What does this rooftop solar payback calculator model?
A rooftop solar decision cannot be reduced to a module price.
Usable roof area, daytime building load, structural reinforcement, waterproofing, grid work, maintenance, and inverter replacement all affect the recovery period.
This calculator first converts roof area into a planning capacity, then links self-consumed generation and confirmed surplus-sale revenue to a year-by-year cash flow.
Results include first-year simple payback, cumulative nominal payback, discounted payback, net present value, and nominal ROI.
A low, base, and high generation comparison makes the estimate useful for challenging an optimistic quote instead of presenting one result as a guarantee.
Factory or warehouse owner
Connect a broad roof and daytime production load to the economics of on-site self-consumption.
Retail or office facility manager
Use actual annual consumption and avoidable energy charges to estimate electricity savings.
Multi-family building manager
Screen a common-area load and roof before commissioning a detailed engineering study.
EPC quote reviewer
Compare structural, grid, support, maintenance, and replacement scope on one basis.
How this differs from a small residential solar ROI estimate
MowaTool already has a residential solar-panel ROI page focused on small home systems and household electricity savings.
This page serves a separate Korea rooftop-building decision: derive capacity from roof area, value daytime self-consumption, align a complete project quote, and include long-term operating cash flow.
It deliberately refuses to award an automatic subsidy or assume that a published market price is the amount the user can receive.
Scope of the rooftop solar payback model and evidence required| Decision area | Calculator treatment | Preferred evidence |
|---|
| System size | Roof area, usable share, and square metres per kW | Stamped or surveyed roof layout and module sheet |
| Electricity value | Self-consumption and surplus are separated | Interval load, bills, and a sale agreement |
| Investment scope | EPC, structural, waterproofing, design, and grid costs | Cash quotes with aligned VAT and exclusions |
| Long-term cost | O&M escalation and one inverter replacement | Service terms and product warranties |
| Economics | Simple, nominal, discounted payback, NPV, and sensitivity | Internal horizon and discount-rate policy |
Korea-specific 2026 context
Jurisdiction note
This English page models a project in South Korea in KRW.
It is a planning tool, not a determination of a generation-business licence, electrical filing or inspection, grid availability, subsidy award, REC status, tax result, or contract entitlement.
The legal and official-source context below was checked on July 28, 2026, and must be rechecked when a project proceeds.
The Korea Energy Agency Green Home page describes a grid-connected solar home and says that a supported system of up to 3 kW needs about 23 m².
Dividing those figures gives about 7.6667 m²/kW, so the editable starting input is rounded to 7.7 m²/kW.
That is not a commercial-roof code rule: module efficiency, row spacing, tilt, fire and maintenance access, parapets, mechanical plant, shade, and structural design can change the layout materially.
The Korea Energy Agency RECloud platform displayed a 14.52% national average utilization rate for reporting-filtered RPS solar facilities for 2026 Q1.
A simple annualization is 14.52% × 8,760 hours = 1,271.952 kWh/kW-year, rounded to the editable 1,272 default.
A first-quarter national average is not an annual yield warranty for a site, so a long-term irradiance study, shading model, EPC production report, or twelve months of measured generation takes priority.
Official and research reference inputs and their limitations| Reference | Observed value | Starting input | Required replacement evidence |
|---|
| KEA residential solar area | 3 kW, about 23 m² | 7.7 m²/kW | Actual module and access layout |
| KEA RPS solar utilization | 14.52% | 1,272 kWh/kW-year | Site study or measured production |
| NREL degradation review | 0.5%/year median | 0.5%/year | Quoted module warranty and model |
| KPX monthly SMP | Variable during 2026 | Not auto-applied | Actual eligibility, contract, and settlement price |
Why support and sale prices remain manual
Article 27 of the Act on the Promotion of the Development, Use and Diffusion of New and Renewable Energy, current MST 268793 and effective February 1, 2026, provides a basis for diffusion and support projects.
It does not guarantee that a particular building is selected or establish the amount this user will receive.
The Korea Energy Agency also published 2026 recruitment material for residential, building, and balcony-style solar support, but program existence is not an award notice for an individual project.
For that reason, confirmed support defaults to KRW 0 and should change only after an approval or agreement defines the amount.
The same discipline applies to surplus electricity: Korea Power Exchange publishes monthly SMP statistics, but market statistics, REC value, intermediary terms, settlement deductions, and the cash price received by a specific facility are not interchangeable.
Keep the sale-price input at zero until a lawful contract or settlement statement supports another value.
Published SMP is a reference, not automatic project revenue
The KPX monthly page showed integrated SMP values of KRW 103.53, 108.52, 109.99, 118.92, 121.32, and 114.10 per kWh for January through June 2026.
EPSIS describes weighted-average SMP as statistical information for market-price trend analysis.
The calculator preserves those facts in its documentation but does not insert their average into a user contract.
How the calculation flows
1. Convert roof area to installable capacity
Usable roof area equals gross roof area multiplied by the usable share.
Installable capacity equals usable area divided by square metres per kW.
A 300 m² roof at 75% usability and 7.7 m²/kW gives 225 m² and about 29.2208 kW before detailed engineering.
2. Split generation between self-consumption and surplus
Year-one generation equals capacity multiplied by annual specific yield.
Self-consumed generation applies the entered share but can never exceed annual building electricity use.
The remainder is surplus; it produces revenue only when a confirmed price is entered.
3. Derive the value of self-consumed electricity
The effective avoided price equals the annual avoidable energy charge divided by annual electricity use.
Fixed basic or demand charges that remain after installation should not inflate this input.
A time-of-use or demand-managed site needs a separate interval analysis because this model uses one annual blended value.
4. Build annual cash flow
Production declines by the entered degradation rate, while avoided electricity value, sale price, and O&M follow their own escalation assumptions.
The inverter replacement cost is deducted once in the chosen year.
Nominal and discounted cumulative cash flow start at negative net investment and add each annual result.
Worked example with the default planning inputs
The default case uses a 300 m² roof, 75% usability, 7.7 m²/kW, and 1,272 kWh/kW-year.
Annual use is 120,000 kWh and the avoidable annual energy charge is KRW 18,000,000, producing an effective value of KRW 150/kWh.
Self-consumption is 70%, surplus sale price is zero, gross project cost is KRW 50,000,000, first-year O&M is KRW 600,000, and a KRW 6,000,000 inverter cost occurs in year 12.
The horizon is 25 years, degradation is 0.5%/year, O&M escalation is 2%/year, and the discount rate is 4%/year.
Default rooftop solar payback worked-example results| Output | Default result | Interpretation |
|---|
| Installable capacity | 29.2208 kW | A pre-layout planning capacity |
| Year-one generation | 37,168.8312 kWh | Replace with the site production study |
| Year-one self-consumption | 26,018.1818 kWh | 70% of generation and below annual use |
| Year-one surplus | 11,150.6494 kWh | Displayed but valued at zero without a contract |
| Year-one electricity savings | KRW 3,902,727 | Self-consumption multiplied by KRW 150/kWh |
| Year-one net benefit | KRW 3,302,727 | Savings less KRW 600,000 O&M |
| Simple payback | 15.1390 years | Net investment divided by year-one net benefit |
| Nominal cash-flow payback | 18.5558 years | Includes degradation, O&M, and inverter replacement |
| Discounted payback | Not recovered in 25 years | Discounted cumulative cash flow stays negative |
| 25-year NPV | -KRW 7,258,889 | Negative at the entered 4% discount rate |
Why the three payback measures diverge
Simple payback divides net investment by the first-year operating benefit, so it is fast but ignores later degradation, escalation, replacement, and the time value of money.
Nominal cash-flow payback follows each annual cash flow, while discounted payback reduces future cash flow to present value.
In this example, a 15.1390-year simple result becomes 18.5558 nominal years and no discounted recovery within 25 years.
Generation sensitivity
The default ±15% band is a user-selected planning range, not a statistical P50 or P90 distribution.
Low generation uses 85% of specific yield, base uses 100%, and high uses 115%, while price, cost, degradation, and discount assumptions remain unchanged.
Default low, base, and high generation sensitivity results| Scenario | Year-one generation | Year-one net benefit | Simple payback | Nominal payback | Discounted payback | 25-year NPV |
|---|
| Low 85% | 31,593.5065 kWh | KRW 2,717,318 | 18.4005 years | 23.5283 years | Not recovered | -KRW 15,962,808 |
| Base 100% | 37,168.8312 kWh | KRW 3,302,727 | 15.1390 years | 18.5558 years | Not recovered | -KRW 7,258,889 |
| High 115% | 42,744.1558 kWh | KRW 3,888,136 | 12.8596 years | 15.4053 years | 23.7361 years | KRW 1,445,031 |
- Check whether the low case still recovers inside the ownership or lease horizon
- Challenge a proposal when only the high case produces a positive NPV
- Do not raise generation and self-consumption optimistically at the same time without interval evidence
- Confirm whether curtailment, outage, soiling, snow, shade, and inverter losses are already in the quoted yield
- Use the same weather period and loss categories when comparing EPC production reports
Step-by-step use
- Define the roof boundary.
Use drawings and a site visit, then exclude plant, fire and maintenance access, setbacks, and shaded zones through the usable-share input.
- Replace the area and yield references.
Change 7.7 m²/kW and 1,272 kWh/kW-year to the actual module layout and site production report.
- Prepare twelve months of load and bills.
Enter use from the same meter and isolate the energy cost that on-site generation can reduce; refine self-consumption with interval load if available.
- Enter sale revenue only from evidence.
Use a contract or settlement value that matches the trading structure, deductions, and tax basis; otherwise retain zero.
- Align total investment scope.
Add EPC, structural work, waterproofing restoration, design, application, inspection, metering, and grid costs, then subtract only confirmed support.
- Stress the long-term result.
Match degradation and replacement to warranties, set an internal discount rate, and review the low case and discounted payback before the headline simple result.
Practical scenarios
Factory or cold-storage daytime load
Continuous daytime production, refrigeration, or HVAC can support a high self-consumption share.
The annual model still cannot capture demand-charge or time-of-use effects, so final approval should use 15-minute interval data.
Retail site with mismatched operating hours
High annual consumption does not guarantee high self-consumption if much of the load occurs at night or on different days.
Keep surplus value at zero without a contract and examine load shifting or storage separately.
Landlord investment and tenant savings
The financial benefit may accrue to a tenant even when the landlord pays for the system.
Roof rights, meter ownership, bill settlement, revenue sharing, lease term, insurance, and reinstatement belong in the contract before the calculated savings are treated as investor cash flow.
Common-area multi-family load
Verify that lifts, pumps, ventilation, and lighting sit behind the meter intended for solar connection.
Resident approvals, reserve planning, waterproofing responsibility, and defects remain separate from this project cash-flow estimate.
Quote-scope checklist
- Module and inverter model, DC and AC capacity, and DC-to-AC ratio
- Structural review, racking, anchoring or non-penetrating method, corrosion, and wind design
- Protection of existing waterproofing, penetration treatment, leak warranty, and reinstatement
- Switchgear reinforcement, cabling, protection, meter, interconnection, and grid charges
- Design, permits, filings, inspection, application service, and VAT inclusion
- Monitoring and communications, periodic inspection, cleaning, insurance, and safety management
- Weather dataset, production losses, outage, degradation, and curtailment assumptions
- Product warranty, workmanship warranty, response time, and financially responsible party
- Contract remedies for support rejection, grid delay, cost increase, or a failed structural review
Licensing, trading, and electrical-safety boundaries
The Korean Law Information Center OPEN API showed the Electric Utility Act as current MST 283981, effective March 10, 2026.
Article 7 addresses licensing for an electric utility business, and Article 31 distinguishes market trading by generation businesses, self-use electrical installations, priority purchase, and other specified participants.
A system size alone is not enough for this calculator to decide licensing or the lawful route to receive revenue.
The Electrical Safety Management Act appeared as current MST 268805, effective February 1, 2026.
Article 8 addresses approval or reporting of work plans for applicable self-use electrical installations, while Article 9 requires applicable work to pass a pre-use inspection before use.
Confirm the project-specific route with the competent authority, KEPCO, the relevant electrical-safety body, and qualified Korean designers and contractors.
Financial payback never replaces roof, fire, or electrical safety
The model does not test roof loading, wind, corrosion, lightning protection, shock and arc hazards, fire spread, firefighter access, waterproofing, or leak risk.
An older structure, sandwich-panel roof, existing leak, undocumented alteration, or mismatch between drawings and the building requires on-site structural, architectural, electrical, and fire-safety review before economics.
Frequently asked questions
How many kW can fit on a 100 m² roof?
At 75% usability and 7.7 m²/kW, the planning result is about 9.74 kW.
The actual result depends on module efficiency, row spacing, tilt, plant, access, setback, shade, and racking, so the detailed layout controls.
Can every Korean site use 1,272 kWh/kW-year?
No.
It is a simple annualization of the 14.52% national average utilization displayed for 2026 Q1, not a site guarantee or statistical P50 result.
Replace it with a site-specific report or measured generation.
Can I add SMP and REC and enter that as the sale price?
A public market price is not automatically the net price available to a project.
Licensing status, trading or intermediary route, fixed-price terms, REC issuance and weighting, fees, settlement, and tax all matter.
Enter a lawful contract or settlement value; otherwise keep zero.
Should I enter the total amount of my electricity bills?
Usually not.
Including fixed basic or demand charges that remain after installation can overstate self-consumption savings.
Isolate the avoidable energy-related portion and model time-of-use and demand impacts separately when material.
How much support should I enter?
Do not enter a value merely because a program exists or an application is planned.
Building use, ownership, program category, budget, approved participant, and selection outcome can change eligibility and amount.
Use only an approval or agreement that confirms the amount.
Does a negative NPV always mean that installation is wrong?
NPV is a financial result under the entered horizon, discount rate, generation, value, and cost scope.
Missing costs can make it worse, while verified support or contract value can change it.
Waterproofing improvement, carbon goals, or RE100 value are not included automatically, but they also should not be monetized without evidence.
What happens when inverter replacement is after the horizon?
No inverter cost appears inside that analysis horizon.
This does not mean replacement is unnecessary, so align the horizon and replacement year with product warranties, expected service life, and the intended holding period.
Sources and maintenance notes
- Korean Law Information Center OPEN API: Renewable Energy Act MST 268793 Article 27, Electric Utility Act MST 283981 Articles 7 and 31, and Electrical Safety Management Act MST 268805 Articles 8 and 9
- Korea Energy Agency Green Home: residential solar up to 3 kW and about 23 m² of installation area
- Korea Energy Agency RECloud: 14.52% national average solar utilization shown for 2026 Q1
- Korea Energy Agency New and Renewable Energy Center: 2026 residential, building, and balcony-style solar support notices and FAQs
- Korea Power Exchange KPX and EPSIS: monthly SMP data and the statistical purpose of weighted-average SMP
- NREL/JA-5200-51664: 0.5%/year median degradation in the reviewed module and system literature
Official context was checked on July 28, 2026.
Recheck effective law, program notices, market rules, agencies, and official data when a project advances.
Actual contracts, engineering, meter data, and quotes always take priority over the demonstration defaults.
Replace the defaults with your roof and your evidence
Use the defaults to understand the model, then enter a roof layout, twelve months of load and bills, aligned EPC quotes, and confirmed contract values.
A decision that remains credible in the low-generation and discounted-payback views is more useful than a headline return built from unsupported assumptions.