Why track credit carryover?

Net metering programs reward solar households for exporting excess electricity to the grid, banking kilowatt-hour credits that offset consumption when the sun is weak. Yet utilities often clear remaining credits annually or after a fixed window, meaning poorly timed surpluses evaporate without delivering value. Homeowners may discover in April that the hefty bank of credits earned during summer has vanished just before the air-conditioning season begins. Utilities that true-up at wholesale rates rather than retail can erode savings further. A carryover forecaster clarifies how monthly usage, seasonal production, and credit expiration rules interact so families can plan major appliance upgrades, vehicle charging, or battery investments with clear financial expectations.

Many solar monitoring portals display production and net meter readings but fail to model future expirations. Some utilities also complicate matters by resetting the calendar year on the customer’s interconnection anniversary rather than December 31. Others offer monthly credits that expire individually after a set number of billing cycles. This planner lets users enter consumption and generation estimates for each month, specify the expiration window, and include any credits already banked. The result is a ledger showing which credits are used, which expire, and when bills are owed. Users can run alternate scenarios—such as shifting loads to winter or adding storage—to see how actions change the ledger.

The math mirrors the accounting used by utilities. Each month’s generation minus consumption determines whether credits are deposited or withdrawals are required. Credits are tracked in discrete buckets with expiration dates. For example, with a twelve-month window, excess generation in March 2024 expires at the end of February 2025 if unused. The planner models this with a first-in, first-out ledger so older credits are consumed first, minimizing expirations. The core formula for credit balance is $C_{\text{end}}=C_{\text{start}}+E_{\text{earned}}-E_{\text{used}}-E_{\text{expired}}$, where credits earned equal positive net exports and credits used satisfy deficits. By applying this identity month after month, the forecast produces a transparent, auditable record.

How the forecast works

After you press the Forecast button, the script parses each month’s consumption and generation, confirming they are non-negative numbers. It converts the credit expiration window into a rolling ledger by storing each deposit’s future expiry month. Starting balances are treated as legacy deposits that already spent part of their lifespan; you specify how many months remain so the ledger knows when those credits disappear. For every month, the tool first removes any credits whose expiry date has arrived, then applies new deposits or draws down the balance to cover deficits. Billable energy appears only when monthly usage exceeds the sum of generation plus available credits.

The output table lists the net exports, credits used, expirations, ending balance, and billable cost at the entered retail rate. For utilities that settle excess credits annually at a wholesale rate, you can replace the retail rate with that settlement value to see how much cash payment to expect. The CSV export allows deeper analysis in spreadsheets—perhaps to compare with utility bills or to aggregate multi-year data. The scenario table demonstrates how modest changes in consumption or production reshape outcomes. For instance, increasing consumption by ten percent might draw down credits sooner, reducing expirations but leading to a modest bill, whereas boosting generation by ten percent might cause more credits to expire unless additional loads such as electric vehicles are added.

Worked example: a mountain home with seasonal swings

Consider a cabin in Colorado with a 9 kW rooftop array. Winter snow and short days limit production from November through February, while long summer days and cool nights allow net exports from April through September. Entering monthly usage of 700 kWh in winter, 550 kWh in spring, and 650 kWh in summer, paired with generation ranging from 300 kWh in January to 1,100 kWh in July, paints a familiar seasonal picture. The owner starts the year with 180 kWh of credit that will expire in six months and faces a twelve-month expiration policy thereafter.

The forecast shows winter deficits consuming the starting credits by March. April’s strong production deposits 250 kWh, May adds 350 kWh, and June and July contribute even more, pushing the balance above 1,200 kWh by mid-summer. Because usage dips in those months, credits continue to accumulate. When October arrives, shorter days and electric heating increase usage to 800 kWh while generation falls to 500 kWh, producing a 300 kWh deficit. The tool automatically applies the oldest credits first, trimming the balance. By December, the household still has 420 kWh banked, but the ledger warns that the earliest of the summer credits will expire the following spring. Armed with this insight, the owners schedule EV charging and a dehumidifier project during late winter to soak up remaining credits before they expire.

Scenario insights and planning strategies

The comparison table contextualizes the base projection. In the example above, the baseline scenario ends the year with 420 kWh, loses 90 kWh to expiration, and pays $0 in bills. Increasing household consumption by ten percent draws the year-end balance down to 180 kWh and eliminates expirations, yet still avoids any bill because credits cover the larger winter deficits. Boosting solar output by ten percent raises the year-end balance above 700 kWh but also increases expirations to 210 kWh, signaling that storage or flexible loads would improve economic value. These scenario contrasts help owners decide whether to add battery storage, preheat water tanks, or time discretionary loads to winter months.

The planner also highlights the effect of retail rates on avoided costs. With a rate of $0.16 per kWh, every 100 kWh of credits offsets $16. If the utility credits overproduction at only $0.04 per kWh during the annual true-up, unused credits lose $12 of potential value per 100 kWh. Users can run the forecast twice—once at the retail rate, once at the settlement rate—to quantify this haircut. If the difference is substantial, investments in load-shifting technologies such as smart thermostats, thermal storage, or EV charging timers may pay for themselves by consuming credits before they expire.

Illustrative comparison of load-shifting strategies

The table below extends the cabin example with three strategies: leaving the system untouched, adding a 10 kWh battery to shift 200 kWh of summer surplus into winter evenings, and adopting a smart water heater that soaks up 150 kWh in late winter months. The numbers demonstrate how targeted interventions reduce expirations and keep the meter spinning backward.

The intervention strategies assume the same generation and usage as the base case but redistribute energy to eliminate expirations. While the battery incurs capital costs, the water heater shift may require only a timer or connected controller. Seeing the credit ledger makes these trade-offs concrete: if the battery reduces expirations by 70 kWh worth $11 annually, it might not justify its price, whereas the water heater strategy that eliminates expirations could deliver outsized savings relative to its cost.

Limitations and assumptions

The forecast simplifies several real-world complexities. It assumes the utility applies credits strictly in first-in, first-out order and that monthly billing cycles align with calendar months. Some providers true-up annually regardless of credit age or pay out unused balances at wholesale rates. Others charge minimum fees or demand charges that persist even when credits cover all energy. The tool does not model time-of-use rates or demand charges, though you can run separate forecasts for peak and off-peak periods by splitting the year into multiple scenarios. Generation and consumption inputs are deterministic, yet weather can deviate significantly from averages. For high-stakes financial decisions, pair this planner with interval data from your utility or monitoring system.

Despite these limitations, the net metering credit carryover forecaster gives solar households a strategic advantage. It connects production forecasts, lifestyle choices, and utility rules into a single timeline. Users can align credit usage with electric vehicle charging plans, heat pump installations, or other electrification projects. Exporting the data supports conversations with installers, battery vendors, and financial planners. By refreshing the model after each billing cycle, homeowners can verify that utility statements match expectations and intervene early if credits risk expiring. The result is a more resilient, informed approach to maximizing the return on a solar investment.