Understanding credit expiration risk
Why seasonal credits create expiration risk even for well-sized subscriptions
A subscription sized to exactly match annual usage still generates far more credit in summer than winter under seasonal allocation, which means summer months build a surplus that needs to carry forward to cover winter shortfalls. If the expiration window is shorter than the gap between peak summer production and peak winter need, some of that surplus can expire before it's ever used. Check the DSIRE database for whether your state sets any minimum rollover period, since this varies by jurisdiction.
How this simulation works
This tool models 24 months of credit generation and bill consumption, tracking each month's credit separately and expiring it if it goes unused past your specified window. Credits are applied oldest first, which mirrors how most utility billing systems handle rollover in practice. See the billing cycle alignment tool for how your specific billing dates interact with this pattern. A grid-connected rooftop system with net billing faces a related use-it-or-carry-it decision, covered by the net billing versus net metering comparison.
What a forfeiture result actually means for your savings
Forfeited credit is savings you paid the subscription discount for but never actually received in bill relief, effectively lowering your real return below what the discount percentage alone would suggest. Even a modest forfeiture rate is worth factoring into your expectations, especially for subscribers near a program's minimum offset. Run the annual true up estimator for how this nets out over a full year. The gap between an offer's advertised and actual savings is exactly what the real versus estimated savings gap calculator is built to catch. The Department of Energy's community solar basics guide recommends this kind of detail be spelled out in plain language in your disclosure documents, so ask to see this in writing.
What to check with your provider
Ask directly about the exact expiration window and whether it's set by the utility or negotiable through the provider. If your simulation shows meaningful forfeiture, consider a lower offset percentage using the subscription size calculator, since a smaller surplus generates less credit at risk of expiring in the first place. If you want the source document behind this, the Department of Energy's community solar basics guide has it.