What one-to-one is worth against a 30 cent rate
Maine Net Energy Billing credits excess generation against household usage on a one-to-one kilowatt hour basis, at the retail rate. An exported kilowatt hour is worth exactly what a purchased one costs.
That is now rare. Georgia pays a fraction of retail for exports, Utah pays roughly a third, Tennessee pays avoided cost, and Connecticut has layered a charge on total generation. Maine has kept the simple arrangement.
The value of keeping it is a function of the retail price, and Maine is among the most expensive states in the country. The EIA put the residential average around 30.4 cents per kWh against a national figure near 18.4.
So the same panel on the same roof delivers substantially more here than in most of the country, which is why Maine solar works despite a northern climate and an installed cost around $3.10 per watt.
The rolling 12-month window
Customers accumulate unused kilowatt hour credits and apply them against usage over a rolling 12-month period. At the end of each rolling 12-month period, accumulated unused credit is eliminated and may not be applied against future usage.
A rolling window is different from a fixed annual true-up. There is no single settlement date in the calendar at which everything resets; instead each month of credit has twelve months to be used before it lapses.
For a Maine household that structure is reasonably kind. Summer surplus is drawn down through a long heating season well inside the window, so credit generated in July is usually consumed before the following July.
It still means an annual surplus is lost rather than banked. Nothing is paid for eliminated credit, so a system generating more than your household consumes across a year is giving the difference away.
What that means for sizing
Build from your last twelve months of bills rather than from available roof area. Ask what percentage of your annual usage the proposed system covers and treat anything meaningfully above 100 percent as needing a reason.
The good reason is a concrete planned increase in load. In Maine a heat pump is the most plausible one, because it genuinely raises winter consumption, and winter is when a Maine array produces least.
That mismatch is worth understanding rather than glossing. Maine winters are dark and snowy, so a system covering 100 percent of annual usage still buys a great deal of grid power in December and January and exports heavily in June.
The rolling window is what makes that work, since summer credit carries into winter. Ask your installer to show the monthly profile with the credit balance tracked, rather than only an annual figure.
Costing it out under the current tariff
The 30 percent federal Residential Clean Energy Credit under Section 25D expired for property placed in service after December 31, 2025, so a cash or loan purchase in Portland receives no federal tax credit, and Maine has no state solar tax credit or rebate.
Section 48E survives at 30 percent for third-party owners under leases and power purchase agreements. Ask what a provider claims and what portion reaches you in the rate, and confirm with a tax advisor.
What exists is one-to-one Net Energy Billing at the retail rate, subject to the rolling 12-month elimination, and the property tax exemption under 36 M.R.S. Section 655, which requires an application to your municipal assessor by April 1.
Then add the electricity you stop buying at your actual rate. In Maine that last term is doing most of the work, which is exactly why the sizing and production assumptions deserve the scrutiny.