Updated: August 28, 2026 · Estimated read time: 10 minutes
You receive a solar proposal and somewhere near the top it says:
Solar Offset: 110%
That sounds good.
After all, 110% must be better than 100%, right?
Not necessarily.
Solar offset is one of the most useful numbers on a proposal—but it's also one of the easiest numbers to misunderstand. A properly designed Illinois solar system shouldn't automatically target 100%, 110%, or any other percentage simply because that's what a salesperson normally uses.
The better question is:
How much electricity should your solar system actually produce for your home?
That depends on your historical usage, future energy needs, roof, utility, net-metering rules, system economics, and when you use electricity—not just one percentage on a proposal.
Quick Answer
Solar offset compares your estimated annual solar production with your annual electricity usage.
If your home uses:
12,000 kWh per year
and your solar system is projected to produce:
12,000 kWh per year
your estimated annual solar offset is:
100%
If the system produces:
13,200 kWh
the estimated offset becomes:
110%
But 110% isn't automatically better than 100%.
Some Illinois homes may reasonably benefit from an offset above 100%, especially when future electricity use is expected to increase.
Other homes may get better economics from a system below 100%.
The correct target depends on the home.
Table of Contents
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What does solar offset actually mean?
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How is solar offset calculated?
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Why doesn't 100% offset mean no electric bill?
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Is 110% solar offset better?
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When does going above 100% make sense?
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Future electricity use matters
-
Why Illinois law recognizes future electrical requirements
-
When oversizing solar can work against you
-
How Illinois net metering changes the calculation
-
Why roof direction and shading matter
-
What about solar-panel degradation?
-
Does adding a battery change the ideal system size?
-
Ameren, ComEd, and CWLP aren't identical
-
How to compare two different solar designs
-
Questions to ask before approving your system size
-
Frequently asked questions
What Does Solar Offset Actually Mean?
Solar offset is a comparison between:
How much electricity your home uses
and:
How much electricity your solar system is projected to produce.
It's usually expressed as a percentage.
For example:
Home A
Annual household usage:
10,000 kWh
Projected solar production:
8,500 kWh
Estimated offset:
85%
Home B
Annual household usage:
10,000 kWh
Projected solar production:
10,000 kWh
Estimated offset:
100%
Home C
Annual household usage:
10,000 kWh
Projected solar production:
11,000 kWh
Estimated offset:
110%
Simple enough.
But here's the first important lesson:
Solar offset is an annual energy comparison—not a prediction of your monthly electric bill.
That's a major distinction.
How Is Solar Offset Calculated?
The basic calculation is:
Projected annual solar production ÷ annual electricity consumption × 100
Let's use a real-world-style example.
Suppose your last 12 months of electricity usage total:
14,500 kWh
Your proposed solar array is estimated to generate:
15,225 kWh
The calculation is:
15,225 ÷ 14,500 × 100 = 105%
So the proposal would show approximately:
105% solar offset
But remember that both parts of that calculation deserve scrutiny.
Is the electricity-usage number accurate?
Ideally, the design should use a full 12 months of actual consumption rather than estimating the year from one unusually high or low bill.
Is the production estimate realistic?
Production depends on:
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System size
-
Panel orientation
-
Roof pitch
-
Shading
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Equipment
-
Weather assumptions
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Geographic location
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System losses
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Solar-resource modeling
A proposal can show 110% offset on paper while the actual system performs differently.
That's why the production number matters just as much as the percentage.
If you're still evaluating your first proposal, start with LuNova's 7 things every Illinois homeowner should check before going solar.
Does 100% Solar Offset Mean My Electric Bill Becomes $0?
No.
This may be the single biggest misconception surrounding solar offset.
A projected 100% annual energy offset does not necessarily mean:
$0 utility bill
for 12 months of the year.
Why?
Because your home doesn't consume electricity on the same schedule your solar panels produce it.
Solar panels generally produce during daylight hours.
Your household may use significant electricity:
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Before sunrise
-
After sunset
-
Overnight
-
During periods of poor solar production
Your utility bill can also contain charges that aren't eliminated simply because annual solar production equals annual usage.
Those can include:
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Customer charges
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Delivery-related charges
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Taxes
-
Riders
-
Other utility-specific charges
This became especially important after Illinois changed net metering for newer systems.
Our complete Illinois Net Metering 2026 guide explains why annual solar production and actual utility billing are not the same thing.
So Is 110% Solar Offset Better Than 100%?
Sometimes.
But not automatically.
Imagine two proposals.
Proposal A
100% projected offset
Proposal B
110% projected offset
Proposal B produces more electricity.
But that alone doesn't tell you which system offers better value.
You still need to know:
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How much more the larger system costs
-
Where the additional panels are being installed
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Whether those panels receive good sunlight
-
How much extra electricity will be exported
-
What your utility credits for those exports
-
Whether your electricity usage is likely to increase
-
Whether financing costs change
-
Whether roof space could be better used differently
-
Whether a battery is involved
The larger system could be an excellent decision.
Or you could be buying additional panels that generate electricity with relatively poor economic value.
More solar isn't automatically more value.
When Does Going Above 100% Make Sense?
There are several legitimate reasons to design a system above today's electricity consumption.
The biggest one is:
Your future home may use more electricity than your current home.
Imagine your household currently consumes:
11,000 kWh per year.
But you're planning to purchase an electric vehicle next year.
Your future electricity usage may increase significantly.
Designing only around today's 11,000 kWh could leave the system undersized almost immediately.
The same applies if you're planning to:
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Purchase an EV
-
Add a second EV
-
Install a heat pump
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Replace gas heat with electric heating
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Replace a gas water heater with an electric unit
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Install a pool
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Add a hot tub
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Finish additional living space
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Add an electric workshop
-
Increase occupancy
-
Add major appliances
Your solar designer should ask about these things before determining the system size.
Not afterward.
Illinois Actually Recognizes Future Electrical Requirements
This isn't simply a LuNova design philosophy.
Illinois' net-metering law specifically recognizes that eligible distributed-generation systems can be intended to offset a customer's current or future electrical requirements.
Current Illinois law also recognizes reasonable future electricity needs associated with things like electric vehicles and switching previously fossil-fuel-powered loads to electricity.
That matters.
A homeowner shouldn't necessarily be forced into designing a system solely around the electricity consumed during the previous 12 months when there are legitimate and reasonable future electrical needs.
But there's an equally important point:
“Future usage” should be based on something real.
“I might someday use more electricity” isn't the same thing as:
“We're purchasing an EV next spring.”
or:
“We're replacing the gas furnace with a heat pump.”
Good system design uses reasonable expectations—not an excuse to sell unnecessary panels.
For the underlying Illinois rules, homeowners can review the Illinois General Assembly net-metering statute.
CHECK MY HOME
Not sure whether your home should be designed for:
90%?
100%?
110%?
or something else entirely?
A useful solar estimate should consider your historical electricity consumption, roof, solar potential, utility, and reasonable future energy needs before deciding how large your system should be.
See what solar could look like for your home
Solar production, offset, utility credits, and savings are estimates and can vary based on weather, equipment, household usage, utility rules, system performance, and other factors.
When Can Oversizing Solar Work Against You?
Here's where the conversation gets more interesting.
Suppose your home uses:
10,000 kWh annually
but you install a system projected to produce:
14,000 kWh
That's roughly:
140% annual offset.
Maybe there's a good reason.
Perhaps you're adding two EVs.
But if your future consumption isn't actually going to increase, you may spend money producing thousands of extra kilowatt-hours primarily for export.
Under some utility structures, exported electricity isn't economically equivalent to electricity used directly inside your home.
That's especially relevant under Illinois' newer net-metering rules.
So the question becomes:
What is the additional solar production actually worth?
That's different from:
How much solar can we physically fit on the roof?
Those are not the same question.
Illinois Net Metering Makes System Sizing More Important
New residential and small-commercial solar systems in Ameren Illinois, ComEd, and MidAmerican territory generally operate under supply-only net metering if they began net-metering service after December 31, 2024.
Illinois Shines explains that newer customers can receive qualifying supply-related credits for excess generation, while delivery charges generally remain on electricity pulled from the grid.
That means the electricity your home uses directly can have a different economic value from electricity exported and later replaced with grid electricity.
For the current consumer explanation, see the Illinois Shines net-metering FAQs.
This creates an important system-sizing question:
If the last 10% of your solar system mostly produces electricity that gets exported, is that additional capacity still worth buying?
Maybe.
But you need to run the numbers.
That's why we shouldn't automatically design:
110% for everyone.
The answer could be:
92%.
103%.
115%.
Or another number entirely.
Self-Consumption Matters Too
Consider two households.
Both use:
12,000 kWh per year.
Both install solar systems producing:
12,000 kWh per year.
Both therefore show:
100% annual offset.
But Household A has people home throughout the day.
They run appliances, HVAC equipment, and other electrical loads during strong solar-production hours.
Household B is largely empty during the day.
Much of its electricity use happens after everyone comes home in the evening.
Same annual electricity usage.
Same solar production.
Same 100% offset.
But potentially different interaction with the grid.
That is why the offset percentage doesn't tell the whole story.
Roof Direction Can Change the Answer
Two 10 kW solar systems do not necessarily produce the same amount of electricity.
Imagine one home has:
A large, unobstructed south-facing roof
while another has:
East- and west-facing roof sections with afternoon tree shade.
Both may have the same number of solar panels.
Both may have the same DC system size.
But annual production can differ.
That's why system size in kilowatts shouldn't be confused with annual production in kilowatt-hours.
kW
How large the solar array is.
kWh
How much energy the system produces over time.
Offset is based on the second number.
A homeowner buying an 11 kW system shouldn't assume it will produce exactly the same amount of electricity as every other 11 kW system in Illinois.
What About Panel Degradation?
Solar panels don't normally produce exactly the same amount of electricity every year forever.
Production can gradually decline as panels age.
The exact degradation profile depends on the equipment and warranty.
Does that mean every system should automatically start at 110% to compensate?
No.
Degradation is simply one design factor.
A good proposal should already include reasonable long-term production assumptions rather than secretly depending on an oversized system to compensate for poor modeling.
Ask:
Does this production estimate account for system losses and expected panel performance?
If not, the pretty offset percentage may be less useful than it appears.
Does a Battery Change the Ideal Solar Offset?
Potentially.
Battery storage gives excess solar electricity another destination.
Instead of immediately doing:
Solar → Grid
some excess electricity can potentially do:
Solar → Battery → Home later
That can increase solar self-consumption.
But a battery doesn't create electricity.
It stores it.
So solar-array size and battery capacity need to make sense together.
A very large battery paired with a system that rarely produces excess electricity may behave differently from a properly matched solar-plus-storage design.
If you're considering storage, read LuNova's Solar Battery or No Battery? Illinois Homeowner Guide.
And always compare:
Solar only
versus:
Solar + battery
before assuming the more complicated design is better.
What About Ameren Illinois?
Ameren Illinois solar projects need to follow the applicable interconnection and net-metering requirements.
Ameren also makes an important point on its customer-owned solar information: solar systems should be sized around the customer's electric requirements, and historical usage information can help an installer evaluate an appropriate system.
You can review current utility information through Ameren's Customer-Owned Solar resources.
For a homeowner-focused explanation, visit LuNova's Ameren Illinois Solar Guide.
What About ComEd?
The general Illinois principles still matter:
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Current electricity consumption
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Reasonable future electricity requirements
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Supply-only net metering for newer systems
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Solar-production timing
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Roof conditions
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System economics
But Ameren and ComEd should never simply be treated as interchangeable utilities.
Tariffs, billing implementation, rates, interconnection processes, and program details can differ.
ComEd homeowners can start with LuNova's ComEd Illinois Solar Guide.
What About Springfield and CWLP?
This distinction is even more important in Springfield.
City Water, Light and Power is a municipal utility.
Do not automatically apply Ameren or ComEd assumptions to a CWLP customer.
Illinois Shines notes that municipal utilities and rural electric cooperatives may have their own policies for calculating the value of electricity sent back to the grid.
So a Springfield homeowner's ideal system size needs to use CWLP-specific rules, not a generic Illinois model.
See LuNova's Springfield and CWLP Solar Guide for more.
Does Financing Affect the Ideal System Size?
It can.
Suppose adding four panels increases production enough to move your projected offset from:
100%
to:
112%
That's one part of the equation.
Now ask:
How much do those four panels add to the project cost?
Then:
How is the project being paid for?
Cash?
Loan?
PPA?
Lease?
The value of additional production can interact with financing costs and ownership structure.
With a PPA, for example, you're generally purchasing the electricity produced by a third-party-owned system rather than buying the equipment outright.
That creates a different economic calculation from cash ownership.
If you're comparing those options, read our Illinois homeowner's guide to solar PPAs.
Don't Forget the Incentives
System size can also affect incentive calculations.
Illinois Shines supports qualifying solar projects through Renewable Energy Credits, while qualifying utility customers may encounter separate distributed-generation incentives.
But an incentive should never become the reason to install unnecessary equipment.
Start with:
What system makes sense?
Then calculate:
Which incentives apply to that system?
Not:
How large can we make the system to maximize an incentive?
For current information, read LuNova's Illinois Solar Incentives in 2026 guide.
How to Compare Two Solar Designs
Let's say two companies quote the same home.
Your annual consumption:
13,000 kWh
Company A
Projected production:
12,350 kWh
Offset:
95%
Company B
Projected production:
14,300 kWh
Offset:
110%
Which proposal is better?
You still don't know.
Ask:
What's the total system cost?
The larger array isn't free.
What's the price per watt?
Compare equipment and installed cost.
Where are the additional panels?
High-production roof area or heavily shaded roof?
What production assumptions were used?
Do both proposals use realistic modeling?
What happens to the additional exported energy?
Know your utility's current rules.
Are future electrical loads expected?
EV? Heat pump? Pool?
Is battery storage involved?
That can change self-consumption.
Are the financing structures identical?
A cash system and 25-year PPA can't be compared simply by offset percentage.
Only after answering those questions does the offset number become useful.
10 Questions to Ask Before Approving Your Solar System Size
1. What was my actual electricity consumption over the last 12 months?
Start with real usage.
2. What annual solar production are you projecting?
Ask for kWh—not just system size in kW.
3. What solar offset does that represent?
And understand how it was calculated.
4. Why did you choose this particular offset?
There should be a better answer than:
“We always do 110%.”
5. What future energy use did you include?
EVs? Heat pump? Pool? Additional occupants?
6. How much electricity is expected to be exported?
Especially important under newer net-metering structures.
7. What happens if you remove the lowest-producing panels?
Sometimes the last few panels contribute disproportionately little value.
8. Does the production estimate account for shading and system losses?
It should.
9. Would a battery change the recommended design?
Compare the scenarios.
10. Can you show me at least two system sizes?
For example:
95% vs. 110%
Then compare the economics.
An informed homeowner should be allowed to see the tradeoff.
Frequently Asked Questions
What does 100% solar offset mean?
A projected 100% offset generally means the solar system is estimated to generate approximately the same amount of electricity annually as the home consumes annually. It does not mean the homeowner will necessarily have a $0 electric bill.
Is 110% solar offset better than 100%?
Not automatically. A larger offset may make sense when future electricity usage is expected to increase or when project economics support additional capacity. In other cases, the added panels may primarily increase exported energy without creating enough additional value.
Can I install more solar than I currently use?
Illinois net-metering rules recognize reasonable future electrical requirements, but applicable utility interconnection rules and project requirements still matter. Future needs should be based on reasonable expectations rather than arbitrary oversizing.
Should I size solar for a future electric vehicle?
If an EV purchase is reasonably expected, its future electricity use should be considered when designing the solar system. Illinois law specifically recognizes future electrical requirements in the net-metering framework.
Does 100% offset eliminate my utility bill?
No. Utility bills can still contain fixed charges, delivery-related charges, taxes, riders, and charges for electricity pulled from the grid.
Does battery storage mean I need a larger solar system?
Not necessarily. Battery capacity, household electricity usage, solar production, backup goals, and utility economics should be evaluated together.
Is 90% solar offset bad?
No. A 90% system can potentially make more financial sense than a 110% system depending on roof conditions, pricing, utility rules, and the homeowner's goals.
Do Ameren, ComEd, and CWLP use the same solar-sizing rules?
No. Illinois-wide rules provide part of the framework, but utility tariffs, interconnection procedures, billing, and net-metering policies can differ. Municipal utilities such as CWLP can use different policies from Ameren Illinois and ComEd.
So What Is the “Perfect” Solar Offset?
There isn't one.
That's the point.
For one Illinois home, the best design might be:
92%
For another:
101%
For a household adding two EVs:
115%
The percentage should be the result of the design process.
It shouldn't be the starting assumption.
A properly designed solar system considers:
-
Your actual electricity usage
-
Reasonable future electricity needs
-
Your roof
-
Shading
-
Production potential
-
Your utility
-
Net-metering rules
-
Self-consumption
-
Battery plans
-
Financing
-
Incentives
-
Long-term goals
Then you determine how much solar makes sense.
Not the other way around.
See How Much Solar Your Home Actually Needs
At LuNova Solar, we don't believe every Illinois homeowner should automatically receive the same offset target.
Your solar design should start with:
Your home.
Your electricity usage.
Your future energy needs.
Your utility.
Then we can look at the system sizes that actually make sense.
No pressure. No arbitrary panel count. Just a clearer look at what your home actually needs.
Related Illinois Solar Guides
Before You Go Solar in Illinois: 7 Things Every Homeowner Should Check
Illinois Net Metering Changed: What Homeowners Need to Know in 2026
Illinois Solar Incentives in 2026
Solar Battery or No Battery? What Illinois Homeowners Actually Need
What Is a Solar PPA? The Illinois Homeowner's Guide
Illinois Solar Questions & Answers
Sources & References
Illinois General Assembly — Net Electricity Metering Law
Illinois Shines — Consumer FAQs and Net Metering Guidance
Ameren — Customer-Owned Solar Information
LuNova Solar provides general educational information and does not provide tax, legal, engineering, or financial advice. Solar production, offset, utility credits, electricity usage, equipment performance, incentives, and savings vary by project. Utility tariffs and program requirements can change. Verify current requirements with your utility and applicable program before making a financial decision.
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