The tailor measured a jacket I had worn for twelve years and told me exactly how large I used to be.
His tape was accurate. The shoulders, sleeves, and waist all agreed. The problem was that the suit we were discussing was for a body in the future, after an injury had changed how I moved and physical therapy had changed the rest. Precision had arrived with the wrong tense.
A stack of twelve electric bills can make the same mistake. It records the house you just finished living in. A solar system will sit on the roof while the house acquires its next car, furnace, water heater, workshop tool, occupant, and summer routine.
Sizing solar from history alone gives a precise answer to an expired question.
Twelve Bills Face Backward
Installers ask for a year of electricity use for good reason. Twelve consecutive months capture the broad seasonal swing that one August bill or one mild April cannot. They show how the household behaved through cooling weather, heating weather, holidays, travel, and ordinary weeks.
ComEd's 2026 net-metering guidance says a participating system should be sized to meet some or all of a customer's current or future energy needs. Those last two words matter. The utility itself recognizes that historical consumption is a starting point rather than a prophecy.
The bills also have blind spots. They do not know that the gasoline car in the driveway has been traded. They cannot see a signed heat-pump estimate in a desk drawer. They treat a college student away from home as permanently absent and a finished basement still under construction as permanently dark.
I would trust twelve real bills over a square-footage shortcut every time. I would still ask the house what is about to change.
A solar array is built from yesterday's evidence and paid for inside tomorrow's habits.
The House Under the Blueprint
Walk through a house scheduled for electrification and the future loads often appear as empty spaces. There is a blank spot on the garage wall for a vehicle charger. A capped line waits where a heat-pump water heater will stand. The gas furnace still runs, but the replacement estimate assumes an electric system.
None of those spaces consumed a kilowatt-hour last year.
The opposite can happen too. A family may be planning to move. A pool pump may be coming out. An adult child may leave. An old resistance heater may be replaced by more efficient equipment. Building a larger array for a load that is disappearing creates generation the household may struggle to use economically.
Name future loads one by one
“We might use more electricity” is too soft for design work. Give each planned change a date, an evidence source, and a range. For an electric vehicle, use expected annual miles and a realistic vehicle efficiency from its official specifications. For a heat pump, use the contractor's load calculation and equipment data rather than a national average. For a remodel, identify the actual appliances and whether gas equipment is being removed.
Some entries will remain uncertain. Mark them that way. A planned purchase with a signed order deserves more weight than a possibility mentioned over dinner.
This simple forecast prevents a common sleight of hand: treating every imagined future appliance as guaranteed consumption so a larger system can be sold today.
Two Futures Are Enough
I prefer two scenarios on a solar proposal.
The first uses the house as documented by the most recent , adjusted only for changes already completed. The second adds the specific future loads the homeowner is reasonably committed to bringing online. Each addition should show its annual estimate and expected start date.
The gap between those scenarios is information. It tells the homeowner how much of the proposed array depends on plans that may move. If the design works only when an electric vehicle begins charging next spring, say so on the page. If the household postpones the car for , the system may export more than expected during those years.
In Illinois, that export matters because new solar systems receive narrower net-metering value than legacy systems. Power consumed in the home avoids a purchase. Excess power exported by a new ComEd or Ameren Illinois customer generally earns supply-side credit rather than full delivery credit. A “future load” that never arrives can therefore weaken the modeled economics even when the panels perform perfectly.
The plan should survive one delayed purchase. If it cannot, the uncertainty belongs in the decision.
Roof Capacity and Bill Capacity Are Different
Solar design software can fill every usable rectangle on a roof. That answer describes physical capacity. It does not establish financial usefulness.
A broad south-facing roof may hold more panels than the household needs. A smaller, interrupted roof may hold fewer than a future all-electric home would use. The roof and the load are independent constraints, and forcing one to impersonate the other produces strange proposals.
Illinois Shines adds another measurement. Its Disclosure Form includes estimated annual kilowatt-hour production along with design and performance information, and its REC calculation relies on projected generation. That estimate describes what the system is expected to make. It does not prove the home will consume the energy at the same time or receive identical value for every export.
Production needs its own weather report
Ask which assumptions created the production estimate: array orientation, tilt, shading, inverter capacity, and modeled losses. Illinois Shines notes that site-specific roof angle and shade guide array placement, while its forms provide a production estimate for comparison.
The consumption forecast should remain separate. One side estimates sunlight converted into electricity. The other estimates how a family will use electricity. A proposal that changes both silently can make almost any system look perfectly matched.
The Cost of Being Too Small
Undersizing has a straightforward consequence. The household buys more electricity from the utility after future loads arrive. That may be acceptable. Solar does not need to cover every kilowatt-hour to provide value, and a roof does not fail because the utility bill survives.
Expansion later can be awkward. New modules may not match. Inverter capacity, roof geometry, permitting, and interconnection rules can constrain an addition. A second mobilization brings another set of fixed costs. If a near-term vehicle or heating conversion is well documented, including it in the first design can be sensible.
Oversizing deserves equal attention. Extra panels cost money now. New-system export credits do not necessarily recover the same bill components as direct consumption. Illinois Shines value rises with modeled production, but an incentive cannot rescue a system whose added cost outruns the useful value it produces for that home.
This is where sales incentives lean toward square footage. A larger system creates a larger contract. The homeowner needs someone willing to leave part of a good roof empty.
Put a Date on the Forecast
Every future-load assumption should carry an “as of” date. Life changes faster than solar engineering.
A family that planned two electric vehicles may accept remote work and need one. A heat-pump project may be redesigned after an electrical-panel inspection. A relative may move in. Six months after the first proposal, the forecast can deserve a new version even if utility rates and program terms have not changed.
The current-house case should update too. Download the newest rather than reusing the packet sent to the first installer. Look for a vacation month, a failed air conditioner, a space heater used during a furnace repair, or a billing estimate corrected later. Outliers are not automatically errors; they need names.
For ComEd and Ameren Illinois homeowners who want the utility, current usage, and dated incentives checked before the sales process expands the roof, The Day Company offers a short eligibility screen and a full review only for homes that appear to fit. The value of that order is restraint. The forecast gets challenged before it becomes hardware.
Let the Proposal Show Its Seams
A trustworthy sizing page can display uncertainty without becoming unreadable. It can show last year's metered use, the identified future changes, expected system production, and the portion of the case that depends on timing. One sentence can explain what happens if the planned load arrives late.
Avoid a single “recommended system size” presented to two decimal places. Roof layouts are built from whole modules. Consumption changes. Weather varies. The proposal can name a useful range and then explain why the selected design sits where it does.
Ask one more question: which decision is difficult to reverse? Waiting for actual vehicle-charging data may improve the model, but it delays the whole project. Building now may capture current state-program terms, but it commits roof space and capital before the new load is measured. There is no universal answer. There should be a visible trade.
The tailor eventually cut extra allowance into the jacket's seams. He did not pretend to know my future measurements. He showed me where the garment could change, charged me for the cloth, and left enough material for one careful alteration.
On the solar proposal, find the seam. Write “next car” beside it, with a date.