One way to think about solar is as a low-risk, long-term, bond-style investment. This calculator is designed to help you compare investing a lump sum against buying solar that offsets your home and fuels an EV on sunshine. Some other ways to think about solar savings are included at the bottom — click through to explore those too!
?Two ways to read that:
| Year | Savings that year | Solar profit (savings reinvested, net of cost) | Cash gain (invested) | Solar − cash |
|---|
Every row re-runs the whole model with one assumption made worse, and shows what the 10-year verdict becomes. Rows are ordered by how much they move the answer — the top row is what your result really hinges on.
Same numbers, a different frame. Each tab reframes your inputs — and a tab turns green when that way of looking at it favors solar.
Prepaid electricity. Treat the system as buying power up front. Its effective price is the total cost spread across every kWh it makes — a number that falls each year as production piles up, while the utility's rate climbs. Amortized over a 25-year panel life, the two lines cross the year solar becomes the cheaper electricity. Drag the vertical line to read the price per kWh in any year.
This tool treats rooftop solar as a long-term, bond-like investment and asks a single question: over your chosen horizon, does buying the system beat investing the same amount of cash? Both sides are measured the same way, so the comparison is apples-to-apples.
Production — the annual kWh figure everything else is built from — should come from a real installer's quote whenever you have one. If you don't yet, ticking "No quote yet? Estimate these" works both production and cost out from panel count, wattage and roof direction; the added-panels section uses the same model for a second roof surface — see how those estimates are derived.
Consumption comes from your bill. If you expect to electrify — a heat pump, a second car, a spa — the future-use panel adds those loads so the system is sized against where you're heading rather than where you've been (more on that here).
The cash side grows the system's price at your expected return, compounded, with tax applied to the gains. The solar side is the yearly bill savings — the electricity you no longer buy, plus the gasoline a solar-charged EV avoids — reinvested each year at that same after-tax return. Because avoided costs aren't taxed but investment gains are, the cash side clears a slightly lower after-tax bar, and the tool accounts for this.
Capture is the heart of the estimate. Solar only earns full value for the energy you actually use or store; the rest is exported — at retail, at a low net-billing rate, or nothing, depending on your utility. The capture percentage is built from your battery size (with a ~10% round-trip storage loss), roof orientation, and the timing of your usage, including whether an EV charges during peak sun. Your net-metering regime doesn't change how much you capture — it sets what the uncaptured remainder is worth. Production you can't use or store is valued at your export rate, and any remaining household demand is still bought from the grid.
Over time, electricity savings grow with your utility escalation rate (which already includes general inflation), gasoline savings grow with a separate fuel-price outlook, and panel output fades with a small annual degradation. All figures are shown in nominal (future) dollars.
The headline measures are payback (the year cumulative savings cover the system cost), the equivalent return (the steady annual return the solar earns on its price, to compare against your cash assumption), and the cost of doing nothing (the escalating utility and fuel bills you'd keep paying without solar). Below those, the stress test re-runs everything with each assumption pushed against you, so you can see which one your answer actually depends on.
These are planning estimates, not a guarantee or financial advice. Rates, incentives, taxes, degradation, and interconnection rules vary — verify the specifics of any quote before relying on these numbers.
When your panels make more than you're using at that moment, the extra flows to the grid — and what your utility pays for it varies enormously by program and region. Rather than name a specific tariff, this calculator asks which of three mechanisms applies and lets you set the rate. Check your own utility's tariff for the exact figure. Full detail & sources →
At (or near) my retail rate. Every exported kWh is credited at (or close to) your full retail rate — your meter effectively runs backward, so a kWh sent at noon offsets a kWh drawn at night. Because of that, capture matters far less here than under the other two: exported and self-consumed energy are worth roughly the same. Two caveats the tool models. First, retail credit is capped at a year of your own consumption; production beyond that is bought out at a low annual true-up rate, so heavily oversizing stops paying. Second, on a time-of-use plan timing still moves the dollars — a battery that shifts energy into the expensive evening window raises the rate your solar is credited at, so it isn't purely a resilience purchase. This is traditional net metering (in California, legacy NEM 1 and NEM 2; still the norm in much of the US).
At a lower set export rate. Exports are credited at a rate below retail — an avoided-cost or value-of-solar figure — while the power you draw still costs full retail. That gap makes using or storing your solar far more valuable than exporting it, so batteries, orientation, and daytime use matter a lot. This is where California's NEM 3 (net billing) and many newer programs sit. Enter your utility's published export rate, or use the $0.06/kWh default.
Little or nothing. A non-export system never pushes power to the grid; anything it can't use or store is curtailed (thrown away). Used where interconnection is limited, or to protect a legacy tariff on a separate system. Here, capturing your own production is everything.
The system's yearly savings come from two very different sources, valued on different terms.
Household savings are the grid electricity you no longer buy. Each kWh of solar you use at home is worth the rate you'd otherwise have paid — the flat "Grid rate" you entered, or, if you've switched on time-of-use, the blended rate the tool works out from your peak and off-peak figures (how that blend is built). So household savings = the kWh you self-consume at home × that rate.
Car savings come from replacing gasoline, not electricity. If a solar-charged EV takes the place of a gas car, each solar kWh that charges it displaces gasoline. I size the EV's yearly need as miles ÷ MPGe × 33.7 kWh — 33.7 kWh is the energy in one gallon of gasoline (the EPA's gasoline-equivalent figure) — and value it at the gas it avoids: (miles ÷ MPG) gallons × your price per gallon. Spread across those kWh, a solar-charged mile is typically worth ~$0.50–0.70 per kWh — well above a ~$0.30–0.40 retail electricity rate. There are other ways of thinking about this. For example, you may have already valued the price of charging vs. gas as a savings on the car itself — but there are compelling reasons to include it here, in a solar savings calculation. Besides being clear, it's also a natural combination: increasingly, electric cars work in concert with home systems to soak up excess solar production, and in the not-too-distant future they may even be able to return power to the house as needed.
Which gets served first, and what happens if there isn't enough. Because a solar kWh is worth more in the car than in the house, the model assigns captured solar to the car first, then sends what's left to the household. If your system doesn't cover the car's full annual need, the gas credit is prorated — cover 70% of the charging and you get 70% of the fuel savings, with the rest of the miles still driven on gasoline you pay for. Any household demand left after that is simply bought from the grid as before.
In short: household savings are valued at avoided grid electricity, car savings at avoided gasoline.
The grid rate is the value of each kWh your solar lets you avoid buying. On a flat plan that's one number off your bill. On a time-of-use (TOU) plan the price changes through the day — usually an expensive "peak" in the late afternoon and evening (often 4–9pm) and a cheaper "off-peak" the rest of the time — and no single number captures that. So the tool takes both rates straight off your bill and works out the blend for you; you don't have to guess it.
Why it needs a blend. Your panels produce during the day, but the pricey peak usually hits after the sun is low. So the value of your solar depends on how much of it lands in that peak window — and two things you've already entered decide that.
A battery stores midday solar and releases it in the evening, right into the peak, so battery-shifted energy earns the peak rate. A west-facing roof produces later in the day, so more of it directly overlaps the peak than a south- or east-facing roof. Everything else is used as it's produced, at the off-peak rate.
The tool adds those up into a peak share — the portion of the solar you use or store that offsets peak-priced power — and blends your two rates by it: effective rate = peak share × peak rate + the remainder × off-peak rate. It shows you the resulting rate and the split, so nothing is hidden. Extra panels on a second roof are folded in the same way, weighted by how much they produce.
Two separate things roof direction does — worth keeping apart. This page is about when your production lands relative to peak pricing. Separately, direction also changes how much energy the array makes in a year, which is modelled from real NREL PVWatts data for your market. Those effects pull in opposite directions for a west-facing roof: it generates meaningfully less total electricity than a south-facing one, but a larger share of what it does generate falls inside the evening peak. Neither effect alone tells you whether west is the better choice — under a flat rate the production loss dominates, while under a steep time-of-use spread the timing gain can outweigh it. The calculator applies both.
Note that the battery figures depend on how big your array is relative to your storage, not just on direction: a large array fills a small battery early on a summer day and spills the rest, so the same battery lifts a small system's peak share further than a large one's. That's why this chart is computed from your own inputs rather than shown as fixed typical values.
This is a planning estimate, not an hour-by-hour simulation: the overlap figures are typical values and partial-peak periods are folded into the two buckets. It gets the direction and rough size right. For an exact figure, your installer's production model or a full year of interval data is the next step.
On the escalation rate. I set a likely-conservative baseline escalation of 5%. The last 10 years have run an annualized increase of about 6.75% by my calculation. Starting in 2027, PG&E is officially requesting steady year-over-year revenue increases. According to analyses of the utility's filing, the projected residential rates per kWh and annual jumps are:
Two places in this tool fill in figures without a real quote: the "No quote yet? Estimate these" checkbox in the System card, and the added-panels surface further down. Both use the same model, described here.
Where the numbers come from. I ran the real NREL PVWatts API — the same physics-based model installers' own software is built on — for 14 major solar markets spanning the country, chosen by real market size and geographic spread rather than picked to represent a "typical" climate. Each market is one specific coordinate, run individually at eight compass directions, a fixed 20° tilt (typical residential roof pitch), standard-efficiency modules, and PVWatts' default 14% system-loss assumption. Those 112 runs were saved as a static table baked into this calculator, rather than called live — so the tool stays a single file that works without an internet connection to a weather-data API, and doesn't depend on an API key that could be rate-limited by other visitors. For any azimuth between the eight reference points, the tool interpolates along a curve fitted to the real PVWatts output — not a plain cosine, since a plain cosine over-penalizes east/west orientations and misses the diffuse skylight those roofs still pick up.
Two different things azimuth affects — worth not mixing up. This model determines how much total energy a given orientation produces over a year. That's separate from the capture/time-of-use engine described in the electricity-rate guide above, which uses azimuth to determine when your already-produced energy lands relative to peak pricing. A west-facing roof produces less total energy than a south-facing one (this guide's effect) but what it does produce lands later in the day, closer to the evening peak (that guide's effect) — both are real and both matter, for different reasons.
What this can't capture. Real PVWatts runs for an exact address account for that site's actual weather history; this table only has 14 points, so anywhere else is an approximation to whichever one is geographically and climatically closest. Every entry is a single coordinate's real result — none are averaged, blended, or used to represent a broader region — but a metro area's own microclimates (coastal fog belts, inland valleys, elevation changes) can still differ from its anchor point. The table also assumes one fixed tilt everywhere, doesn't know about shading from trees or nearby structures, and doesn't model inverter clipping or a specific panel model's real-world derate.
Panel wattage defaults to 460W for the primary system (matching this tool's seeded example quote) and 430W for added panels (a common current panel spec) — both are just starting points. Check your panel's actual spec sheet or your installer's proposal for the real number; it's usually printed right on the quote.
Estimating cost. If you have no quote, the tool can price the system as array + battery. The array uses a dollar-per-watt figure — the unit installers themselves quote in — applied to panel count × wattage. The default of $3.00/W sits deliberately between two credible benchmarks that disagree: competitive marketplace quotes averaged roughly $2.60–2.75/W in early 2026, while Berkeley Lab's national median for cash purchases runs nearer $3.50/W. That spread isn't measurement error, it's real variation between shopping aggressively and taking the first offer. The battery is priced separately at $1,150/kWh installed, from marketplace data putting a 13.5 kWh install near $15,600 before incentives.
Treat the cost estimate as a budgeting placeholder, not a price. Roof complexity, electrical panel upgrades, trenching, permitting and installer margin move real quotes far more than system size does — two identical arrays on two houses can differ by thousands. Anyone within a per-watt band of the default is in normal territory; the only way to know your number is two or three real quotes. Note also that the federal residential credit (Section 25D) expired at the end of 2025, so what this shows is what you'd actually pay, with no credit netted out. If you expect a state or utility rebate, subtract it yourself and enter the net figure.
For a more accurate number: run PVWatts directly with your exact address, roof tilt, and shading — it's free and takes a couple of minutes — or get a real quote from an installer, whose software already accounts for all of this plus your specific equipment. Either way, you can paste the resulting number straight into "Annual production" above and leave this estimate mode off.
One way to read the shortfall. When your system produces less than your household needs, the gap shown in the energy summary is the electricity you still buy from the utility at retail — and it's worth thinking of as the size of the remaining opportunity. Every kWh of that gap is one you're paying full price for, that a larger system might have covered instead. The closer that number gets to zero, the more completely you've taken advantage of the solar opportunity available to you. It's a different question from whether the system pays for itself: a small system can have an excellent return and still leave most of the opportunity on the table.
Why this matters more than it used to. Sizing solar against last year's bill quietly assumes your usage stays flat — which is exactly wrong for most people considering solar, since electrification and solar tend to arrive together. Add a heat pump, a second EV, or a spa after the system is installed, and a array that once covered everything suddenly covers a fraction. The panel below the household field lets you add expected loads so the shortfall reflects where you're heading, not just where you've been.
Typical additions, for scale. Every one of these swings widely with climate, equipment and habits — they're starting points, not predictions:
Figures are annual added electricity when replacing a gas appliance, or total consumption for a new load. Sources and ranges are listed below.
Heat pump water heater — 1,200 kWh/yr. Real-world reports cluster around 900–1,500 kWh/yr; the Department of Energy's category average runs higher at about 2,195 kWh/yr, likely reflecting larger households. Applies when replacing gas — if you're replacing electric resistance, a heat pump unit cuts usage by roughly 60–70% instead of adding.
Heat pump space heating — 2,500 / 4,000 / 7,000 kWh/yr. Whole-house heat pump consumption typically runs 3,000–8,500 kWh/yr, with mild climates near 3,000–4,500 and cold upper-Midwest homes reaching 6,500–9,000. The presets are heating-only: if you already run central air, a heat pump handles cooling at comparable efficiency, so the added load is essentially the winter half. For a firm number, a Manual J load calculation beats any rule of thumb.
Induction range — 500 kWh/yr. Typical household cooking runs roughly 500–700 kWh/yr on electric, and induction uses about 30–40% less than a standard electric coil. Small relative to everything else here.
Electric clothes dryer — 600 kWh/yr. A resistance dryer runs about 500–800 kWh/yr for five to eight loads a week. A heat pump dryer cuts that by 40–60%, to roughly 200–400 kWh/yr.
Pool pump — 2,500 kWh/yr and hot tub — 1,800 kWh/yr. A single-speed pool pump draws 1,500–2,500W running 8–12 hours a day in season; variable-speed models use 50–70% less, making this the widest range on the list. A spa consumes roughly 3–7.5 kWh/day, driven more by cover quality and outdoor temperature than tub size.
Always-on machines. Computed from the draw and runtime you enter, because this category has no typical value. For scale: a current-generation mining rig draws roughly 3,500W, which running continuously is about 30,000 kWh a year — roughly three times an average US home, and more than most rooftop systems generate.
One deliberate conservatism worth naming: for gas-replacement rows the model adds the new electricity but doesn't credit back the gas you stop buying, so your real net cost is better than shown. The EV is the exception — it's handled separately above, where avoided gasoline is counted, because that's the whole point of that calculation.
Every number above rests on assumptions, and some of them matter far more than others. Rather than ask you to take them on faith, this panel re-runs the entire model with one assumption pushed against you at a time, and reports what your headline verdict becomes. Rows are ordered by impact, so the top row is the assumption your result actually depends on.
These are adverse cases, not forecasts. Each row is a deliberate “what if this goes worse than I expected” — not a prediction, and not a median outcome. The final row stacks all of them at once, which is a genuinely pessimistic scenario and unlikely to happen in full. If the answer survives that row, the conclusion is robust.
The headline sentence is the more useful number. It solves for the lowest rate of utility increases at which solar still beats the cash alternative — so instead of trusting the escalation figure you entered, you can see how much slack there is. If it says solar wins even at 0%, the escalation assumption isn't doing the work; if the threshold sits close to what you entered, it is.
Rates vs. fixed fees. One row models the utility shifting revenue from per-kWh rates into a fixed monthly connection charge. This matters because solar can only offset the part of your bill you're billed per kWh — a fixed charge stays whether you produce or not. It isn't hypothetical: PG&E restructured this way in 2026, moving roughly $24/month into a Base Services Charge and lowering the per-kWh price. If that trend continues, bills keep rising while solar savings lag behind them.
Battery end of life. Batteries are the least-warrantied, most expensive component, so this is the one equipment risk modeled here. Inverters are deliberately excluded — warranties have lengthened (many microinverters now carry 25 years) and replacement costs have fallen, making them a much smaller risk than they used to be. When the battery reaches end of life, the model compares two options and takes whichever costs less over your remaining window: replace it, or run solar-only without it. Under full net metering it will usually choose to run without, since exports already earn retail; under net billing or zero export it will usually replace, since captured energy is where the value lives. Note the economic comparison doesn't price the loss of backup power, so “cheaper” isn't automatically “better.”
On the failure year and replacement cost. The default of year 12 is a deliberately harsh case, not an expectation. Tesla's Powerwall warranty guarantees at least 70% of original capacity at 10 years, real-world degradation typically runs 2–3% per year, and most units are expected to last 15–20 years before significant decline — so outright failure at 12 sits well into the unlucky tail. The replacement figure uses a 2026 installed price for an event a decade or more away, which cuts both ways: battery prices have fallen steadily in real terms, so this is likely conservative, but no federal residential tax credit applies to a replacement under current law, which pushes the other way. Both the year and the price are adjustable so you can test your own view.
These are the fixed values and modeling choices the form doesn't ask about. Where a figure comes from a published source it's linked; the rest are my own modeling estimates — reasonable engineering approximations, labeled as such. The same list, as plain paragraphs, is on the methodology page. Verify anything material against a real quote and your own bill.