Calculator overview

Inputs and outputs

This summary comes from the calculator's published input and output contract.

Inputs

Panel Degradation: About this input

The yearly loss of panel output, as a percent per year, that reduces production over the system life. Around 0.5 percent per year is typical.

Unit %/yr Default 0.5 Range 0 to 100
Selfconsumption Fraction: About this input

The share of generation used on site rather than exported, as a percent from 0 to 100. The rest is exported and valued at the export rate.

Unit % Default 40 Range 0 to 100
Other Rebates: About this input

Any additional up-front rebates or incentives, in dollars, beyond the tax credit.

Unit $ Default 0
Net Metering Treatment: About this input

Selects how exported energy is valued: full retail net metering, a separate export rate, or self-consumption only. It sets which of the export inputs are used.

Default Full retail net metering Allowed Full retail net metering, Net billing (exports at a lower rate), Self-consumption only (no export credit)
Operations And Maintenance: About this input

The yearly operating and maintenance cost, in dollars per kilowatt of system size per year.

Unit $/kW/yr Default 15
Tax Credit Rate: About this input

The investment tax credit as a percent of eligible cost. It is illustrative; confirm the rate and eligibility that apply to your project.

Unit % Default 0 Range 0 to 100
Utility Electricity Rate: About this input

The retail price of grid electricity the system offsets, in dollars per kilowatt-hour.

Unit $/kWh Default 0.16
System Size: About this input

The system size, in kilowatts of direct-current panel capacity, which drives cost and production.

Unit kW DC Default 8
Share Of Net Cost Financed: About this input

The portion of the net cost covered by a loan, as a percent from 0 to 100, with the rest paid up front.

Unit % Default 0 Range 0 to 100
Specific Yield: About this input

The annual energy produced per unit of installed capacity, in kilowatt-hours per kilowatt per year, set by the site's solar resource. It is illustrative; take a site figure from an irradiance source.

Unit kWh/kW/yr Default 1400
Location Preset: About this input

Selects a location preset that fills the specific yield and related defaults. Override the individual values if you have site-specific data.

Default Custom (type the yield below) Allowed Custom (type the yield below), Desert southwest, illustrative, Southern sunbelt, illustrative, Mid-latitude temperate, illustrative, Northern temperate, illustrative, Coastal marine, illustrative, Northern Europe, illustrative, Mediterranean, illustrative, Tropical, illustrative
Export Rate: About this input

The price paid for energy exported to the grid, in dollars per kilowatt-hour, used when export is valued separately from retail.

Unit $/kWh Default 0.05
Financing: About this input

Selects whether the system is paid in cash or financed with a loan. It controls whether the loan inputs apply.

Default Cash purchase Allowed Cash purchase, Loan
Discount Rate: About this input

The annual discount rate used for the net present value and discounted payback, as a percent, reflecting the time value of money.

Unit % Default 5
Annual Rate Escalation: About this input

The yearly rise in the utility electricity rate, as a percent per year, applied over the analysis period.

Unit % Default 3 Range At least 0
Currency: About this input

Selects the currency used for all monetary inputs and results.

Default US dollar Allowed US dollar, Pound sterling, Euro, Canadian dollar, Australian dollar
Loan Interest Rate: About this input

The annual interest rate on the loan, as a percent, used to compute the loan payment.

Unit % Default 6.9
Loan Term: About this input

The length of the loan, in years, over which it is repaid.

Unit years Default 15
Inverter Replacement Year: About this input

The year in the system life when the inverter is replaced, used to place its replacement cost in the cash flow.

Unit year Default 15
Installed Cost Per Watt: About this input

The all-in installed system cost, in dollars per watt of direct-current capacity, before incentives.

Unit $/W Default 3
Inverter Replacement Cost: About this input

The cost to replace the inverter, in dollars, incurred in the replacement year.

Unit $ Default 2500

Outputs

Net Present Value: About this output

The present value of all savings and costs over the system life, in dollars, discounted at the discount rate. A positive value means the project earns more than the discount rate.

Unit $
Return On Investment: About this output

The lifetime net benefit as a percent of the net installed cost, an undiscounted measure of total return.

Unit %
Model Status: About this output

The overall check on your entries, shown above the results. It reads OK when the inputs are usable, NOT VALID with a reason when an entry makes the model meaningless, or CHECK with a reason when a result is valid but worth a second look. Read it before you trust the numbers below.

No unit declared
Net Installed Cost After Incentives: About this output

The system cost after the tax credit and other rebates, in dollars.

Unit $
Simple Payback: About this output

The number of years for cumulative undiscounted savings to repay the net cost, in years. It ignores the time value of money.

Unit years
Year 1 Bill Savings: About this output

The first-year saving on the electricity bill, in dollars, from self-consumed and exported energy at the applicable rates.

Unit $
Year 1 Production: About this output

The energy the system produces in its first year, in kilowatt-hours, from system size and specific yield.

Unit kWh
Tax Credit Value: About this output

The value of the tax credit, in dollars: the credit rate applied to the eligible cost.

Unit $
Upfront Cash Required: About this output

The cash needed at installation, in dollars, after incentives and any amount financed.

Unit $
Discounted Payback: About this output

The number of years for cumulative savings, discounted at the discount rate, to repay the net cost, in years.

Unit years
Gross Installed Cost: About this output

The total installed cost before incentives, in dollars: system size times installed cost per watt.

Unit $
Amount Financed: About this output

The portion of the net cost covered by the loan, in dollars.

Unit $
Annual Loan Payment: About this output

The level yearly loan repayment, in dollars, from the amount financed, interest rate and term.

Unit $
Internal Rate Of Return: About this output

The annual rate of return at which the project's net present value is zero, as a percent.

Unit %
Lifetime Net Benefit: About this output

The lifetime bill savings less all costs, in dollars, the undiscounted net gain over the system life.

Unit $
Lifetime Production: About this output

The total energy produced over the system life, in kilowatt-hours, after degradation.

Unit kWh
Levelised Cost Of Energy: About this output

The lifetime cost divided by lifetime production, in dollars per kilowatt-hour, the average cost of the energy the system produces. Compare it against the utility rate.

Unit $/kWh
Lifetime Bill Savings: About this output

The total bill savings over the system life, in dollars, after rate escalation and degradation.

Unit $

Methodology

Purpose and model boundary

This model projects a 25-year owned residential solar-PV cash flow and reports simple and discounted payback, net present value, internal rate of return, levelised cost of energy, lifetime production, savings, and net benefit. It compares cash and loan financing and three treatments of exported energy.

It is a scenario model, not tax, legal, investment, lending, tariff, engineering, or performance advice. It does not size electrical equipment or establish eligibility for any incentive.

Inputs and units

System size is entered in kW DC; installed cost in currency per watt; specific yield in kWh/kW/yr; degradation and escalation in percent per year; and operations/maintenance in currency per kW per year. A location preset supplies an illustrative yield unless the custom option is selected.

Energy-value inputs include retail utility rate, export rate, self-consumption share, and net-metering treatment. Financing inputs include financed share of net cost, annual interest rate, loan term, and discount rate. The selected currency changes labels only; the workbook performs no exchange-rate conversion.

Governing relationships

Gross cost, incentive, financing, and payment are

gross cost = system kW x 1000 x installed cost per W;
tax credit value = gross cost x entered credit rate;
net installed cost = gross cost - credit - other rebates;
amount financed = net installed cost x financed share;
up-front cash = net installed cost - amount financed;
for positive rate and term, annual payment = principal x r / [1 - (1 + r)^(-n)]; at a zero rate it is principal/n.

For analysis year t from 1 through 25:

production_t = system kW x specific yield x (1 - degradation)^(t-1).

Retail and export rates escalate as their year-1 rate times (1 + escalation)^(t-1). Bill savings are selected by net-metering treatment:

full retail net metering: production x retail rate;
net billing: production x self-consumption x retail rate + production x exported share x export rate;
self-consumption only: production x self-consumption x retail rate.

Annual O&M is O&M per kW x system kW x (1 + escalation)^(t-1). Inverter replacement cost appears only in the selected year. Loan payment appears through the loan term. Net cash flow is savings less O&M, inverter cost, and loan payment.

Simple cumulative cash flow begins at negative up-front cash and adds each annual net cash flow. Discounted cash flow multiplies year t by 1/(1 + discount rate)^t. Each payback is the first zero crossing with linear interpolation inside the crossing year. NPV is negative up-front cash plus the sum of discounted annual cash flows. IRR uses Excel's IRR over the year-zero outflow and 25 annual flows. Levelised cost is discounted lifetime cost divided by discounted lifetime production.

Calculation sequence

Resolve location, net-metering, financing, and currency lookups and convert percentages to fractions.
Calculate gross cost, entered incentives, net cost, financed amount, up-front cash, and annual loan payment.
Build 25 annual rows for degrading production, escalating rates/O&M, bill savings, replacement cost, loan payment, and net cash flow.
Carry forward simple and discounted cumulative cash flow and find the first zero crossings.
Calculate NPV, IRR, LCOE, lifetime totals, ROI, and year-1 values.
Evaluate Model_Status in the exact order below.

Outputs and interpretation

Simple payback ignores the time value of money; discounted payback includes the entered discount rate. When a crossing does not occur within 25 years, the public numeric payback output is blank and status explains the horizon. NPV is the discounted value after up-front cash. IRR is blank when Excel cannot define a root. ROI is lifetime net benefit divided by net installed cost.

The chart is the same cumulative cash-flow series used by the payback calculation. Bars below the zero reference are adverse and bars at or above zero show break-even/positive cumulative value.

Validation and status logic

The workbook returns the first matching status.

Condition	Returned status
System size is less than or equal to zero	`NOT VALID: system size must be greater than zero`
Resolved specific yield is less than or equal to zero	`NOT VALID: specific yield must be greater than zero`
Utility electricity rate is less than or equal to zero	`NOT VALID: utility rate must be greater than zero`
Amount financed is positive and loan term is less than or equal to zero	`NOT VALID: loan term must be greater than zero when an amount is financed`
Entered tax-credit rate is above zero	`CHECK: a tax credit above zero is assumed; confirm you qualify`
Simple or discounted payback exceeds the 25-year horizon	`CHECK: the system does not pay back within the 25-year analysis horizon`
Net present value is negative	`CHECK: net present value is negative at this discount rate`
None of the preceding conditions applies	`OK`

Assumptions and limitations

The model assumes deterministic annual production, one constant degradation rate, one escalation rate shared by utility/export rates and O&M, a single replacement event, annual rather than monthly cash flows, and no residual value. Location yields are illustrative and do not model tilt, azimuth, shading, snow, soiling, outages, clipping, weather variability, or equipment-specific losses.

Tariffs, export compensation, financing terms, tax eligibility, and incentive law can change. The workbook models an owned system and defaults the federal credit to zero based on its documented post-2025 Section 25D assumption; users must verify current law and project eligibility. The workbook does not model depreciation, tax basis, renewable certificates, demand charges, battery storage, lease/PPA economics, inflation distinct from rate escalation, or financing fees.

IRR can have no result or more than one mathematical root for nonconventional cash-flow patterns. The workbook reports Excel's IRR result when defined and independently checks its discounted balance in validation tests; it does not search for all possible roots.

Restrictions and non-computing states

The published limits constrain most physical and percentage inputs, and the workbook requires positive system size, yield, and utility rate. A financed project requires a positive loan term. The analysis horizon is fixed at 25 years; payback beyond that is not extrapolated. Inverter replacement year is bounded to the horizon. A zero or negative discount denominator or escalation at/below -100% is outside ordinary model use; annual escalation has a published minimum of -100%.

Errors and warnings

NOT VALID means a core production/value or financing denominator prevents meaningful analysis. CHECK preserves a calculation but flags an assumed tax credit, horizon failure, or negative NPV. Input checking errors happen before the workbook. A network or calculation-service failure is an availability problem and is not investment evidence.

References

The workbook's References sheet documents standard discounted cash flow, interpolated zero-crossing payback, explicit NPV discounting, Excel IRR, discounted-cost/production LCOE, and algebraic level-annuity payment. It also records the incentive assumption and instructs users to verify the current policy position and the Disclaimer date before relying on it.

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