Accumulate, then forfeit
Credits accumulate across the year rather than being settled each billing period, which is genuinely useful in a state where generation and consumption do not line up month to month.
A strong summer builds credit; a cold, dark Montana winter draws it down. That carry is what makes the arrangement work for a northern household.
At the annual settle-up month, any remaining unused kilowatt hour credit from the previous twelve months is granted to NorthWestern Energy without compensation.
So there is no payout at avoided cost, no rollover into the next year and no discretion. Credit that has not been consumed by that point simply stops being yours.
Sizing so little reaches the settle-up
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.
Installations under 50 kW are eligible on the NorthWestern system, which is far above household need, so the programme ceiling is not what should constrain you. Your consumption is.
Ask for the monthly credit balance tracked through to the settle-up month. A balance that rises through summer and returns toward zero describes a well-matched system.
A balance that only climbs describes one that is too large, and the excess is being handed over each year for nothing.
When building larger is still right
A concrete planned increase in load with a timeline. An electric vehicle, a heat pump, an addition, a shop. Those genuinely raise future consumption.
In Montana a heat pump is a particularly plausible case, because it raises winter consumption substantially, and winter is when a Montana array produces least.
That mismatch is exactly what the annual credit accumulation exists to bridge, so sizing ahead of a heat pump is more defensible here than a general expectation of using more power.
Ask what specific future load the extra capacity is intended to serve, and ask for a smaller system modelled alongside so you can compare returns rather than assume.
What survives, and what to ask for
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 Billings receives no federal tax credit. Confirm with the Montana Department of Revenue whether any state credit currently applies.
Section 48E survives at 30 percent for third-party owners under leases and power purchase agreements, so ask what a provider claims and what reaches you in the rate.
What exists is net metering under the NorthWestern arrangement, with credits accumulating across the year and unused credit granted to the utility at the annual settle-up.
Ask for the monthly credit balance tracked through to the settle-up month and the percentage of annual usage stated, rather than an annual savings figure alone.