Will adding panels, a battery or a new owner cost you your grandfathered net metering?

What grandfathering protects, and what it does not

Regulators created the older net metering tariffs when solar was a small niche. When they replaced those tariffs with less generous ones, they promised the people who had already invested that they could keep the old terms for a defined period. That promise is called grandfathering in some places and legacy treatment in others. It is a promise about a rate structure: how your exports are credited against your imports. It is not a promise about the rest of your bill, and it is not a promise that survives every change you make to the hardware.

Three separate questions decide whether you keep it. The first is the clock: how many years the protection lasts and when it starts. The second is the perimeter: how much you may change the system before it counts as a new system. The third is the owner: whether the protection belongs to you personally or to the equipment on your roof. Most expensive mistakes come from assuming the answer to the second and third questions is the same as the answer in another state.

The Idaho Public Utilities Commission put the underlying principle bluntly in Order No. 34854, issued December 1, 2020: "tariffs are not contracts and are subject to change." The California commission, in Decision 22-12-056, concluded that it had the authority to revise the older tariffs and chose to leave them intact. Both statements matter. Grandfathering is a regulatory decision that can be narrowed in a later proceeding, which is one reason to avoid spending money on the assumption that it will last forever.

• The clock is set from a specific date in your paperwork, such as the day the utility granted permission to operate, and it is not reset by routine repairs.
• The perimeter is usually a size limit on how much capacity you can add, measured against the nameplate as it stood when the system was first connected.
• The owner question is answered in tariff text, and in the three places covered here the answer is that the status follows the system rather than the person.

California under PG&E's NEM2 tariff, rule by rule

California is the best-documented case, because the tariff text is public and the commission decision that closed the program is on the record. The rules below come from PG&E's Electric Schedule NEM2, Special Condition 8 (NEM2 Legacy Provisions), as published in the tariff book and checked on October 22, 2026. The relevant sheets carry an effective date of January 22, 2025 (Advice 7489-E). Southern California Edison and San Diego Gas & Electric have their own schedules; their sheets were not read for this guide, so a customer of either should ask for the matching clause rather than assume it reads the same.

The closing of NEM2 itself is covered in the California net billing guide. In brief, interconnection applications submitted after April 14, 2023 go to the Net Billing Tariff, and customers who were already in before that date keep the older terms for a limited legacy period. The decision that made that choice, D.22-12-056, adopted December 15, 2022, concluded that the NEM 1.0 and NEM 2.0 tariffs "should remain intact."

The 20-year clock starts at permission to operate

Special Condition 8.a gives NEM2 customers service on the schedule for 20 years from the date PG&E originally granted permission to operate, running through the end of the billing period that ends on or after the 20th anniversary. The tariff's own example: a customer who first received permission to operate on April 1, 2018 stays eligible through the conclusion of the relevant period ending on or after March 31, 2038. The date that matters is therefore the original permission to operate letter, which is worth locating now and filing with the house documents.

A customer who moved from the older NEM schedule to NEM2 did not get a fresh 20 years. The tariff says such a customer continues from the original permission to operate date and cannot restart the transition period. At the end of the period the customer moves to the Net Billing Tariff.

The expansion allowance: the greater of 10 percent or 1 kW

Special Condition 8.b says a generating facility that is modified or repaired remains eligible for the rest of its 20 years as long as the change does not increase the facility by more than the greater of two numbers: 10 percent of the nameplate capacity as established when the facility was originally interconnected, or 1 kW. A further condition appears in the same clause: the modification must not leave the system larger than the customer's annual onsite load.

Because the larger of the two numbers applies, any system under 10 kW gets a 1 kW allowance, and a system of 10 kW or more gets 10 percent. A 4 kW system may grow by 1 kW, which is 25 percent. A 12 kW system may grow by 1.2 kW. The allowance is measured against the original nameplate, so ask your utility whether earlier additions count against it; the tariff text does not say that a second small addition resets the baseline, and a cautious reading is that the cap is cumulative from the original figure.

What happens if you go past the limit

A footnote to the same clause gives two options for an addition that exceeds the limit. The first is to interconnect the addition separately under another eligible tariff, with Rule 21 non-export given as the example; in that case the original 20-year or 9-year transition period is not altered. The second is to have the entire system served under the Net Billing Tariff. A customer who makes that choice becomes eligible for the same 9-year legacy period as a new net billing customer, and for the 9-year export rate lock and the ACC Plus adder tied to the calendar year the upgraded system received permission to operate.

PG&E's public Solar Billing Plan page puts the consequence in one sentence: customers who "modify or expand their legacy NEM/NEM2 system above the tariff threshold levels" move to the Solar Billing Plan at their annual true-up date. The thresholds it names match the tariff: 10 percent of original nameplate or more than 1 kW. What lands on that settlement statement is walked through in the true-up bill guide.

Non-export additions and the departing load wrinkle

The non-export route is real and, according to PG&E, widely used. In Advice Letter 7915-E, filed May 22, 2026, PG&E reports nearly 2,500 applications for what it calls NEM Non-Export Expansion projects since the sunset, and describes them as residential systems of 30 kW or less where a customer adds non-export capacity above the modification threshold and stays on NEM or NEM2 because the added capacity cannot send power to the grid. The letter says the additions need a certified power control system.

The same letter shows the cost of that route. Non-export generation is subject to PG&E's departing customer generation schedule, which carries nonbypassable charges for the load the new array serves. The advice letter proposes a default billing method, Option 5, that estimates the departing load from the added capacity. The letter is a utility filing, not a commission decision, and its status after filing was not checked, so read it as a description of how the route works in practice and a warning that the added array comes with a billing line the original array never had. Ask for that charge in writing, per month, before you sign.

Adding a battery

Special Condition 8.b also addresses storage directly. Citing D.22-12-056, it says the addition of energy storage to the existing generating facility shall not disqualify the customer from the remainder of the 20-year transition period. There is one exception with a date attached: a customer who adds storage through the Self-Generation Incentive Program and has a reservation accepted on or after June 4, 2024 is required to move to the Net Billing Tariff under D.24-03-071. Three SGIP budget categories are exempt from that requirement: Residential Solar and Storage Equity, low-income qualified Equity Resiliency residential, and San Joaquin Valley Residential.

In plain terms, a battery paid for without an SGIP incentive does not trigger the move. A battery paid for with an SGIP incentive reserved on or after June 4, 2024 does, unless it falls in an exempt category. Whether the incentive is worth that trade is a question for your own numbers. The sibling guide on adding a battery to existing solar covers coupling choices, permits and the economics of the battery itself; this guide only settles what the battery does to your tariff.

Selling the house or changing the account holder

Special Condition 8.c says a facility eligible for the 20-year transition period does not lose eligibility if it is transferred to a new owner, operator or PG&E account, provided it remains at its original location. A footnote adds the other half: moving an existing facility to a new location is treated as a new installation requiring a new interconnection agreement under whatever tariffs apply when that request is completed.

So the legacy status passes to the person who buys the house, for the years that remain on the clock. This is different from the successor tariff. Under D.22-12-056, the net billing lock is tied to the original customer rather than the system, a difference the net billing guide covers in full. If you are buying a house with panels installed after April 14, 2023, do not assume the older, system-tied rule applies; ask which tariff the system is on.

A cheat sheet for common changes to a California legacy system

The table maps everyday projects onto the PG&E text. Each row states the rule and what to ask your utility; none replaces a written answer from the utility about your own account.

Idaho: legacy treatment tied to the system at the meter

Idaho Power moved new on-site generation customers from retail net metering to net billing, and the Idaho Public Utilities Commission protected the people already in. The order that explains the terms in the most usable detail is Order No. 34854 in Case IPC-E-20-26, issued December 1, 2020. It dealt with commercial, industrial and irrigation customers, and it states that those customers receive the same terms the commission had given residential and small general service customers in an earlier case, IPC-E-18-15. That sentence is the reason the order is a useful window onto the residential rules. The residential orders themselves were not retrievable in this run, so treat the residential terms below as described in Order 34854 rather than read from the original.

As Order 34854 describes them, the terms are: a long period of 25 years from the order's service date for the commercial customers, the grandfathered status "stays with the system at the meter site," the status is forfeited if the system is offline for more than six months or is moved to another site, and the customer may increase capacity by no more than 10 percent of the originally installed nameplate capacity or 1 kW, whichever is greater, to allow for replacement of degraded or broken panels. The order adds that if a customer expands beyond those limits, "the new portion of their system would not qualify for legacy treatment."

That last sentence is different from California. In Idaho the legacy status covers the old portion of the system and the new portion is treated as new. In California, the PG&E footnote offers a choice between a separate tariff for the addition and moving the entire system to net billing. The distinction matters for the economics, because in Idaho you keep the old array's credit and the new array earns the current, lower credit, while in California a separate non-export addition keeps the old array unchanged but cannot export at all; the controls and approval screens such an addition must meet are set out in the guide to zero-export interconnection.

What the difference is worth in Idaho

Idaho Power's own customer brochure, which carries a 2026 copyright, states that net energy metering compensates exports with a one-for-one kilowatt-hour credit and applies to customers with legacy systems. For everyone else the brochure lists export credit rates by season and time of day, effective October 1, 2025: in the summer, 15.6836 cents per kilowatt-hour on peak and 3.3920 cents off peak; in the non-summer months, 2.9019 cents off peak. It adds that Idaho Power will file a proposed update to the export credit rate on April 1, 2028. A legacy system that exports in the afternoon and early evening is therefore credited at a retail-offset value, not at a few cents.

The brochure also lists what counts as a modification that must go through the interconnection process with a new application: adding or replacing solar panels or generation equipment, replacing an inverter, or adding batteries. It does not say that a battery ends legacy status, and the order text cited above does not list batteries among the forfeiture triggers. It does say that an account with a disabled system is removed from the on-site generation pricing schedule after six months, which matches the six-month offline rule in the order. If your inverter dies in winter, replace it promptly.

Illinois: lifetime of the system, with a different expansion test

Illinois ended full retail-rate net metering for new customers on January 1, 2025. Customers registered before that date keep it, and the Illinois Power Agency's program administrator, which runs the Illinois Shines program, answers the questions in an FAQ dated July 26, 2024. That document is an administrator's guidance, not a statute, and the official statute text on the General Assembly site could not be read in this run because of a certificate error. A search listing for the statute page also indicated a later amendment by Public Act 104-458 that this guide did not read. Check the current statute and your utility tariff before relying on the details below.

As the administrator describes it, systems registered for full retail net metering before January 1, 2025 continue to receive it for the lifetime of the system, regardless of whether the customer changes electricity providers or whether the customer receiving the benefit from the system changes. Asked whether a project with full retail net metering loses it on a change of ownership, the FAQ answers no: the net metering services stay with the system itself. It gives the same answer to a customer who buys out a leased or power purchase agreement system, so a lease buyout does not cost the system its legacy rate.

Where the lines are drawn for expansion

Illinois draws the expansion line by utility. According to the FAQ, a ComEd system loses its legacy status if the change requires a new interconnection application, and an Ameren system loses it if the nameplate capacity increases by more than 100 percent. Those are very different tests. In ComEd territory almost any addition requires a new application, so the safer assumption is that adding panels ends the status. In Ameren territory, a system could in principle double before the test is met. MidAmerican has its own rule, and the FAQ says its test is whether a completed net metering application was submitted before January 1, 2025.

A second condition is easy to miss. The FAQ says a customer who requested a Distributed Generation rebate from the utility in 2024 was not eligible for full retail net metering and receives supply-only net metering instead. If you are buying a house in Illinois with panels, find out whether the previous owner took that rebate, because the answer decides which tariff the system is on.

The three places side by side

The table collects the answers to the three questions from each source. It is a map of what was found, not a complete survey; other utilities and states have their own rules, and several carry no grandfathering at all.

Buying or selling a house with a legacy system

If the status transfers with the system, it has a value that belongs in the sale conversation. The way to convert it into a number is to compare what the system earns on the legacy tariff with what the same system would earn on the tariff a new system gets today, then multiply by the years left on the clock. That gives a buyer a reason to pay for the panels, and a seller a reason to document them. The framework for pricing it, with the usual cautions about appraisals and leased equipment, sits in payback if you move or sell, and the leased-equipment version is in the guide on selling a house with leased solar.

The paperwork that proves the status is short. It is the permission to operate letter (for the start date of the clock), the interconnection agreement (for the original nameplate and the tariff), a recent bill that names the rate schedule, and a record of any modification since. If an addition was made, the interconnection record should show that it fit inside the allowance or went on a separate tariff. A buyer who cannot get these documents is buying an assumption.

• Sellers: put the permission to operate date, the tariff name and the remaining years into the listing packet, and say plainly whether any addition was made.
• Buyers: ask the utility, not the seller, which schedule the account is on, and whether the transfer of the account triggers any review. Do not assume a system installed after the older tariff closed has any legacy period at all.
• Leased systems: the lessor decides most changes, so a buyer taking over a lease should ask whether the lease permits additions and who bears the cost of losing the tariff.

How long the protection can be relied on

Grandfathering is a bet on regulatory stability. In California, the commission's 2022 decision records that parties argued for moving older customers onto the successor tariff after eight years, and the commission declined. It explained that although it had the authority to revise the older tariffs, doing so would create an inequity for one group of ratepayers or the other, and it left the older tariffs intact. In Idaho, the commission said that legacy treatment is "only appropriate in limited circumstances" and gave the reason it applied: customers who made a significant investment relying on program stability.

Two practical consequences follow. First, a date in the tariff is the best available evidence of how long the protection lasts, but a future commission could change it. Second, the protection pays for itself fastest when the system's cost is recovered early. A system that pays back in ten years on the legacy rate has its investment covered before most of the clock has run, and the remaining years are a bonus. A purchase decision that only works if the legacy rate lasts the full term should be examined with extra care. Where rates themselves are the question, the rate context sits on EnergyBS; the time-of-use rates guide there explains the rate plans that determine how much an export credit is worth in the first place.