the same; their usefulness had separated quietly.
Illinois electricity now carries a similar label. A kilowatt-hour of solar used inside your home at noon can have a different financial value from a kilowatt-hour exported at noon and bought back after dinner. The meter records the boundary crossing. The 2025 net-metering rules decide which parts of the bill that crossing can touch.
Annual production alone no longer tells the whole economic story for a new ComEd or Ameren Illinois solar system.
The Meter Has Two Doors
A modern bidirectional utility meter measures electricity coming from the grid and excess electricity flowing from the home to the grid. It does not see all solar production. If the array is producing while the air conditioner, refrigerator, and office computer are using power, that energy serves the home before it reaches the meter.
ComEd explains this directly in its 2026 net-metering FAQ: the utility meter is not connected to the panels and does not measure their total output. Ameren Illinois says the same thing in practical terms. Solar generation meets the home's electric requirements first; only the surplus travels outward.
Picture two doors in a train station. Imported electricity enters through one. Exported electricity leaves through the other. Solar consumed in the house never steps onto the platform.
That hidden third path is where the current Illinois rules become interesting.
The most valuable solar kilowatt-hour may be the one your utility meter never gets the chance to count.
Changed the Export Door
For new residential and small-load ComEd customers interconnected on or after , excess solar earns credits that offset charges in the supply section of the bill. ComEd customers interconnected by generally retain legacy full-retail net metering for the life of the system, which the utility describes as . ComEd's FAQ warns that taking its generation rebate ends legacy status; taking the storage rebate alone does not.
Ameren Illinois uses slightly different administrative language and bill presentation, but the statewide direction is the same. New applications after receive excess-generation value on the supply side; Ameren includes transmission in its description. Delivery charges no longer disappear merely because the home exported an equal number of kilowatt-hours earlier.
Net metering continues with a narrower export credit for new systems.
That sentence belongs beside every 2026 savings estimate. A proposal that models exported power at the same value as power consumed instantly inside the home is using an economic shortcut. It may still produce a plausible-looking curve. The curve is labeling both dishes with the same hour.
Self-Consumption Avoids a Purchase
Suppose the panels produce one kilowatt-hour while the house needs one kilowatt-hour. The energy serves the load directly. The homeowner buys one fewer kilowatt-hour from the utility, so the usage-based supply and delivery costs associated with that purchase never occur.
Now suppose the house is nearly idle at noon. The solar kilowatt-hour goes through the meter as an export. Later, the family cooks dinner and imports a kilowatt-hour. Under supply-only net metering, the earlier credit can help with the energy-supply side of that later purchase. Usage-based delivery charges remain.
The panels produced the same amount in both examples. Household timing changed the value.
A battery is one possible clock
Storage can hold midday production for evening use, but battery economics include equipment cost, conversion losses, warranty limits, operating settings, and any utility-program commitments. Moving flexible loads into solar hours can also raise self-consumption. An electric vehicle can charge in the afternoon when the car is home. A dishwasher can run after lunch. Pre-cooling may shift some air-conditioning demand without turning the living room into a laboratory.
Many loads refuse to move. Dinner happens at dinner. A medical device runs when needed. Families should not reorganize the house around a spreadsheet that saves less than it costs in inconvenience.
The point is to measure the load shape before assigning value to a solution.
Annual Offset Can Hide the Clock
Solar proposals love the annual offset percentage. A system expected to produce 10,000 kilowatt-hours against 10,000 kilowatt-hours of annual household use can be displayed as 100 percent offset. The symmetry is visually satisfying.
Those totals say nothing about how much production meets demand at the same moment. A commuter's empty house may export heavily on clear weekdays and import during the morning and evening. A retired couple with daytime cooking, laundry, cooling, and workshop use may consume a larger share directly. Their annual bills could show the same total usage before solar and respond differently to the same array.
Annual production remains central to sizing, Illinois Shines estimates, and long-term planning. The model needs another line: expected self-consumption under the household's actual schedule.
Ask a proposal to show at least two flows. How many projected kilowatt-hours will the home use directly? How many will cross the meter as exports? If the software cannot separate them, ask what self-consumption assumption it used. Silence is also an assumption, usually the most favorable one.
Read a weekday before reading
Hourly or interval usage data can reveal when the home imports power. Solar-monitoring data, once a system exists, can show total generation. The utility bill alone usually shows monthly totals, which flatten breakfast, an August afternoon, and a January evening into one number.
One ordinary weekday and one weekend will not predict a year. They can expose whether a proposal's story resembles the house.
A Smaller Bill Still Has a Floor
Net-metering credits do not erase every line. ComEd states that its customer charge and standard metering charge are unaffected by net-metering credits. Ameren bills also retain charges outside the export-credit calculation.
This matters because “zero electric bill” still appears in sales language. A system can substantially reduce purchased energy and leave fixed charges, delivery components, taxes, fees, or supplier-related treatment on the page. The exact floor depends on the utility, rate, supplier, and account.
There is a psychological trap here. Homeowners see a credit balance and assume it behaves like cash. Utilities account for credits under tariff rules. ComEd says credits for customers interconnected in 2025 or later do not expire, whether tracked in kilowatt-hours or money. Ameren also carries new-program excess credits forward, while requiring the applicant to choose a kilowatt-hour or monetary method under its rules.
Carryforward can be useful. Chronic overproduction can still leave value marooned on the account if household use never catches up. An oversized roof is a poor savings account when withdrawals are restricted to future electricity.
ComEd and Ameren Use Different Ledgers
Illinois policy changed statewide, yet utility details remain local.
ComEd places new-system credits in the supply section and offers credit methods tied to the customer's rate arrangement. Hourly Pricing values exports according to the hour in which they occur. Fixed-price, Time of Day, and third-party supplier accounts have their own handling.
Ameren Illinois asks new applicants to make a one-time choice between kilowatt-hour usage netting and monetary-value netting. Its public customer-owned solar page says that choice cannot later be changed. The monetary method depends on current supply pricing, including the price from an alternate retail supplier when applicable.
A national solar calculator rarely knows which election will be made on an Ameren application or which ComEd supply product sits behind a Chicago-suburb bill. That is why utility territory belongs near the beginning of an evaluation rather than in a footnote.
For Illinois homeowners who want the current rule applied before an installation conversation, The Day Company screens only ComEd and Ameren homes and publishes dated, source-linked program information. The useful outcome may be a full review. It may also be discovering that the proposal's export assumption does not fit the account.
Read the House as a Twenty-Four-Hour Place
Before accepting a savings curve, draw one day.
Mark when the home is usually empty, when heating or cooling works hardest, when an electric vehicle is connected, and which loads can move without making life ridiculous. Then ask the installer to place expected solar production over that day for a clear summer period and a shorter winter period. The exercise will be approximate. Its assumptions will at least be visible.
Do the same with any battery proposal. Identify which energy is intended for outage backup, which is intended for daily shifting, and whether a utility program can dispatch the battery. One box in the garage can be assigned several jobs; its usable capacity cannot perform all of them at once.
The masking tape on my grandmother's glass dishes was never elegant. It contained a date, an hour, and sometimes one uncertain word such as “gravy?” That small label made the contents more useful than a spotless lid ever could.
Write the hour beside the kilowatt-hour. Then the solar estimate can describe the home that will actually pay the bill.