Direct answers to the questions and pushback users are most likely to raise. SIREN is a planning model, not a utility settlement engine or tax opinion. If a result surprises you, compare the input assumptions line-by-line against your installer proposal, utility tariff, and tax situation.
What kind of model is SIREN?
SIREN is a 12-month solar ROI and dispatch planning model. It takes your annual production estimate, spreads it across the selected monthly production curve, compares each month with monthly household usage, estimates direct self-use, surplus export, and battery shifting, then rolls the first-year value into a 30-year financial model. It is stronger than a simple annual-average calculator, but it is not a full hourly or 15-minute utility-grade simulation.
Is this an hourly battery dispatch model?
No. SIREN uses monthly buckets. It captures seasonal mismatch and month-level self-consumption, export, and battery-shift value without requiring interval data. It does not know the exact hour your EV charges, individual cloudy days, weekend versus weekday TOU rules, or hourly export-credit tables. For final design, compare the result with installer software, your utility tariff, or an interval-data simulation.
Why did changing the monthly production curve change my ROI only a little — or not at all?
The curve does not create more annual solar. It changes when the annual kWh are assumed to occur. ROI changes only when that new timing changes the value of the energy: more direct self-use, less low-value export, more usable battery shifting, or better matching with seasonal household usage. If your load is high enough to absorb solar in most months, your battery is not binding, export credit is close to retail, or the compared curves are similar, the ROI movement may be small. For the strongest estimate, enter both a custom monthly production curve from your proposal and actual monthly kWh usage from your bills. For example: shifting solar production from winter into summer only helps if you actually use more power in summer (say, running AC). If that extra summer solar would just be exported to the grid for a few cents, moving the curve barely changes your ROI.
Does entering monthly kWh usage actually matter?
Yes. Monthly usage entries define the load shape used by the 12-month dispatch layer. Two homes can use the same annual kWh but have different ROI if one uses more power during high-solar months and the other uses more during low-solar months. If you leave monthly usage blank, SIREN spreads annual usage evenly across the year. That is acceptable for a quick estimate but weaker than using actual monthly bill data.
Why does SIREN assume electricity rates rise 4%/yr? That's too high.
It is a default, not a fact. Utility escalation varies by region, tariff, regulation, and time period. A 4% default is meant to represent a moderately high-cost utility environment, but you should replace it with your own assumption if you believe rates will rise more slowly. The Skeptic's View button re-runs the model with a lower utility-escalation assumption, a lower market-return assumption, and no resale premium so you can see whether the thesis survives a more conservative case. Why it matters: at 4%/yr, a $200/month power bill grows to about $430/month in 20 years — so the savings solar locks in keep getting bigger. If you expect slower increases, lower the number and watch the result move.
A 10% S&P 500 return for 30 years is fantasy.
10% is a conventional long-run nominal benchmark, not a prediction. SIREN uses it so the solar project is compared against a recognizable alternative use of capital. The comparison is after-tax: federal and state capital-gains assumptions reduce the market side, while solar bill savings are treated as tax-free avoided cost. You can change the benchmark return, and Skeptic's View lowers it automatically for a conservative test. The headline "Vs benchmark" figure is apples-to-apples: it compares solar with your post-payback bill savings reinvested at the same market return against simply investing the cash in the market — not raw solar net gain versus the market lump. The un-reinvested solar net gain is still shown alongside (as "Net gain") for reference.
Why do capital-gains tax changes affect "Vs Benchmark" but not Payback or IRR?
Because those are different calculations. Solar payback, IRR, and solar net gain are based on project cash flows: avoided utility purchases, export value, battery-shift value, electrification savings, maintenance, degradation, and replacement assumptions. Capital-gains tax applies to the market-investment comparison, not to electricity you avoid buying from the utility. So changing federal or state capital-gains tax should move the after-tax benchmark and "Vs Benchmark" result, but it should not change the solar project's own payback or IRR. For example: raise your capital-gains tax and the stock-market side gets taxed more, so the gap in "Vs Benchmark" shrinks — but your panels still cut the exact same amount off your power bill, so payback and IRR don't move at all.
Did you account for the time value of money — a dollar later being worth less than a dollar now?
Yes — that's exactly what IRR does. Plain version: $100 today is worth more than $100 ten years from now, because today's $100 could be invested and grow in the meantime. So when SIREN reports a return, it isn't just adding up dollars — IRR already weighs when each dollar of savings shows up, counting early savings more than far-off ones. For example: solar that saves $2,000 a year for 30 years is not simply "worth $60,000" — the early years count for more, and IRR builds that in. SIREN also shows the plain, un-adjusted total as "Net gain," so you see both numbers: the simple sum and the time-adjusted return. And the whole reason we compare against the stock market is to answer the real time-value question — "could this money have grown more somewhere else?"
The headline IRR seems absurdly high. Is this real?
Headline IRR is configuration-specific. A modest solar-only system in a mild climate may show single-to-low-double-digit IRR. A much higher IRR usually means the configuration includes large electrification streams, such as EVs displacing gasoline, heat pumps displacing gas, or pool-heater conversion. Those streams convert fuel spending into electricity demand that can be supplied by solar. The results break down the value drivers so you can see whether the result is coming from solar-only savings, battery shifting, exports, electrification, resale value, or reinvestment assumptions. For example: the panels alone might show about 8% IRR. Add two EVs that replace $4,000/yr of gasoline and the IRR can jump into the teens — not because the panels changed, but because the same sunshine is now replacing expensive gasoline, not just grid power.
Where's the 30% federal tax credit?
SIREN works from your Net project cost — the price after any incentives you actually receive — so enter that figure, not the gross sticker price. This tool does not assume or promise any specific tax credit. Whether you qualify for a tax credit, and how much, depends on your individual tax situation — for tax credits you may qualify for, consult a qualified tax professional. If you are unsure whether an incentive applies to you, enter the higher gross cost for a conservative result.
"Self-consumed solar value" is much bigger than my old electric bill. Is something double-counted?
Not necessarily. The self-consumed solar value is the retail value of solar electricity used by home loads, including new electrical loads such as EVs, heat pumps, pool equipment, or other electrification. That value can exceed the old electric bill if the home now uses more electricity because former gasoline or gas loads have moved onto the electric side. SIREN is designed to offset this by treating electrification savings as fuel avoided minus added electricity required. The Year-1 savings breakdown and the Drivers section show whether value is coming from avoided grid purchases, export credits, battery shifting, or non-electric fuel displacement. For example: say your old power bill was $2,000/yr. You add an EV and a heat pump, so the home now uses much more electricity — but you've stopped buying roughly $1,500/yr of gasoline and $800/yr of natural gas. Solar now covers the electricity that replaces all three, so the "solar value" can be around $4,000 — larger than the old $2,000 bill because it's also replacing the gas and gasoline, not double-counting them.
I changed panel efficiency / wattage and ROI didn't move. Broken?
Usually intentional. Your annual production figure should come from a production model or installer proposal, and that production figure should already reflect panel wattage, panel count, roof geometry, shading, inverter limits, and climate assumptions. If SIREN also recalculated production from panel efficiency, it could double-count. The panel spec that directly affects long-term ROI is degradation, because it changes production over the 30-year horizon. Use the panel comparison tools for educational side-by-side comparisons, but treat entered annual production as the controlling production input unless you are using a dedicated production designer.
My installer's production estimate looks optimistic.
It might be — installers sometimes quote best-case annual production. SIREN trusts the annual production figure you enter, so the result is only as honest as that number. Cross-check it against an independent estimate (for example, NREL's free PVWatts tool) using your address, array size, tilt, azimuth, and shading. If the two disagree, enter the lower figure — or haircut your installer's number by 5–10% — to stress-test the project. Production is one of the two biggest ROI drivers (alongside your electricity rate), so a few percent here matters.
Battery replacement at $500/kWh is too cheap. Why not today's $700–900?
The replacement cost is editable because no one knows the installed residential battery price 10–20 years from now. Today's cost may be higher, but future chemistry, installation practices, competition, and warranty structures may be different. Use $500/kWh as a planning midpoint only if it fits your view. For a conservative case, enter a higher number. For an optimistic technology-cost decline case, enter a lower number. The important point is that SIREN shows the assumption instead of hiding it.
Can too many batteries make ROI worse?
Yes. More batteries can improve backup power, reduce export, and shift more solar into expensive evening hours. But once the battery bank is larger than the home's usable surplus solar and evening demand, additional units may add cost without adding much annual value. If the replacement option is enabled, the future refresh cost also scales with the modeled battery bank. Oversized storage may be reasonable for resilience, medical equipment, outage protection, or whole-home backup, but it should not be assumed to improve financial ROI unless the monthly dispatch results show the extra capacity is actually used.
You only model one battery replacement. What about a second one?
The default is one refresh at the chosen battery replacement year. A second replacement option can be enabled for a more conservative long-horizon case. It is usually off by default because projecting battery chemistry and installed pack prices two decades out is highly uncertain. If you want the worst-case view, turn it on and use conservative replacement pricing.
The default export rate is wrong for me.
It may be. Export compensation varies by utility, tariff, hour, season, retail plan, and country. SIREN's default export value is a placeholder when no better value is entered. Replace it with your installer's modeled annual-average export compensation rate, your utility's published estimate, or a conservative value you want to test. In the monthly model, the export rate affects the value of surplus solar that remains after monthly self-use and estimated battery shifting.
What does "curtailed" or "no export value" mean?
It means SIREN gives no credit to surplus solar that exceeds monthly household load and usable battery shifting. This can represent a tariff that does not compensate exports, a system that is export-limited, or a conservative stress test. If your utility does pay for exported energy, use an export-credit setting instead. The choice can materially affect ROI for oversized systems.
What about fixed charges, minimum bills, or solar grid fees?
Use the monthly residual / minimum bill field. Most utilities charge a fixed connection or customer charge you pay regardless of solar, and some impose minimum bills or solar-specific grid-access fees. Entering that amount reduces the savings SIREN credits to solar each month, so the model does not assume your bill drops to zero. If your utility is proposing new solar fees, model a higher residual to see whether the project still holds up.
The home resale premium feels speculative.
That is fair. Resale value is uncertain and market-specific. SIREN treats it as optional and conservative: it contributes only if a sale year or resale assumption is enabled. Skeptic's View removes it. Treat the resale table as a directional estimate, not an appraisal.
Is this a sales funnel for a specific installer or warranty product?
No. Installer quality, warranty structure, and company stability matter because solar is a long-lived asset, but those sections are meant to teach users what to verify, not to force a vendor choice. The financial model should not depend on a particular installer name unless the user enters costs, warranties, or service assumptions from that installer.
What if the solar company goes out of business?
It happens — even big installers fail (SunPower, one of the largest, went bankrupt in 2024). Your panels keep making power, but the warranty that was supposed to cover repairs can become worthless, and there may be no one left to call for service. That's an "orphan system." For example: a few years in, your inverter dies — if the installer is gone, the free replacement they promised is now your bill, often $1,500–$2,500. Two protections: (1) make sure the workmanship warranty is backed by a separate company, not just the installer; and (2) register the panel and inverter manufacturer warranties in your own name, so they survive even if the installer doesn't. And if you financed, you still owe the loan even if the installer vanishes — so keep the loan independent of the installer where you can.
Single HTML file — is this safe? Where's the source?
It is a single self-contained file so the model can be inspected. Right-click and choose View Source to see the calculations, labels, and assumptions. There is no required backend for the calculator logic. If a future hosted version adds analytics, saving, accounts, or external services, that should be disclosed separately.
What about replacing the roof? Pulling panels off and back on isn't free.
Valid — and commonly omitted by solar calculators. If your roof will need replacing within the hold period, budget the panel removal-and-reinstall cost: typically about $1,500–$5,000+ depending on system size and roof complexity. SIREN does not add this automatically because roof age varies enormously from home to home — fold it into your Net project cost or the repair reserve. Best practice: install on a roof with at least 15 years of life left, so the system and the roof age out together and you avoid a mid-life removal.
You didn't account for [inverter / O&M / insurance / SRECs]…
Many of these are already modeled or editable: panel degradation, battery degradation and refresh, repair reserve, insurance increase, inverter replacement treatment, SREC or incentive income where applicable, EV maintenance differential, and EV insurance taper. The Audit & context section should be used to check what is active in the current scenario. If a factor is important for your project and not explicitly modeled, treat the result as incomplete until you add it manually or adjust the assumptions.
Can I get a printable report to keep or share?
Yes — and it's worth doing. On the results bar, click 🖨 Save / Print and choose “Save as PDF” in your browser's print dialog. SIREN builds a clean, dated, publication-style report, and every page opens with a plain-English line explaining what it shows — no solar expertise needed. Full Analysis generates the richer version: an executive summary and the four ways owning your power protects you (rate hikes, outages, fuel spikes, car upkeep); a “life with solar & electrification” before/after; how you use power day vs. evening; your car and your home compared gas-vs-electric (the 10-year ownership and appliance math); the 30-year money and a “what if you sell early?” resale page; solar vs. the market and cash-vs-finance at your own loan terms; resilience & seasonality with a battery right-sizing check; a “how solid are these numbers?” input-confidence read (with an honest “what this does not model” note); a year-by-year cash flow and a full assumptions audit; and a fill-in installer-quote comparison worksheet with a climate-based panel-temperature tip. Want a one-pager to forward? Flip the Full / Brief toggle on the results bar for a 2-page Executive Brief. Quick Estimate prints a concise leave-behind. Lease or PPA plans get a dedicated “should you own instead?” review.
Have a question that is not here? The best next step is to take SIREN's output to your installer, utility tariff, CPA, or qualified financial advisor and ask them to defend any number that surprises you. The purpose of the tool is to help you ask better questions, not to replace project-specific professional review.
