Formula used and calculation methodology
How the estimator converts inputs into energy, costs, bills, financing, cash flow, and investment metrics.
System sizing
Direct mode uses the entered DC capacity. Consumption mode divides the desired annual solar output by modeled annual yield per installed kilowatt. Area mode divides usable roof or land area by panel area, rounds down to a whole number of panels, and converts the result into DC capacity. Budget mode applies a preliminary cost-per-watt assumption. Bill mode estimates annual energy spending after fixed charges and converts it into approximate annual electricity use.
Solar production
Annual production begins with panel DC capacity and average peak sun hours. The calculator applies inverter efficiency, shading, soiling, snow, wiring, mismatch, availability, temperature, clipping, and other losses. It also applies simplified orientation, tilt, tracking, and bifacial factors. These approximations are suitable for feasibility screening, but hourly weather and validated simulation software should be used for final design.
Self-consumption, storage, exports, and imports
Direct solar consumption is limited by the entered daytime load share. Excess generation can charge the battery within usable-capacity, reserve, cycle, and efficiency constraints. Battery discharge supplies nighttime load. Remaining generation is exported unless zero-export or off-grid mode is selected. Remaining consumption becomes grid import.
Utility-bill calculation
The baseline bill combines energy charges, fixed monthly charges, optional demand charges, taxes, and surcharges. The post-solar bill values imported energy at the purchase rate and exported energy at the export credit, while preserving fixed charges, demand charges, and any minimum monthly bill. Time-of-use mode uses a weighted average of peak, shoulder, and off-peak rates.
Installed cost
Panels, inverter, battery, mounting, hardware, labor, engineering, permits, inspections, interconnection, shipping, site work, roof work, trenching, electrical upgrades, monitoring, transformer, and metering costs are combined. Contractor margin, contingency, and installation tax are then applied. A quote override replaces the detailed total when a complete installer quotation is available.
Incentives
Fixed rebates, grants, property-tax benefits, and sales-tax benefits are deducted before the percentage tax credit. Recurring production incentives and renewable certificate income are included in annual cash flow. The calculator does not determine legal eligibility, tax liability, incentive caps, placed-in-service dates, or stacking restrictions.
Financing
Cash ownership records the net project cost in year zero. Loan mode calculates a level monthly payment from principal, origination fee, interest rate, and term. Lease mode escalates the entered monthly payment. PPA mode values annual system generation at the contract rate and escalator. Balloon payments, extra annual payments, and end-of-term buyout costs are modeled as entered.
Cash flow and investment metrics
Annual cash flow equals utility-bill savings plus recurring project income, minus operating costs, replacement expenses, and financing payments. Simple payback finds the first fractional year in which nominal cumulative cash flow becomes nonnegative. Discounted payback performs the same test after discounting. Net present value discounts each cash flow to year zero. Internal rate of return is estimated through a bisection search when the cash-flow series includes positive and negative values.
Levelized cost of energy
Levelized energy cost divides discounted project costs by discounted solar generation. This implementation includes net upfront cost, operating cost, and replacement cost. It does not currently apply tax depreciation benefits to after-tax cash flow, although depreciation assumptions are retained in the project record.
Environmental estimates
Avoided emissions equal self-consumed solar electricity multiplied by the entered grid emissions factor. The optional grid-decarbonization rate reduces this factor over time. Tree, fuel, and carbon-value outputs are illustrative estimates rather than audited carbon claims.
How to use this calculator
A practical workflow for producing and documenting a preliminary solar estimate.
1. Gather electricity data
2. Confirm the current utility tariff
3. Choose a sizing method
4. Enter production assumptions
5. Build the project cost
6. Add incentives carefully
7. Select financing
8. Review lifetime costs
9. Compare scenarios
10. Export and retain assumptions
Professional checks before installation
Important matters outside a preliminary browser calculator.
Confirm roof condition, dead and live loads, wind uplift, seismic requirements, attachment design, drainage, and fire-access pathways.
Verify service capacity, breakers, conductors, voltage rise, grounding, protection, rapid shutdown, arc-fault requirements, and utility rules.
Use a site survey, drone model, lidar, or shading instrument to capture seasonal obstacles and row-to-row effects.
Utility rates, export compensation, net-metering rules, fixed charges, demand charges, and taxes may change.
Credits, depreciation, grants, rebates, and ownership benefits depend on eligibility, timing, jurisdiction, and tax liability.
Actual dispatch depends on hourly load, weather, power limits, firmware, reserve settings, thermal conditions, warranties, and utility programs.