sunsolarcalculator.com

How this calculator works

My reasoning and methods, then every fixed number the calculator uses and where it comes from.

Open the calculator →Read this alongside the tool, or come back to it when a result surprises you.

The core comparison

The calculator treats rooftop solar as a long-term use of cash and asks one question: over your horizon, does buying the system beat investing the same money? It's closer to prepaying a bill than to buying a bond: bond interest is usually taxed, avoided bills aren't.

The cash side grows the system's price at your expected return, with tax on the gains. The solar side is the electricity you no longer buy, reinvested each year at the same after-tax return. What an EV saves on gasoline is shown separately.

The headline measures:

Electricity savings grow with your utility escalation rate (which includes inflation), gasoline with a separate fuel outlook. Panels lose a little output each year, a battery a little more. All dollars are future (nominal) dollars.

What this is not

This is capital budgeting: money out now against costs avoided later. It isn't depreciation, book income or simple ROI, which ignores when savings arrive. The system's resale value is left out of the return and looked at separately under Home equity. It models an owner-occupied cash purchase only: no financing, no rental or business tax treatment.

Capture: the heart of the estimate

Solar earns full value only for the energy you use or store. The rest is exported, at retail, at a low net-billing rate, or for nothing. Everything comes from one yearly energy ledger that counts each kWh once:

The rooftop calculator fills the ledger hour by hour. It takes five homes with your usage pattern, simulated by NLR's ResStock from real housing in your area, scales them to your yearly use, and runs them through 180 days of real weather, 15 a month, with each hour's production from NLR PVWatts for your roof direction. Each hour the panels serve the house first, the surplus fills the battery, the battery covers the evening and night until it's empty, and what's left is exported. So a small array under a large always-on load is used almost whole, and a large array over a small daytime load exports most of itself.

Battery losses, and energy thrown away because it can't be exported, are worth $0. What the house still needs is bought from the grid, and that figure, not "usage minus production", is the electricity you're still buying.

The balcony calculator runs on the same hourly ledger, with production for steep panels of its own. Its differences are listed at the end of the fixed values.

Your own meter data

With a Green Button file the calculator runs on your home's own year instead: every real day, hour by hour, paired with the production of the matching day in your area's typical weather, with the battery's charge carried from day to day. If the file has readings every 15 minutes (or any part of an hour), solar use is counted reading by reading: hourly averages hide the minutes a home draws less than the panels make, and overstated a kit's use by 13% on one real year. Months the file doesn't cover come from the typical homes. If you already have solar, what those panels supplied directly is estimated hour by hour and held to what they made less what was sent back. On 50 real homes' full years, savings landed within half a percent of an exact calculation, with or without a battery, and within about 1% with an existing array. The file is read in your browser and goes nowhere.

If you already have solar

Adding panels or a battery is a decision about the next check, not the one already written. Under The solar you already own, savings become value(existing + new) − value(existing only), two runs of the same ledger, and the cost is the new invoice alone. There's no field for what the old system cost: it's sunk.

The usage field changes meaning here. A bill from a house with panels shows only what the utility delivered; solar used as it was made never crossed the meter. So the field asks for that delivered figure and works out the real use. If you also enter what you sent back, it's arithmetic: use = bought + produced − sent back. Without it, the calculator finds the household whose modelled purchases match your bill. Don't enter bought minus sent back: that's neither your use nor your bill.

Both runs share one household, so the new project's value depends on how the old system is credited. On net billing or zero export, the old panels already cover much of midday, so new panels earn only the use they add, often well under their output. That's when a battery or a daytime load like an EV charging at noon changes the answer.

Two things the calculator also states on screen:

How exported solar is credited

Your location sets your utility's usual rule for new systems, which you can change. For PG&E and SCE customers on net billing, the credit is the utility's published price for each hour (see below).

The $0.06/kWh export-credit default

For net billing the calculator fills in $0.06/kWh, which you can change. It's a production-weighted yearly average: each hour's export value weighted by how much of a typical system's exports land in that hour. Without a battery most exports land at low-value middays, which is why serious estimates fall in the $0.05–0.08/kWh band.

Sources: PG&E's Energy Export Credit value sheets (hourly data at pge.com/eecvalues), SDG&E's export-pricing files, both from the CPUC's Avoided Cost Calculator. Elsewhere, look for your state's value-of-solar study or your utility's avoided-cost schedule (DSIRE lists state policies). Background: Berkeley Lab's A Review of Value of Solar Studies and One Year In: Tracking the Impacts of NEM 3.0.

Household savings vs. car savings

Solar's yearly saving reads straight off the ledger: solar used as produced × its rate, plus what the battery gives back × its rate, plus exports × their credit.

The car is valued separately, never at the price of gas. Its yearly need is miles ÷ MPGe × 33.7 kWh (the EPA's energy in a gallon of gasoline). Solar that charges it is priced like any other solar: at the electricity it replaces. Whether an electric car beats a gas car is a different question, so the gas saving, (miles ÷ MPG) gallons × your gas price less the cost of charging, is its own line.

A car's kWh are added to your household usage unless you check "This car already charges at home", which tells the calculator they're already in your bill's number. Either way they're timed on their own: from the panels while the sun is up by default (the rest in the cheapest night hours), overnight, or as soon as it's plugged in.

Time-of-use rates

For PG&E, LADWP, APS, SRP and NV Energy the calculator starts on the plan new solar customers there actually get, priced for every month and hour: prices from the Utility Rate Database, hours from each utility's tariff. You can pick another published plan, set your own summer and winter prices, or use one peak and one off-peak price all year.

Each kWh is priced at its hour: solar used as it's made, battery output, and what you still buy. So a battery and a west-facing roof earn their value in the hours they actually move. Your bill without solar is priced from when the house uses power, so turning the roof can't change the cost of doing nothing. Exports are priced by the hour too: at retail under net metering, at PG&E's or SCE's published price under California net billing.

On the escalation rate

I set a likely-conservative 5% a year. The last 10 years ran about 6.75% a year by my calculation. Starting in 2027, PG&E is asking for steady yearly increases; analyses of the filing project about 8% in 2027 (to ~38.4¢/kWh) and about 6.1% a year through 2030 (to ~42¢/kWh). Enter what you can defend from your own bill history.

Under the hood: fixed values and modeling choices

Hour by hour — how much solar you use or store

Five homesFor each area and usage pattern (home during the day, mornings and evenings, out most days), five typical homes from NLR's ResStock simulation of that metro's housing. Each is scaled to your yearly kWh with its always-on level held: a bigger household has taller peaks, not a bigger fridge.
180 sample daysFifteen a month from the typical weather year, each standing for its share of the month. Hourly production from NLR PVWatts v8 for your metro and roof direction, scaled to your quote's yearly total or the estimate's.
Electric heatingThe heating share of your use moves to the heating hours of homes that heat the same way: winter mornings and nights. A heat pump carries 75% of a resistance-heated home's share. Without this, electrically heated homes in cold metros came out about 15% too high.
Always-on loadsSpread evenly over all 8,760 hours.
Car chargingFrom the panels while the sun is up, the rest in the cheapest night hours, never from the grid at peak; overnight in the cheapest hours; or as soon as it's plugged in, 6–10pm. A 7.2 kW home charger.
Future loadsEach follows its appliance's timing from NLR's electrification runs for your metro; ones on a timer can be given a run window.

Battery

90%Round-trip efficiency: energy stored and drawn later loses ~10%. NLR's Annual Technology Baseline puts lithium at ~85–90% (NLR ATB).
92%Availability: imperfect dispatch and capacity held back for backup. Not a depth-of-discharge derate, since headline capacities (a Powerwall's 13.5 kWh) are already usable figures. My estimate.
Charges from solar, covers the next hoursThe battery charges only from surplus solar, never the grid, and covers the next hours the panels can't until it's empty. Where a California export sheet applies it may also sell: each hour it ranks the uses of a stored kWh until the panels refill it (the house at retail, the grid at that hour's export price, up to 5 kW) and takes the most valuable first. It sells only once the panels stop exporting, and only when the export price beats the day's cheapest retail price. On 50 Marin homes over a year it earned 80–85% of what a perfect year-long schedule would add, within about $30–50 a year. Capacity a day never uses is backup, not return. My dispatch assumption.
2%/yr battery fadeUsable capacity falls about 2% a year; warranties promise 70% after ten years, the worst case. The battery's value is recalculated at the capacity left in years 10 and 20 and read between, so fade costs little when the battery seldom fills and more when it does. A replacement starts full. My estimate; adjustable next to panel degradation.
0.5%/yrPanel output lost per year. Jordan and Kurtz's compendium of field data from thousands of systems puts the median near 0.5%/yr, and NLR's PV Lifetime Project keeps measuring it. Adjustable.

Prices

Published plans, month by hourA price for every month and hour from the Utility Rate Database, hours from the utility's own tariff, weekends and part-peak folded in, fixed charges left out: PG&E E-ELEC and EV2-A, LADWP R-1(B), APS R-TOU-E, SRP E-13, NV Energy ORS-TOU.
Peak 4–9pmThe peak window for one peak/off-peak pair, or for your own summer and winter prices.
Marginal ratesA location's flat rate is the price of the kWh solar replaces (above baseline, or the top tier), not the bill's average.
California export pricesUnder net billing, PG&E and SCE credit an exported kWh at a published price for its month, hour and day type, fixed for nine years from the year you connect: under a cent at spring middays, around $1 on August evenings in the first years. The calculator uses each 2026 schedule's nine-year average with weekends folded in (PG&E's August-evening price averages 88¢), held flat in dollars rather than rising with power prices. In Marin, MCE sets the generation part of the credit for its customers; the calculator uses PG&E's full credit. Sources: PG&E, SCE.
Net metering by the hourEach exported kWh is credited at its hour's retail price, up to what you buy in a year; the rest at the true-up rate.

Not modelled

Export prices elsewhereOutside PG&E's and SCE's areas every exported kWh earns the one credit you enter, and the battery doesn't sell.
Backup, events, demandWhat backup power is worth to you, energy a battery program uses during its events, demand charges, and usage tiers below the marginal rate.
Shading, by your estimateProduction estimates include PVWatts' standard 14% losses (soiling, wiring, a little shade). More shade is the percentage you enter when building a system; the calculator doesn't work it out from your roof. A quote's production figure already includes your roof's real shade.
Good and bad weather yearsProduction is one typical weather year. The stress test's 'panels underperform' row gives a sense of the range.
Loans, leases and PPAsCash purchase only. Financed systems need their own numbers.
Your utility's fine printCaps on additions to a legacy net-metering system, export limits and interconnection rules vary by utility and contract and aren't checked. The notes on existing systems say where they matter.

The balcony calculator

Same homes, same daysThe rooftop calculator's hourly ledger: five real homes for the way you use power (or your own Green Button file), on the area's sample days. Nothing sent out earns anything.
20° to 90°, hour by hourProduction by the hour from NLR PVWatts v8 at 20, 30, 45, 60, 75 and 90° for the eight facings in every area, read between the nearest two. Each month is then set to the 896-run tilt table, so yearly totals come from the measured table.
Hours of direct sunTaken as the panel's best run of that many hours in each month's average day. Outside them it keeps a quarter of what it would make, from open sky. My estimate.
Inverter limit, hour by hourEach hour is held to the kit's limit. Output swings within an hour, so this slightly undercounts what an undersized inverter loses.
Always-on powerIf you enter it, it replaces each home's own quietest level and the rest of the home's use is scaled to your yearly kWh. It can't exceed half of that year.
Beside rooftop solarYou pick the plan: net metering, net billing or zero export. With your meter file, the kit runs against what the file shows you buying, so your panels' size isn't needed; under net metering what you already send back is netted first, and past a year's net purchases a kit kWh earns the $0.03 true-up rate. Without a file, your panels' size and facing rebuild your use from what you buy, as on the rooftop calculator. Net billing's credit starts at the area's rule for new systems, which older systems may not be on.
Hourly readingsThe typical homes are hour by hour, which smooths over the minutes when a home draws less than the panels make, so they flatter a kit a little. Your own Green Button file with 15-minute readings is counted reading by reading: on one real year that cut the kit's use by 13% against the same file taken hour by hour.

Exports & tariff mechanics

$0.06/kWhDefault export credit for net billing outside PG&E and SCE: a production-weighted yearly average. A planning midpoint, not your number: look up your utility's export-rate schedule and enter that instead. See the derivation and sources above.
$0.03/kWhYearly true-up rate under net metering, paid only on production beyond a year of your own use. Real buyout rates run about 2–4¢/kWh (some pay $0). It only affects oversized systems on retail net metering; a system sized at or below your use never reaches it.
Credit capped at annual useUnder net metering, retail-rate credit is limited to a year of your consumption, the standard true-up rule (EIA rates). Net billing applies its rate to all exports, uncapped.
Cheapest-to-export firstWhen a second array is credited differently from the main one, the house uses the array with the lower export value first. My dispatch assumption.

EV & gasoline

33.7 kWh/galEnergy in a gallon of gasoline (EPA gasoline-equivalent), used to size the EV's kWh need; see fueleconomy.gov.
Car credited by incrementSolar is credited only with the use the car adds over a house-only run, priced as electricity, never as gasoline avoided. Gas versus electric is a separate, car-side figure. Full reasoning in the calculator's savings guide. My allocation choice.
1% / 3% / 5%Gas-price outlook presets (Low / Base / High) per year. Base ≈ flat in real terms per EIA's long-run outlook (EIA AEO); the exact figures are mine.

Financial engine

Nominal dollarsNot inflation-adjusted; rate escalation already includes inflation.
After-tax returnBoth sides use your after-tax return, with solar savings reinvested at it. This drives the solar-vs-cash verdict and the year-by-year profit column only; payback is undiscounted and the equivalent return discounts raw savings, so neither moves with it.
Existing system is sunkWith solar you already own, savings are value(existing + new) − value(existing only) and the cost is the new invoice. The old system's price is never asked; its production and tariff are, because they change what the new system is worth. The cost of doing nothing is the bill with the old system still running.
Use worked out from a solar billWith existing solar, the usage field is kWh delivered by the utility. With exports entered, use = bought + produced − sent back (exact). Without them, use is solved so the modelled purchases match the bill, on the same ledger as the headline. A Green Button file replaces this with the meter's own hours. Future loads and a planned EV are added afterwards, since neither is on last year's bill.
No export pass-throughOn retail net metering, if the house still buys power, a new array earns close to retail whatever its own tariff, because each kWh it supplies frees an old kWh to export at retail. If a site export limit would curtail the new array instead, the note on existing systems works out that case and shows its payback and return beside the main ones. A separate cap control would give the same answer, because the old array's export earnings appear in both runs and cancel.
Expansion of a legacy tariffThe "California Gold" sample adds panels to an NEM 1 / NEM 2 array and credits them at retail, as the grandfathering allowance permits: generally up to 1 kW or 10% of the original system, whichever is greater. The calculator doesn't enforce that limit, because it varies by utility, and going over it typically moves the whole system to the current export tariff. Size an expansion against your utility's rule.
Seeded quote is itemisedThe opening figures are one real 2026 quote: $11,500 panels + $1,461 EV charger + $545 plans + $500 permits = $14,006 of solar and soft costs, plus $12,800 for a 13.5 kWh Powerwall 3. Removing the battery subtracts the quoted $12,800, and $/W without the battery comes from the $14,006. Type your own price over either and the calculator says the split is now a guess.
One shared battery packA retrofit battery charges from either array's surplus after the house is served. A pack tied to the new array alone isn't modelled, and the calculator says so on screen. My dispatch assumption.
$0 residual valueThe equivalent return counts the system as worth nothing after your analysis year: no resale value, no credit for production still to come. A 10-year window on a system that pays back in 13 reads negative by design. Remaining value is in the Home equity tab.
≥ 25-year scheduleThe model always runs at least 25 years (a typical panel life); the headline compares at your horizon.
Federal tax creditNot applied. Enter your net cost for an incentive you expect, or the gross cost for the unsubsidized picture.

Home-equity lens: would a sale make you whole?

This premium lives here only. Don't add it to the equivalent return, which already sets resale to $0; that would double-count it, especially with a longer analysis window.

Resale value = ~60% of cost, fading to zeroWhat owned solar has actually added to home prices: paired-sales studies put the premium for a new owned system near 60% of installed cost (Zillow's +4.1% of home value works out to about that for a typical system), falling straight-line to ~$0 by year 25. This is what's realizable at closing, not the income value.
Green when bill savings + resale ≥ costThe lens turns green when the bill savings kept by the sale year (a plain running total, not reinvested, so "whole" means nominal dollars back) plus that resale value cover what you paid. The slider explores selling sooner or later, starting at your horizon. This differs from the headline verdict, which is solar's margin over investing the same cash.
Appraisal income value (context)For reference: the present value of the remaining production, the basis of the Appraisal-Institute-endorsed PV Value® method, valued at your captured rate (self-use at retail, surplus at the export rate), discounted at an appraisal-grade 7%, degraded, capped at year 25. It's always higher than the market pays, because buyers and appraisers don't fully count future savings.

Defaults

Pre-filled figuresThe rooftop calculator opens on the first of four worked samples, each on its area's published plan (PG&E E-ELEC for the Marin examples). Behind them the form is seeded from a real Tesla quote (6,423 kWh, $26,806, 10 panels, 4.6 kW, 13.5 kWh battery). Replace them with your own quote and bill.
Location presetsPicking a location fills in its rate, export rule and rate plan from the card under the field, each with its source and the date it was checked; red marks anything unknown or over a year old.

Location & the panel-spec production estimate

Without a quote, the calculator estimates first-year production from panel count, wattage and location. The 14 locations are real coordinates, one per major US solar market. Each was run through the NLR PVWatts v8 API with standard modules and PVWatts' default 14% system loss, at eight tilts and eight directions: 896 runs, saved as a table. The roof model reads the 20° tilt, a typical pitch. Between the eight directions it follows a smooth curve through the measured points; a cosine over-penalizes east and west roofs, which still collect diffuse skylight. Each direction has its own monthly shares, so a Seattle winter isn't treated like a Miami one.

This sets how much a roof produces in a year; the hourly ledger sets when. A west roof makes less than a south one but more of it near the evening peak. Anywhere else is approximated by the nearest of the 14 points, and local microclimates can differ. For a real number, run PVWatts for your address or get a quote and choose "I have a quote".

$3.00/W, $948/kWhCost-estimate defaults when no quote is entered. Per-watt sits between competitive marketplace averages (~$2.60–2.75/W, early 2026) and Berkeley Lab's national median for cash purchases (~$3.50/W); the battery figure is the seeded quote's own, $12,800 for a 13.5 kWh Powerwall 3, so switching the battery off subtracts a real line item. Budgeting placeholders, not prices — roof complexity, panel upgrades, trenching and permitting move real quotes more than system size does. The federal residential credit (Section 25D) expired at the end of 2025, so the estimate shows what you'd pay with no credit netted out.
460W / 430WDefault panel wattage for the primary system / added panels. Starting points — use your panel's actual spec sheet.

Planning for future electricity use

Sizing solar to last year's bill assumes your use stays flat, which is usually wrong: electrification and solar tend to arrive together. You can add expected loads so the shortfall reflects where you're heading. The presets are mid-range starting points that vary widely with climate, equipment and habits: heat-pump water heater ~1,200 kWh/yr replacing gas (real-world reports cluster 900–1,500; DOE's category average ~2,195); heat-pump space heating 2,500 / 4,000 / 7,000 kWh/yr for mild / moderate / cold climates (whole-house figures run 3,000–8,500); induction range ~500; electric dryer ~600; pool pump ~2,500 (single-speed; variable-speed 50–70% less); spa ~1,800. Always-on machines use the draw and hours you enter. For gas replacements the calculator adds the new electricity but doesn't credit the gas you stop buying, so your real cost is better than shown.

The stress test

The stress test re-runs the whole model with one assumption made worse at a time, ordered by impact, and shows the new verdict. These are deliberate bad cases, not forecasts; the last row stacks them all, which is meant to be unlikely. If the answer survives that, I trust it. The headline gives the lowest rate escalation at which solar still wins, so you can see how much slack you have.

Rates → fixed fees: solar only offsets the per-kWh part of a bill, so a utility moving charges into a fixed monthly fee erodes savings even as bills rise. PG&E did this in 2026 (~$24/month Base Services Charge). Battery end of life: the one equipment failure modelled (inverter warranties now run ~25 years). The calculator takes whichever is cheaper over the remaining years: replace the battery or run without it. The default failure year (12) and the 2026 replacement price are harsh on purpose and adjustable. It doesn't price losing backup power, so cheaper isn't automatically better.

The "Our future" lens: carbon & social value

Carbon payback compares the CO₂ to make the system with the grid emissions it offsets. Manufacturing is estimated at ~1,600 kg CO₂e per kW for a typical mostly-China-made system (module, inverter, racking; ≈45 gCO₂/kWh over its life, consistent with NLR's life-cycle harmonization and IEA PVPS Task 12), or ~1,000 kg/kW if made on a low-carbon grid; both editable. Each year's offset is your production (degraded) × your grid's carbon intensity, from EPA eGRID2023 for each market's subregion (average, not marginal), editable. The grid gets cleaner each year, so later years offset less: 2.5% a year by default, close to the US average's recent fall (0.855 lb CO₂/kWh in 2021 to 0.81 in 2023, EIA), and editable. On a very clean grid the system may never pay back its carbon, and the lens shows that. The equivalents use EPA figures: ~2,500 miles in an average gas car per tonne, and 8.887 kg CO₂ per gallon of gasoline (~113 gallons per tonne).

Social value puts a dollar figure on the climate part only: lifetime tonnes avoided × the U.S. federal social cost of carbon, set on a slider (default ~$190; from near zero, through the 2021 federal ~$51 and EPA's 2023 ~$190, to $300+ in lower-discount-rate academic work). It's value to society, not money you receive, and it leaves out health damages from coal and gas, which are real.

The lens turns green when carbon payback falls inside your horizon, which it nearly always does; the lifetime figure is the point. The Resilience lens turns green when your system includes a battery.

These are planning estimates, not a guarantee or financial advice. Rates, incentives, taxes, degradation and interconnection rules vary; check the details of any quote before relying on these numbers.