Calculation workbook and sources
Test version 2026-09-25.2. Source review: September 25, 2026.
The workbook contains editable inputs and formulas for the monthly estimate, ten-year payments, remaining loan balances, future scheduled payments, REC income and sensitivity to production and remaining utility charges. Editing it does not update the website.
What the arithmetic confirms
For the selected example, 17 panels of 440 W produce an assumed 8,976 kWh/year. At $2.85/W, the installed price is $21,318. A 15-year loan at 6.5% costs $185.70/month. Adding $19.81 and subtracting $22/month average REC income gives $183.51 net cost and $26.78 savings against a $210.29 baseline bill.
The prepaid financed option requires $23,156.91 in payments during the first ten years, excluding RECs. This includes the separate $1,000 year-five buyout. At year ten the remaining principal is approximately $8,424, with approximately $9,890 in later scheduled loan payments. Those are different measures of the same remaining loan; do not add both.
Dated tariff evidence
Eversource rate summary, downloaded September 25, 2026 — the document is dated August 1, 2026. Official source.
| Residential Rate R component | ¢/kWh |
|---|---|
| Distribution | 6.727 |
| Transmission | 4.445 |
| Stranded cost recovery | −0.148 |
| System benefits | 0.618 |
| Regulatory reconciliation | 0.296 |
| Pole plant adjustment | −0.029 |
| Energy | 14.009 |
| Total of published components | 25.918 |
The published monthly customer charge is $19.81, matching this model. The selected test rate remains 25.8336¢/kWh. The difference is about $0.62/month for 8,848 kWh/year. The model’s supply/delivery split of 14.0086 + 11.825 cents was derived to match the selected total, rather than verified from a bill. Confirm the actual supplier and detailed tariff before replacing it.
Supplier choice also matters. Rye’s July 27, 2026 rate notice lists a 14.949¢/kWh Granite Basic supply rate effective August 1. The selected example should not be described as the verified tariff for every Rye household.
Pricing documents still needed
| Selected assumption | Evidence status |
|---|---|
| $2.85/W installation | User-selected. A dated installer quote, equipment specification and exclusions have not been supplied. |
| $18,922 prepaid lease and $1,000 year-five buyout | A dated proposal and contract are needed to confirm buyout eligibility, price, ownership and REC rights after buyout. |
| 6.5% loan for 15 years; $130/month lease | Illustrative terms. Confirm fees, escalators, collateral, early payoff and transfer obligations. |
| $33 per REC | User-selected. CPCNH’s program page requires NH-certified RECs and updates prices/vintages annually. The available price-table content did not substantiate $33. Confirm the applicable vintage and aggregation agreement. |
Production and utility-bill assumptions
The example is a full-offset scenario, not a validated post-solar utility bill. Its calculation assumes no variable utility charges remain. Matching annual generation to annual use does not establish that result.
Eversource’s net-metering explanation distinguishes gross purchases, monthly net purchases and net sales. Certain charges remain tied to gross purchases. For applicable small generators, net sales receive energy-service, transmission and 25% of distribution components; customers using a competitive supplier do not receive Eversource’s default energy-service portion. The fixed customer charge remains. Confirm supplier-specific export compensation and the customer’s eligibility.
The assumed 1.2 kWh/W/year production factor is a screening input. An installer production report or a PVWatts calculation needs location, system rating, tilt, azimuth and losses. No property-specific production model has been provided. With the same 7.48 kW system, a factor of 1.0 gives 7,480 kWh/year; 0.8 gives 5,984 kWh/year.
To finish validation, provide 12 monthly usage bills, the supplier/rate class and net-metering eligibility, and an installer or PVWatts monthly generation estimate. The workbook shows that an additional $20/month variable bill reduces the illustrated $26.78 savings to $6.78; $40/month changes it to $13.22 additional cost.
Comparison conventions
The introduction and homeowner PDF include REC income in net cost and savings. The financing table compares payments excluding RECs and states each option’s assumed REC rights. REC receipts are separate income, not reductions of the actual loan payment or utility bill. Prepaid financing is a loan repayment and is not the lowest monthly payment shown.
Long-term comparisons assume unchanged prices, generation and rates. They exclude degradation, maintenance, equipment replacement, taxes, incentives and financing or transfer fees. These are illustrative cash-payment comparisons, not discounted ownership-cost forecasts.
Version agreement
The website is built from one shared benchmark configuration. Its calculator link carries a version and assumption fingerprint. The Google calculator compares both before allowing calculation; a changed server assumption also invalidates an in-progress estimate or PDF request. This prevents matching-looking pages from silently using different benchmark inputs.