solar-breakeven

Solar Breakeven Skill

Every solar quote comes with a payback claim, and the claim always assumes the sunny version: full incentive eligibility, generous net metering forever, zero maintenance. This skill runs the honest model — net cost after verified incentives, offset that degrades ~0.5%/yr, electricity prices that inflate, and the inverter that dies around year 12 — and reports the breakeven year with its assumptions labeled. The biggest risk stays outside every model and gets named instead: net-metering policy is a regulatory decision that can change under you, and it moves paybacks by years.

What This Skill Produces

• The year-by-year table — savings, cumulative, vs-cost — from the script
• The breakeven year — and the net gain at the 25-year warranty horizon
• The quote check — the installer's payback claim vs. this model, with the assumption gaps named
• The not-modeled list — net-metering risk first, financing interest, roof interactions

Required Inputs

Ask for these if not provided:

• The quote — installed cost, claimed incentives (flagged verify-eligibility — incentives have income, tax-liability, and program caps), the claimed payback for comparison
• The bill — current monthly, and the offset % the installer claims (their number, tested; 80–95% is typical for a well-sized system)
• Ownership horizon — moving in 6 years changes everything; solar's value transfer at sale is uncertain and the model says so
• Financing — cash or loan; a loan adds interest the breakeven must also clear (run [the loan math] separately and add it — the script models the cash case)

Programmatic Helper

python3 scripts/solar_breakeven.py --cost 22000 --incentive 6600 --bill 190
python3 scripts/solar_breakeven.py --cost 22000 --incentive 6600 --bill 190 --offset 90 --json

Deterministic. Defaults: 85% offset, 3% electricity inflation, 0.5%/yr degradation, $2,000 inverter at year 12, 25-year horizon — every one overridable to match the quote's claims, which is how quotes get tested.

Framework: The Honest-Model Rules

1. Net cost means verified incentives: tax credits require tax liability to credit against; rebates have program caps and expiry — the model runs at face value with the verify flag, and a second run at zero-incentive shows what eligibility risk costs.
2. Test the quote by adopting its assumptions: run the script with the installer's offset and inflation numbers — if their payback claim still doesn't reproduce, the gap is usually missing degradation, the inverter, or arithmetic optimism; show the diff line by line.
3. Electricity inflation cuts both ways: it's the assumption doing the most quiet work — at 1% vs 5% the breakeven moves by years; the sensitivity line runs both and says which side of the bet the buyer is taking.
4. Net metering is the named elephant: the model implicitly assumes exported power keeps earning bill-rate credit; jurisdictions have changed these terms with real paybacks stranded mid-curve. It can't be modeled honestly — it gets stated honestly, every time, with "check your utility's current tariff and its grandfathering terms" as homework.
5. The horizon question is the ownership question: breakeven at year 9 is great for a 20-year owner and speculative for a 5-year one — resale value transfer is genuinely uncertain (and leased systems complicate sales enough to deserve their own warning). The read is delivered against their stated horizon.

Output Format

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Solar Breakeven: [system cost] — [bill]/mo

The Table and the Breakeven

[Script output: years 1–5 + milestones · breakeven year · net gain at horizon]

Quote Check

[Their claimed payback vs. this model at their assumptions · the gap, itemized]

Sensitivity

[Breakeven at electricity inflation 1% / 3% / 5% · at zero incentives — one line each]

What This Model Ignores

Net-metering policy risk (the big one — verify the current tariff and grandfathering) · financing interest if loaned · roof repairs under panels · resale-value transfer uncertainty · lease-vs-own complications.

Incentive eligibility and utility tariffs are jurisdiction-specific and change — verify both before signing. Educational model, not financial advice.

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Quality Checks

• [ ] Incentives carry the verify-eligibility flag and a zero-incentive sensitivity run
• [ ] The quote's claim is reproduced-or-diffed at its own assumptions
• [ ] Degradation and the inverter replacement are in the model
• [ ] Net-metering risk is stated with its homework, unmodeled and unhidden
• [ ] The breakeven is read against the user's stated ownership horizon

Anti-Patterns

• [ ] Do not accept the quote's payback as the baseline — reproduce it or show why it doesn't reproduce
• [ ] Do not model net metering as eternal — name it as policy, not physics
• [ ] Do not ignore the inverter — a known cost at a known-ish year is not a surprise
• [ ] Do not run only the sunny case — the zero-incentive and low-inflation runs are the honesty
• [ ] Do not moralize either way — solar pencils brilliantly on some roofs and poorly on others; the table decides, not the vibe