I Fed the Same Solar Quote Into Six Calculators. Here's What Came Back.
I built Sun Solar Calculator because I went looking for a free tool that would let me actually see the math behind a solar quote, and I couldn't find one I trusted. So I built one, with Claude at my side to level up my very basic coding skills. Along the way I got curious about the tools that already existed. This post is that research: the same handful of inputs, run through six different calculators, compared against a reference model built from public data (EIA, NREL, PVWatts).
None of these tools are trying to mislead anyone, but it's worth noting there's a real incentive to simplify — or maybe a real dis-incentive to include the complexity. Solar math is genuinely hard to compress into a form with three fields, and every calculator has to pick shortcuts somewhere. The point of this comparison isn't "which one is best" — it's showing where each one simplified, so you know what to double-check no matter which calculator you use, including mine.
The transparency problem
What assumptions and arithmetic lie hidden behind the form? How much electricity does your roof actually produce per kilowatt of panels? What happens to the power you generate but don't use in the moment — is it credited at full retail price, a wholesale rate, or something in between? Is there still a 30% federal tax credit in the model (there shouldn't be, for systems installed after 2025 — more on that below)?
Those aren't small details, but they're hard to build into a simple tool, and once you do try to add them, there's a lot more to get right downstream. In the comparison below, the same roughly-12-kilowatt system, evaluated by different tools, produced verdicts ranging from "264% ROI, obvious yes" to "you'd come out ahead just investing the cash instead" — dramatically different answers, because each tool is quietly making different assumptions about the parts it doesn't ask you about.
The data was there for the digging
Here's the useful part: none of this is hidden particularly well. Every one of these calculators will tell you a system size and a production number if you give it a bill and a location. From there you can back into what it's assuming — divide production by system size and you get its assumed solar yield (kWh per kW); divide savings by production and you get its assumed electricity rate; compare its stated cost to public installer benchmarks and you find out whether it's using something realistic. Run the same inputs through several tools, watch what changes and what stays suspiciously fixed, and the hidden assumptions surface pretty quickly.
I ran a small test matrix — the same set of monthly bills ($100–$600), the same ZIP code, incentives toggled on and off — across six free tools, and matched every result against a first-principles reference model with every assumption exposed and sourced. Here's what came out.
One important, dateable fact first
The 30% federal residential solar tax credit (Section 25D) was repealed for systems placed in service after December 31, 2025, under the One Big Beautiful Bill Act (July 2025). Two of the six tools tested still apply it automatically as of this writing, and a third contradicts itself — one part of the page says it doesn't apply, another part deducts it anyway. That single assumption swings a quoted price by close to a third, so it's worth checking on any tool before you trust its "net cost" number.
The six tools, side by side
| Calculator | What you enter | What it's genuinely good at | What to watch for |
|---|---|---|---|
| solarpanelcostestimator.com | Monthly bill only | Fast, no address or contact info required | Almost every number is a fixed, invisible constant — one flat yield for the whole country, a flat 30% credit applied whether or not you ask for it, and a 6 kW minimum system size that makes a $90 bill and a $200 bill come back identical. Payback (9 years) and ROI (264%) never change no matter what you enter. |
| solarreviews.com | ZIP code + monthly bill | Real per-ZIP climate and usage modeling — noticeably better grounded than a bill-only tool | Still assumes the expired 30% credit, and the number you actually want — installed cost — is hidden behind a "compare local prices" lead form. Effectively no accounting for exported surplus power; the model treats close to all production as full-retail-value. |
| energyscout.org | ZIP code + monthly bill, with editable rate, coverage %, $/W, and battery $/kWh sliders | Genuinely transparent, formula-driven sizing math that checks out exactly against what it shows you. Also the only tool besides mine that compares solar to a market alternative — it runs cash vs. loan vs. lease vs. investing the same money in the S&P 500 (not my favorite thing to compare to, but still interesting). | Contradicts itself on incentives: one panel correctly says a cash buyer doesn't get the 30% credit, a separate panel on the same page, same inputs, deducts it anyway — two different net-cost answers. Also recommended a Sacramento-only battery incentive for a Marin County address, and has a hardcoded minimum system size that throws off some smaller system calculations. |
| cleanenergycalc.com | System size, state, annual usage (+ optional shading, direction, export rate, escalation) | The most transparent of the group on its own math — publishes its formulas and cites EIA and NREL sources on the page. Correctly treats the federal credit as gone for 2026. Real state-level electricity rates rather than one national number. | Self-consumption defaults to a flat 40% of production regardless of whether your system is sized to your usage, so it won't itself flag an oversized array. Sun-hours are state-wide, so it can't tell a foggy coastline from a sunny inland valley — a limitation mine shares. |
| nem3calculator.com | Real street address (via Google's Solar API) + system and battery size | By far the most physically accurate energy modeling of the group — actual rooftop geometry, and genuine hour-by-hour utility rate and battery-arbitrage modeling against 576 real PG&E rate periods. | Installed cost is a flat per-watt number with no location adjustment, despite being address-based. No rate escalation or panel degradation modeled at all, so a payback number is really just "if today's conditions held forever." PG&E service territory only. |
| Sun Solar Calculator (mine) | Annual usage + a real quote (or a build-your-own system) | Models adding a second system under a different export credit, models a battery and export credit under net billing (NEM 3.0), and — the thing none of the others do except energyscout.org — compares the whole investment to simply investing the same money at a market return. | It's not a one-field tool: you bring your own usage, rate, and quote rather than typing one number and getting an instant answer. Its built-in location presets lean toward California, because that's what I know best. |
The same system, different verdicts
To make the differences concrete: take a roughly 12-kilowatt system, sized against a $300/month bill, and feed it into the tools that will accept a fixed system and bill.
| solarpanelcostestimator.com | solarreviews.com | cleanenergycalc.com | Sun Solar Calculator | |
|---|---|---|---|---|
| System size | 12.0 kW | 7.2 kW | 12.4 kW | 12.4 kW |
| Annual production | 16,800 kWh | 11,184 kWh | 22,612 kWh | 19,416 kWh |
| Assumed house usage | not asked | 11,469 kWh | 7,000 kWh | 7,000 kWh |
| Production vs. usage | — | 0.98× | 3.2× | 2.8× |
| Payback | 9.0 yrs (fixed) | 4.0 yrs | 9.9 yrs | 7.9 yrs |
| 25-year verdict | 264% ROI | $95,903 net savings | $76,956 net savings | cash investing wins by $6,881 |
The gap between the top row and the bottom row is almost entirely about one question none of the bill-only tools ask: how much of what this system produces will you actually use, versus sell back at a much lower export rate? Under net billing (NEM 3.0), a system sized well above your own household usage is generating a lot of power that's worth six cents a kilowatt-hour, not thirty-nine cents. Tools that assume every kilowatt-hour offsets your bill at full retail price will always look better than tools that don't — right up until you get your first bill.
Two tools don't fit into that table at all, because they size their own system rather than accepting a fixed 12 kW input: nem3calculator.com (which needs a real street address) and energyscout.org (which sizes to a coverage percentage of your usage). energyscout.org is worth a specific mention here. Fed the same $300/month bill, it sized a smaller, better-matched 9.75 kW system, priced it at $27,310, and ran its own 25-year comparison against the S&P 500 — and in that comparison, investing the money wins too: $130,706 versus $75,315 for the solar system, even on a right-sized array. Comparing to the S&P seems like the wrong comparison to me, from a risk perspective, but I appreciate the thinking. It's also the tool with the self-contradiction on the tax credit noted above — the same page shows $27,310 net in one place and $19,117 net in another for identical inputs.
What to check, whichever calculator you use
Regardless of which tool you land on, it's worth pulling out these five numbers before you trust the result:
- Solar yield (kWh per kW per year). A single national number is a red flag — this should vary by climate, roughly 1,150 kWh/kW in cloudy regions up to 1,700+ in the desert Southwest.
- Installed cost per watt. Real cash-purchase pricing runs roughly $2.50–$3.50/W before any credit; anything wildly outside that range, especially if it's not shown at all, deserves a second look.
- Whether a federal tax credit is applied — and whether it should be. As of systems installed after 2025-12-31, it shouldn't be, unless something changes again. Check for internal consistency too — some tools apply it in one place and not another.
- What happens to power you export. Full retail credit, a lower net-billing rate, or nothing — this one assumption moves the answer more than any other input.
- Whether the model compares to anything else. A payback period tells you when you break even. It doesn't tell you whether the money would have done better somewhere else in the meantime.
I've tried to make all five of those assumptions visible and editable in my own tool, with sources cited on the methodology page — and I'd say the same to anyone using any of the tools above: if a number isn't sourced, treat it as a placeholder, not an answer.