The sizing principle under a partial credit
Electricity your household uses at the moment it is generated is worth the full retail rate, because you simply do not buy it. Electricity you export is credited at 100 percent of supply, 100 percent of transmission and 25 percent of distribution.
The difference is not dramatic, which is why New Hampshire sizing advice is less severe than Georgia or Tennessee advice. But it is real, and it compounds over twenty-five years.
So the design should start from your last twelve months of bills rather than from available roof area. Ask what percentage of your annual usage the proposed system covers.
A design meaningfully above 100 percent of your usage is producing electricity that will be exported at the partial rate, having cost full price to generate. That needs a specific justification.
What counts as a good reason to size ahead
A concrete planned increase in load with a timeline. An electric vehicle, a heat pump, an addition, a workshop. Those genuinely raise future consumption and sizing for them is sound engineering.
In New Hampshire a heat pump is the most plausible of these, because it raises winter consumption substantially, and winter is when the array produces least. The two do not line up, but the export credit bridges part of the gap.
What does not count is a general expectation of using more electricity later. That is a hope rather than a plan, and it is the most common justification offered for a system larger than the household needs.
Ask your installer what specifically the extra capacity is intended to serve, and ask to see the design at a smaller size alongside it so you can compare returns directly.
Raising the value without changing the array
Every kilowatt hour you shift from evening to daylight moves from a partially credited export to a fully avoided purchase. That is free money and it does not require any equipment.
Dishwasher, washing machine and dryer are the easy ones. So is a pool pump if you have one, and so is charging an electric vehicle during the day rather than overnight.
The gain per kilowatt hour is the distribution component you would otherwise forgo, which is smaller here than in an avoided-cost state but still positive on every unit shifted.
Ask your installer what self-consumption share the projection assumed, and whether that assumption was based on your actual usage pattern or a generic figure. It is the assumption that determines how much of your production earns the full rate.
The pieces that remain, kept separate
The 30 percent federal Residential Clean Energy Credit under Section 25D expired for property placed in service after December 31, 2025, and the New Hampshire rebate was repealed by SB 303 in 2024, so there is no cash or tax incentive at either level for a cash or loan purchase.
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 2.0 at the partial credit rate, the RSA 72:62 property tax exemption where a municipality has adopted it, no sales tax on the equipment, and the electricity you stop buying at roughly 24 to 27 cents per kWh.
Ask for the design built from twelve months of your own bills, the self-consumption share stated, and a smaller system modelled alongside the proposal.