Methodology
The “Lab” in Ballpark Lab. Here is exactly how every number is produced, where the underlying data comes from, and how often we refresh it. Nothing is hidden behind an email gate.
Solar data last updated June 30, 2026· Pool data last updated June 30, 2026.
How the solar calculator works
We start from how much electricity you use, then work forward to system size, production, price, incentives, and payback. There are two paths — a quick estimate from your bill (Tier 1) and a satellite roof analysis via the Google Solar API (Tier 2) — but they only differ in how system size and yearly production are derived. From there both share one chain, so the tax logic stays consistent.
- Annual usage.If you give kilowatt-hours we use them directly; if you give a monthly bill we divide by your state’s electricity price and annualize.
- System size. We size the array to cover your target share of usage given local peak sun hours and a performance ratio for real-world losses, then round to a whole number of panels.
- Production.Tier 1 estimates yearly output from size and sun hours. Tier 2 uses the roof’s actual modeled output (orientation, pitch, and shade), converted from DC to AC delivery.
- Gross price. System watts × your state’s installed $ per watt (adjusted for roof material), plus battery cost if selected, plus a fixed $2,500of soft costs — permits, interconnection, design, and inspection — that every install carries regardless of size. Because those don’t scale with the system, a smaller array has a higher effective $ per watt and pays back more slowly, which is why your payback responds to how much power you use.
- Incentives (the 25D reality). For cash and loan buyers the federal residential credit is $0 — Section 25D expired on 2025-12-31. We then subtract any state incentive to get net cost. A loan carries that net cost at 6.5% APR over 12 years, so you repay the principal plus interest — which is why a financed system pays back later than paying cash. Lease/PPA customers pay $0 upfront; the installer claims the commercial credit and passes it through as a lower indicative monthly payment.
- Savings, payback, and lifetime. Each year’s savings reflect panel degradation, electricity-price escalation, and your net-metering factor. Payback is the year cumulative savings cover net cost; lifetime savings sum the horizon and subtract net cost.
| Assumption | Value |
|---|---|
| Default panel wattage (Tier 1) | 400 W |
| Performance ratio | 0.8 |
| DC→AC derate | 0.85 |
| Electricity price escalation | 3.5% / yr |
| Panel degradation | 0.5% / yr |
| Lifetime horizon | 25 years |
| Federal residential credit (25D) | 0% (expired) |
| Federal commercial credit (Sec. 48, lease/PPA) | 30% |
| Fixed soft costs (permits, interconnection, design) | $2,500 |
| Loan APR / term (financed purchases) | 6.5% / 12 yr |
Estimate for pre-qualification, not a final engineering design.
How the pool calculator works
Pool cost does not scale cleanly like solar, so the model is additive: a base shell plus features. Tier 1 uses length × width from the form; Tier 2 uses the area and perimeter of a polygon you draw on a satellite map. Both feed the same chain.
- Surface area. Length × width, or the measured area of your drawn shape.
- Base shell.A fixed base for your pool type (gunite, fiberglass, or vinyl) plus area × the per-square-foot rate, adjusted by a regional labor multiplier and a depth factor.
- Sitework. The shell is scaled by an excavation multiplier for flat, sloped, difficult-access, or rocky ground.
- Features. Heating, decking, saltwater, automation, lighting, a spa, water features, a cover, and fencing are each added from published price ranges.
- Total as a range. We present the total as a low/high band rather than a single false- precision number, because real bids vary by contractor and season.
Estimate for pre-qualification, not a quote; price varies by contractor, location, and season.
Data sources
We cite primary sources, not other blogs. Figures are calibrated from:
- U.S. DOE / EIA
- Residential retail electricity prices by state ($/kWh).
- EnergySage Solar Marketplace data
- Installed solar system pricing ($ per watt) by state.
- NREL (National Renewable Energy Laboratory)
- Average daily peak sun hours used for production estimates.
- Pool & Hot Tub Alliance and contractor pricing surveys
- Pool shell, sitework, and feature cost ranges by type.
- IRS guidance and the One Big Beautiful Bill Act (OBBBA)
- Federal tax credit status — the residential 25D credit expired 2025-12-31.
- State energy offices
- State-level incentives and net-metering treatment.
Update cadence
Pricing, electricity rates, and incentives are reviewed on a quarterly schedule, and immediately whenever a policy change (such as the 25D expiration) materially affects the math. Every calculator and cost page shows a “last updated” date so you can judge freshness at a glance.
- Solar data: last updated June 30, 2026.
- Pool data: last updated June 30, 2026.