How accurate is this calculator vs. an actual solar quote?

Methodologically the same approach NREL and SolarReviews use: production from PVWatts kWh/kW/yr, degradation per LBNL, ITC at IRS 30%. Simplifications: location insolation (default 1,300 kWh/kW/yr; Phoenix 1,650, Seattle 1,050; run NREL PVWatts for your zip); installer-quote variance ($2.30-$4.00/W in 2024-25; calc splits $2.50 basic vs $3.50 premium); state/utility rebates beyond ITC (check DSIRE database). Use as strategic gut-check before quotes; budget plus or minus 15-25% variance vs installed reality.

Why does net-metering policy dominate the result so heavily?

Because it determines what your utility pays for excess generation, and most residential systems generate 50-80% above instant consumption. Three regimes: full retail 1:1 (every kWh exported earns import rate, grid acts as free battery, fastest payback); partial / TOU export (CA NEM 3.0, NV: excess at avoided-cost ~$0.05/kWh vs $0.30 import, payback 2-4 yrs longer); none (TX, AL, KY: only consumed energy counts, 30-40% without battery, battery economically necessary). Same hardware, three different paybacks.

Is the 30% federal tax credit included?

Yes, the calculator nets the IRS Section 25D Residential Clean Energy Credit at 30% off gross system + battery. It applies to systems placed in service Jan 1, 2022 through Dec 31, 2032; steps down to 26% in 2033, 22% in 2034, then expires unless Congress extends. Claim on IRS Form 5695 the year you turn on. The credit is nonrefundable but rolls forward up to 5 yrs. State and utility rebates are additional and not modeled; check DSIRE for your zip.

What payback period is ‘good’?

Industry rule of thumb: 6-9 yrs is excellent (high tariff, sunny climate, full net metering); 9-12 yrs typical for the average US installer + state combination; 12-15 yrs borderline; 15+ yrs requires utility rate hikes faster than current trends. Average homeowner stays ~13 years; if payback exceeds that, the resale-value bump (Zillow: solar adds ~4% to home value, recovering ~70% of unamortized cost) closes the loop. Real-world payback tracks plus or minus 15% of the calc number.

Why is the IRR sometimes higher than expected stock-market returns?

Three reasons. Solar IRR is tax-free in the US: avoided bill savings aren’t taxable, so after-tax-equivalent equity return is ~1.5x headline IRR for high earners. A 7% solar IRR equals a 10-11% pre-tax stock return. Electricity-rate appreciation locks in: rate-of-return bumps every year prices rise. System value transfers if you sell: Zillow shows solar homes sell ~4% higher, recovering ~70% of unamortized cost. Solar typically beats bonds and ties or beats average index funds.

When does battery make economic sense?

Batteries don’t pay back on energy savings alone in most US markets; they’re a value-add, not value-creator. They make sense in three cases: partial / no net metering (excess generation is worthless without storage; under CA NEM 3.0 battery payback 9-12 yrs vs solar-only 7-9 yrs); steep TOU rates (arbitrage off-peak $0.10-0.15 vs peak $0.40-0.60; 8-10 yr payback feasible); backup power (frequent outages, hurricane, wildfire-PSPS zones). Skip battery under full 1:1 net metering.

How do I size the system correctly?

Two constraints: usage and roof. Usage sizing: divide annual kWh by 1,300 (or local NREL PVWatts) to get kW that exactly offsets your bill. Roof sizing: usable south/west-facing area divided by 70 sq ft/kW for max physical fit. Pick the smaller. Three strategies: 100% offset (matches usage, fastest payback under full net metering); oversize 110-120% (anticipates EV/heat-pump/electrification; only worth it under retail-credit net metering); undersize 70-90% (budget or shaded roofs).

What’s the realistic system lifetime?

Panels: 25-30 yrs warranty-backed, with premium tiers guaranteeing 90% of nameplate at year 25 and 80% at year 30. LBNL field studies of 50,000+ systems show median capacity at year 20 still north of 90%. Inverters: string inverters need replacement at year 10-15 ($1,500-3,000); microinverters and DC optimizers carry 25-yr warranties matching panels. Roof: ideally re-roof BEFORE installing solar; removing/reinstalling for re-roof costs $2-5K. Mounting hardware outlasts panels.

Will solar actually save me money in a low-tariff state like Texas?

Marginally, but the math changes. Texas residential tariff ~$0.13/kWh combined with mostly partial / no net metering means payback often runs 11-14 yrs vs. 7-9 in California or New England. The case rests on three factors: self-consumption ratio (EV or large daytime load = 60-80% self-consumed at retail); tariff trajectory (Texas rates rose 25% from 2020-2024); battery + TOU plans (free-nights paired with battery compresses payback to 8-10 yrs). Run with realistic Texas inputs and check IRR vs alternatives.

How does the calculator handle electricity-price appreciation — that’s a big assumption?

It compounds your year-1 savings forward at the appreciation rate you specify. Default 3%/yr matches EIA long-run US average; 2020-2024 ran 4-6%/yr. Sensitivity rule of thumb: every 1 pp of appreciation = ~0.5 yr off payback and ~$3-5K extra over 20 yrs. Conservative case (1.5-2%/yr) is the right test for ‘does this still make sense in a flat-rate world?’. Aggressive case (5%/yr+) is reasonable in CA, MA, HI, NY where state policy drives structural rate increases.

What happens to my payback if I sell the house?

Three scenarios. Sell BEFORE payback: Zillow / Berkeley Lab show solar homes sell ~4% more ($15-25K bump on a typical home), recovering 60-80% of unamortized cost. So if you’re 5 yrs into a 9-yr payback and sell, you don’t lose 4 yrs; you recoup most as resale premium. Sell AFTER payback: pure upside; the system is paid off and the premium is gravy. Owe a loan: most solar loans are assumable; leases / PPAs can complicate sales because the buyer must qualify.

Why doesn’t the calculator factor in shading, roof tilt, or azimuth?

Because those are site-specific and the calculator is a strategic gut-check, not an installer-grade quote. NREL PVWatts accounts for them, and a real installer site visit lands plus or minus 15% of NREL after measuring tilt, azimuth, and shade. The 1,300 kWh/kW/yr default approximates a moderately oriented residential roof in moderate-insolation US; south-facing AZ/NM/FL = 1,500-1,650, east-west or shaded PNW = 950-1,100. Tune by running NREL PVWatts free or requesting 2-3 installer quotes.

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Solar ROI Calculator — Payback, 20-Year Savings & IRR

Drop your monthly bill, local kWh tariff, roof area, panel tier, battery option, financing, net-metering policy, electricity-price appreciation, panel degradation, and time horizon. Calculator sizes the system to your usage (or roof, whichever is smaller), applies the IRS Section 25D 30% federal tax credit, simulates production with degradation and tariff escalation, and returns payback period, 20-yr savings, IRR, battery payback delta, and lifetime CO₂ avoided. Anchored to NREL PVWatts insolation, LBNL field-study degradation curves, and EIA grid-emissions data.

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Reviewed by CalcBold EditorialLast verified May 4, 2026Methodology

Monthly electricity bill

Average across the last 12 months — solar offsets your annual usage, not just summer or winter peaks. Pull the figure from your utility’s online dashboard. Bills under $80/mo rarely justify residential solar (system cost overshoots savings); $150+ is the typical sweet spot.

Local kWh tariff

All-in rate per kWh including delivery + supply + fees. EIA US average ~$0.16/kWh (2024); Hawaii $0.40+, California $0.30+, Texas $0.13, Pacific Northwest $0.10. Higher tariff = faster payback; below $0.13 typically pushes solar payback past 12 yrs unless full retail net metering.

Available roof area

Usable south- or west-facing roof area, free of shade or obstructions. Rule of thumb: ~70 sq ft per kW of panel capacity. 800 sq ft fits ~11 kW; 400 sq ft fits ~5.7 kW. Calculator sizes to your usage if roof allows, or caps at roof if you’re constrained.

Panel tier

Basic = standard 400W mono-Si panels with central string inverter — cheapest, slightly more shade-sensitive. Premium = high-efficiency 425-440W panels with per-panel microinverters or DC optimizers — better partial-shade performance, longer warranties, more granular monitoring. Premium pays for itself only on shaded or complex roofs.

Battery option

Battery is rarely worth it under full 1:1 net metering — the grid acts as a free battery. Battery makes economic sense when (a) net metering is partial / none and excess generation has no value, or (b) your utility has steep time-of-use rates with high peak / low off-peak (battery arbitrages the spread), or (c) grid reliability is poor and you want backup-power coverage.

Financing

Cash gives the highest IRR (no financing drag); loan smooths the cost into monthly payments often lower than your previous bill (cash-flow positive day 1) but reduces effective IRR by ~1.5-2 pp. PPA / leases are a third option (zero upfront, savings split with installer) — they aren’t modeled here because the homeowner ROI is typically poor; they’re an option only when ITC / rebates can’t be claimed.

Net-metering policy

Single biggest variable in solar ROI. Full retail (1:1) — every excess kWh exported earns the same rate you’d pay; payback fastest. Partial — excess credited at avoided-cost / wholesale (CA NEM 3.0 ~$0.05/kWh export vs ~$0.30/kWh import); payback 2-4 yrs longer. None — only self-consumed energy counts (typically 30-40% without battery, 70-90% with); battery becomes economically necessary. Check DSIRE database for your state’s current rule.

Electricity-price appreciation

EIA long-run US residential rate appreciation ~2-3%/yr nominal; recent 2020-2024 stretch ran 4-6%/yr (post-pandemic + grid investment + extreme weather). Conservative: 2.5%; baseline: 3%; aggressive (high-growth states like CA, MA): 4-5%. Higher appreciation = solar locks in today’s rate, savings compound — payback shortens materially.

Panel degradation

Annual capacity loss. LBNL field studies of 50,000+ residential systems: median 0.45%/yr for crystalline silicon, 25th-percentile 0.3%/yr (premium panels), 75th-percentile 0.7%/yr (older / lower-tier). Premium-tier warranties: 90% capacity at year 25. Use 0.5% as the safe default; 0.3% for premium panels with strong warranties; 0.8% for budget Tier-2/3 panels.

Time horizon

How many years you plan to model. 20 yrs is the standard analysis horizon (matches inverter warranty + typical homeowner intent). 25 yrs matches panel warranty floor (most crystalline panels carry a 25-yr 80-90% capacity warranty). 30 yrs is aggressive but reasonable for premium systems. Shorter horizons (10-15 yrs) suit homeowners who plan to sell — the system value transfers but payback may not have completed.

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What This Calculator Does

The Solar ROI Calculator answers the question installer pitch decks rarely answer honestly: given my actual electricity bill, my state’s net-metering policy, and realistic panel degradation — what’s the payback period, the 20-year savings, and the IRR I can defend to a CPA? Drop your monthly bill, kWh tariff, roof area, panel tier, battery option, financing choice, net-metering policy, electricity-price appreciation, panel degradation, and time horizon. The calculator sizes the system to your usage (or roof, whichever is smaller), applies the IRS Section 25D 30% federal tax credit, simulates annual production with degradation and tariff escalation, and returns payback period, 20-yr savings, IRR, battery payback delta, and lifetime CO₂ avoided.

Most online solar calculators are vendor marketing tools — optimized to show the biggest-possible savings number to convert you into a lead. They tend to assume full retail net metering when most US states no longer have it, ignore panel degradation, and skip the inverter replacement budget at year 12. CalcBold’s Solar ROI calculator surfaces every assumption explicitly — every default is a starting point you can override, and the page links the source for the underlying number (NREL PVWatts for production, LBNL field studies for degradation, EIA grid-emissions data for CO₂, IRS Form 5695 for the credit). Use this calc as a strategic gut-check before requesting installer quotes; your real-world payback should land within ±15-25% of the number here.

The Math — Payback, 20-Yr Savings, IRR

Three layers compound the result. Sizing sets the system to offset 100% of annual kWh usage if the roof allows, or caps at the maximum kW that physically fits at ~70 sq ft/kW. Production applies NREL PVWatts moderate-US default of 1,300 kWh per kW per year, then degrades the panels at your specified rate (LBNL median 0.5%/yr, premium panels 0.3%/yr, budget panels 0.7%/yr). Tariff appreciates at your specified rate (EIA long-run average 3%/yr). Cash flownets the IRS 30% federal credit off the gross system + battery cost, deducts a string-inverter replacement budget at year 12 ($200/kW typical), and accumulates net savings against the upfront outflow until they cross zero — that’s payback.

IRR is solved by bisection on the cashflow sequence [-netUpfront, savings_1, savings_2, …, savings_N]— the rate that makes the net-present-value of the stream equal zero. Solar IRR is tax-free in the US (the savings you avoid on your bill aren’t taxable income), so the after-tax- equivalent equity return is roughly 1.5× the headline IRR for high earners. A 7% solar IRR is roughly equivalent to a 10-11% pre-tax stock return. That comparison usually decides whether the capital is better deployed in solar or in a broad-market index fund.

A Worked Example — “$148/mo Bill in California”

Suppose a California homeowner with a $148/mo electricity bill, $0.18/kWh tariff (bundled delivery + supply post-NEM 3.0), 800 sq ft of usable south-facing roof, basic-tier panels at $2.50/W, no battery, cash financing, and partial net metering (NEM 3.0 export rate ~60% of retail). The calculator derives an 8.0 kW system (annual usage 9,867 kWh ÷ 1,300 = 7.6 kW, capped at 11.4 kW roof-max), gross cost $20,000, net of 30% ITC $14,000. Year-1 savings at 80% net metering: 8.0 kW × 1,300 kWh × $0.18 × 0.8 = $1,498. With 3%/yr tariff appreciation and 0.5%/yr panel degradation, the cumulative cash position crosses zero around year 9.4 — the payback period. 20-year cumulative savings net of upfront: approximately $22,000(sensitive to tariff appreciation — at 5%/yr appreciation it’s $30K+, at 1%/yr it’s $14K). IRR for the 20-year horizon: roughly 7.4% — a tax-free 7.4% beats most index-fund forward expectations on after-tax basis.

Now flip one variable: switch net metering to full retail (1:1) — common in many states still on legacy NEM rules. Year-1 savings becomes 8.0 × 1,300 × $0.18 × 1.0 = $1,872, and payback compresses to 7.5 yrs; IRR rises to 9.3%; 20-yr savings climb to $32K. Same hardware, same sun — net-metering policy alone moves payback by ~2 years.

Why Net-Metering Policy Dominates the Result

Most residential solar systems generate 50-80% of annual production above what the home consumes in the moment. That excess gets pushed back to the grid — and what the utility credits you for it determines the bulk of your savings. Three regimes:

Full retail (1:1)— every excess kWh exported earns the same rate you’d pay to import. The grid acts as a free battery with infinite capacity. Payback fastest (typically 7-9 yrs in sunny + tariff $0.18+), IRR highest. Examples: most US states on legacy net-metering rules pre-2023; many northeastern states still on full NEM today.
Partial / time-of-use export — excess credited at avoided-cost or wholesale rate (CA NEM 3.0 averages ~$0.05/kWh export vs ~$0.30/kWh import — a 6× spread). Payback stretches by 2-4 yrs vs full retail. Examples: California (NEM 3.0 since April 2023), Nevada (post-2017 rule changes), parts of Arizona.
None / self-consumption-only— only the energy you use during generation counts toward savings. Without storage, that’s typically 30-40% of total production; with battery, 70-90%. Payback longest, battery becomes economically necessary. Examples: Texas (most utilities), Alabama, Kentucky, parts of South Carolina.

The DSIRE database (dsireusa.org) is the authoritative public source for your state’s current net-metering rule. It updates as legislation changes — California flipped from NEM 2.0 (full retail) to NEM 3.0 (partial + time-of-use) in April 2023, and that single rule change moved typical payback from 7-8 yrs to 10-13 yrs in the state. Always check DSIRE before locking in your assumptions; the calculator’s default is full retail because legacy rules still cover ~30% of US homes.

When Battery Makes Sense (and When It Doesn’t)

Batteries don’t pay back on energy savings alone in most US markets — they’re a value-add, not a value-creator. The calculator’s “battery added X yrs to payback” row makes this trade-off explicit. Battery makes economic sense in three specific cases.

Partial / no net metering — your excess generation is worthless without storage, so battery captures it at retail rate vs. exporting at wholesale. Under CA NEM 3.0 a battery typically pays back in 9-12 yrs vs. solar-only 7-9 yrs.
Steep time-of-use rates — batteries arbitrage the spread between off-peak ($0.10-0.15/kWh) and peak ($0.40-0.60/kWh in CA, summer NY); 8-10 yr payback feasible if the spread is wide and you have an EV-charging or large evening load.
Backup-power need— frequent grid outages (hurricane country, wildfire-PSPS zones, rural areas) where the battery’s real value is keeping lights on, not direct $-savings; calc this as insurance, not investment.

Skip battery under full 1:1 net metering — the grid is a free battery with infinite capacity. Skip battery if your only motivation is “more green” — adding battery typically extends the system’s carbon-payback period by 1-2 years (battery production is carbon-intensive) and rarely meaningfully increases the electricity offset. Under partial net metering a battery is genuinely earning its keep; under full net metering it’s a $9-16K luxury with poor economics.

Common Mistakes That Distort the Answer

Ignoring net-metering policy. The single biggest variable. A homeowner who plugs in California numbers but assumes full retail (NEM 2.0 legacy) instead of NEM 3.0 partial export will overstate savings by 30-40%. Always check DSIRE for your current state rule.
Assuming aggressive electricity-price appreciation. Recent 2020-2024 stretch ran 4-6%/yr — that’s unusual; long-run EIA average is 2-3%/yr. Run the conservative case (1.5-2%/yr) to test whether the project still makes sense in a flat-rate world.
Forgetting panel degradation. Vendor calculators routinely model panels at 100% capacity for 25 years. Real degradation per LBNL field studies of 50,000+ residential systems is 0.45-0.5%/yr median — meaning year 25 production is ~88% of year-1. Over a 20-year horizon that’s ~5% of cumulative savings shaved off; model it.
Skipping inverter replacement. String inverters (basic tier) typically need replacement at year 10-15 ($1,500-$3,000 cost). Calculator bakes a $200/kW year-12 replacement into the cashflow stream; microinverters / DC optimizers (premium tier) carry 25-yr warranties matching panels and don’t need replacement.
Sizing for current usage without considering EV / heat-pump electrification. If you plan to add an EV (adds ~3-4 MWh/yr) or replace a gas furnace with a heat pump (adds ~5-8 MWh/yr in cold climates), size the system 110-130% of current usage. Net-metering rules may cap how much you can oversize, so check your utility’s policy first.
Comparing solar IRR to pre-tax stock returns. Solar savings are tax-free; index-fund returns are taxed at LTCG rates (15-20%) plus state tax. Adjust the comparison: a 7% solar IRR is roughly a 10-11% pre-tax-equivalent stock return for high earners. Most solar projects beat equities on after-tax basis under full net metering; they tie or lose under partial net metering in low-tariff states.

Related Calculators

Pair the Solar ROI Calculator with the EV vs Gas Cost Calculator — solar bonds hard with EV ownership, since every kWh you generate displaces gas at the gas-station price (effectively $0.50-0.80 per equivalent-kWh). Run the EV TCO with home- charging assumed at your solar marginal-cost (essentially zero past payback) and the EV case becomes much stronger. Solar is also a capital- allocation decision: $15-30K of cash competing with mortgage paydown, equities, or a cash buffer. Run the Payoff Mortgage vs Invest Calculator to anchor on the after-tax-equivalent return on extra mortgage principal (~3-7% depending on your rate); compare to your solar IRR to see whether solar wins on pure return. Run your net upfront cost through the Compound Interest Calculator at 7% market return for 20 years to see the alternative-investment case explicitly — solar usually wins on after-tax basis, but the compound calc makes the trade-off concrete rather than assumed. If financing the system, run the loan payment through the Can I Afford This? Calculator to confirm the monthly is comfortable — solar loans are usually cash-flow-positive day 1 (savings exceed payment), but only if your tariff and net-metering match this calc’s assumptions.

How to Read the Verdict

The honest comparison is between solar IRR and what the same cash would earn invested elsewhere. Solar wins on a risk-adjusted basis when the after-credit IRR clears the alternative — but the answer is highly sensitive to your net-metering policy and tariff appreciation assumption.

Payback under 8 years & full net metering (1:1). Strong buy. The 25-year warranty horizon gives you 17+ free years post-payback at typical 0.5%/yr degradation.
Payback 8-12 years. Compare 20-yr IRR to your marginal investing alternative — if solar IRR clears 7% real and you plan to stay in the home, take solar; if you plan to sell within 5 years, the value capitalizes into resale imperfectly.
Net-billing or avoided-cost state (CA NEM 3.0, HI). Battery becomes mandatory for the math to work — without storage, exported kWh sells for 25-30% of retail and payback stretches past 15 years.
Payback over 15 years. Pass on solar; spend the install premium on insulation or a heat pump first — both have shorter paybacks at most US tariffs.

Frequently Asked Questions

The most common questions we get about this calculator — each answer is kept under 60 words so you can scan.

How accurate is this calculator vs. an actual solar quote?
Methodologically the same approach NREL and SolarReviews use: production from PVWatts kWh/kW/yr, degradation per LBNL, ITC at IRS 30%. Simplifications: location insolation (default 1,300 kWh/kW/yr; Phoenix 1,650, Seattle 1,050; run NREL PVWatts for your zip); installer-quote variance ($2.30-$4.00/W in 2024-25; calc splits $2.50 basic vs $3.50 premium); state/utility rebates beyond ITC (check DSIRE database). Use as strategic gut-check before quotes; budget plus or minus 15-25% variance vs installed reality.
Why does net-metering policy dominate the result so heavily?
Because it determines what your utility pays for excess generation, and most residential systems generate 50-80% above instant consumption. Three regimes: full retail 1:1 (every kWh exported earns import rate, grid acts as free battery, fastest payback); partial / TOU export (CA NEM 3.0, NV: excess at avoided-cost ~$0.05/kWh vs $0.30 import, payback 2-4 yrs longer); none (TX, AL, KY: only consumed energy counts, 30-40% without battery, battery economically necessary). Same hardware, three different paybacks.
Is the 30% federal tax credit included?
Yes, the calculator nets the IRS Section 25D Residential Clean Energy Credit at 30% off gross system + battery. It applies to systems placed in service Jan 1, 2022 through Dec 31, 2032; steps down to 26% in 2033, 22% in 2034, then expires unless Congress extends. Claim on IRS Form 5695 the year you turn on. The credit is nonrefundable but rolls forward up to 5 yrs. State and utility rebates are additional and not modeled; check DSIRE for your zip.
What payback period is ‘good’?
Industry rule of thumb: 6-9 yrs is excellent (high tariff, sunny climate, full net metering); 9-12 yrs typical for the average US installer + state combination; 12-15 yrs borderline; 15+ yrs requires utility rate hikes faster than current trends. Average homeowner stays ~13 years; if payback exceeds that, the resale-value bump (Zillow: solar adds ~4% to home value, recovering ~70% of unamortized cost) closes the loop. Real-world payback tracks plus or minus 15% of the calc number.
Why is the IRR sometimes higher than expected stock-market returns?
Three reasons. Solar IRR is tax-free in the US: avoided bill savings aren’t taxable, so after-tax-equivalent equity return is ~1.5x headline IRR for high earners. A 7% solar IRR equals a 10-11% pre-tax stock return. Electricity-rate appreciation locks in: rate-of-return bumps every year prices rise. System value transfers if you sell: Zillow shows solar homes sell ~4% higher, recovering ~70% of unamortized cost. Solar typically beats bonds and ties or beats average index funds.
When does battery make economic sense?
Batteries don’t pay back on energy savings alone in most US markets; they’re a value-add, not value-creator. They make sense in three cases: partial / no net metering (excess generation is worthless without storage; under CA NEM 3.0 battery payback 9-12 yrs vs solar-only 7-9 yrs); steep TOU rates (arbitrage off-peak $0.10-0.15 vs peak $0.40-0.60; 8-10 yr payback feasible); backup power (frequent outages, hurricane, wildfire-PSPS zones). Skip battery under full 1:1 net metering.
How do I size the system correctly?
Two constraints: usage and roof. Usage sizing: divide annual kWh by 1,300 (or local NREL PVWatts) to get kW that exactly offsets your bill. Roof sizing: usable south/west-facing area divided by 70 sq ft/kW for max physical fit. Pick the smaller. Three strategies: 100% offset (matches usage, fastest payback under full net metering); oversize 110-120% (anticipates EV/heat-pump/electrification; only worth it under retail-credit net metering); undersize 70-90% (budget or shaded roofs).
What’s the realistic system lifetime?
Panels: 25-30 yrs warranty-backed, with premium tiers guaranteeing 90% of nameplate at year 25 and 80% at year 30. LBNL field studies of 50,000+ systems show median capacity at year 20 still north of 90%. Inverters: string inverters need replacement at year 10-15 ($1,500-3,000); microinverters and DC optimizers carry 25-yr warranties matching panels. Roof: ideally re-roof BEFORE installing solar; removing/reinstalling for re-roof costs $2-5K. Mounting hardware outlasts panels.
Will solar actually save me money in a low-tariff state like Texas?
Marginally, but the math changes. Texas residential tariff ~$0.13/kWh combined with mostly partial / no net metering means payback often runs 11-14 yrs vs. 7-9 in California or New England. The case rests on three factors: self-consumption ratio (EV or large daytime load = 60-80% self-consumed at retail); tariff trajectory (Texas rates rose 25% from 2020-2024); battery + TOU plans (free-nights paired with battery compresses payback to 8-10 yrs). Run with realistic Texas inputs and check IRR vs alternatives.
How does the calculator handle electricity-price appreciation — that’s a big assumption?
It compounds your year-1 savings forward at the appreciation rate you specify. Default 3%/yr matches EIA long-run US average; 2020-2024 ran 4-6%/yr. Sensitivity rule of thumb: every 1 pp of appreciation = ~0.5 yr off payback and ~$3-5K extra over 20 yrs. Conservative case (1.5-2%/yr) is the right test for ‘does this still make sense in a flat-rate world?’. Aggressive case (5%/yr+) is reasonable in CA, MA, HI, NY where state policy drives structural rate increases.
What happens to my payback if I sell the house?
Three scenarios. Sell BEFORE payback: Zillow / Berkeley Lab show solar homes sell ~4% more ($15-25K bump on a typical home), recovering 60-80% of unamortized cost. So if you’re 5 yrs into a 9-yr payback and sell, you don’t lose 4 yrs; you recoup most as resale premium. Sell AFTER payback: pure upside; the system is paid off and the premium is gravy. Owe a loan: most solar loans are assumable; leases / PPAs can complicate sales because the buyer must qualify.
Why doesn’t the calculator factor in shading, roof tilt, or azimuth?
Because those are site-specific and the calculator is a strategic gut-check, not an installer-grade quote. NREL PVWatts accounts for them, and a real installer site visit lands plus or minus 15% of NREL after measuring tilt, azimuth, and shade. The 1,300 kWh/kW/yr default approximates a moderately oriented residential roof in moderate-insolation US; south-facing AZ/NM/FL = 1,500-1,650, east-west or shaded PNW = 950-1,100. Tune by running NREL PVWatts free or requesting 2-3 installer quotes.