How our calculators work.
Every calculator on this site runs a transparent, editable formula directly in your browser. Here's the math, the default assumptions, and the limitations behind each one.
EV Charging Cost Calculator
Battery capacity in kWh is multiplied by the percentage-point difference between your target and current charge to get energy added. That figure is divided by (1 − charging loss) to estimate the larger amount of energy actually drawn from the outlet, then multiplied by your electricity rate.
Battery: 60 kWh, charging 20% → 80%
Energy added: 36 kWh · Charging loss: 10%
Energy from outlet: 40 kWh · Rate: $0.16/kWh
Result: about $6.40 per charge
Limitations: this is an estimate, not a bill. Real-world results vary with battery temperature, charger and vehicle efficiency, utility fees, demand charges, and time-of-use pricing.
EV Charging Time Calculator
Energy added (kWh) is divided by your charger's effective power — the lower of your charger's rating and your vehicle's maximum charging rate, reduced for losses. Most EVs also slow down above 80% charge to protect battery health, so the calculator splits the session into a normal-speed segment up to 80% and a roughly half-speed segment beyond it.
Limitations: real charging curves taper more gradually than this two-segment model, and cold battery temperature or shared fast-charger infrastructure can further slow real-world sessions.
EV vs Gas Savings Calculator
Annual cost = (annual miles ÷ efficiency) × price per unit of energy, calculated separately for electricity (per kWh) and gasoline (per gallon), then compared to estimate savings.
Limitations: this compares fuel cost only by default; it does not include maintenance, insurance, purchase price, or incentives unless you factor those in yourself.
EV Range Calculator
Usable battery energy — capacity times current charge minus a reserve percentage you set aside — is multiplied by your vehicle's real-world efficiency in miles per kWh, adjusted for weather or driving-condition inputs.
Limitations: this is a planning estimate using a single efficiency figure, not a live reading; your vehicle's dashboard accounts for real-time factors this calculator can't see.
RV Solar Calculator
Daily energy use is inflated by your system loss percentage, then divided by peak sun hours to size the solar panel array. Battery capacity multiplies that adjusted daily energy by your days of autonomy, divided by system voltage and depth of discharge. Charge controller amperage and inverter wattage each apply a 125% continuous-duty safety margin, following the same margin NEC 690.8 requires for photovoltaic circuit conductors and equipment.
Daily use: 2,000 Wh · System loss: 20% · Sun hours: 4.5
Panel: ~556 W · Battery: ~417 Ah (12V, 50% DoD, 1 day autonomy)
Controller: ~57.9 A · Inverter: 1,875 W (1,500 W peak load)
Limitations: this is a component-sizing estimate, not an engineering or electrical-code compliance document. Have a qualified installer verify wiring, overcurrent protection, and code compliance for your specific system.
Solar Loan Calculator
The standard loan amortization formula converts your system cost, down payment, interest rate, and term into a monthly payment: loan amount × monthly rate ÷ (1 − (1 + monthly rate)⁻ⁿ), where n is your total number of monthly payments.
Loan: $25,000, 0% down · Rate: 6.5% APR · Term: 15 years
Monthly payment: ~$217.78 · Total interest: ~$14,200
Net monthly cost (with $150/mo savings): ~$67.78
Limitations: this estimates payments on the amount you enter; it does not include loan fees, tax credits applied to principal, or your lender's specific terms. Review your loan documents for exact figures.
Solar Panel Snow Load Calculator
A baseline flat-surface snow load is calculated from ground snow load, exposure, thermal (mounting) factor, and risk category, following ASCE 7's flat-roof snow load structure. A slope factor, based on your array's tilt angle and the panel glass's slippery surface, then reduces that baseline for the design load your racking needs to support.
Ground load: 30 psf · Exposure: partial · Mounting: unheated space below · Tilt: 20°
Flat load: 23.1 psf · Slope factor: 0.83
Design load: ~19.3 psf (~94 kg/m²)
Limitations: this uses simplified representative factors for planning, not the full ASCE 7 methodology with site-specific values. Have a qualified structural engineer verify the design load for any permitted installation.
Solar Payback Period & ROI Calculator
Net system cost is gross installed cost (kW × 1,000 × cost per watt) less the federal credit percentage and any state or utility incentive. Annual production is system size × peak sun hours × 365 × performance ratio, reduced each year by the panel degradation rate. Each year's savings split production between energy consumed on site, valued at your retail rate, and energy exported, valued at your export credit rate, with both escalated forward at your rate escalation. Annual O&M and a one-time inverter replacement are subtracted to give net cash flow. Payback is the year cumulative net cash flow first covers net cost; ROI is total net profit ÷ net cost; IRR is solved by bisection as the discount rate at which the 25-year cash-flow series nets to zero; LCOE is lifetime cost ÷ lifetime production.
System: 8 kW · Cost: $2.53/W · California, no incentives
Net cost: $20,240 · Production: ~12,366 kWh/yr
40% self-consumed at $0.3325, 60% exported at $0.05
Year 1 savings: ~$2,016 · Result: ~10-year payback, ~10% IRR
Sources: default electricity rates are the U.S. Energy Information Administration's average residential retail price by state (Electric Power Monthly, Table 5.6.A); installed cost defaults are state-average residential benchmarks before incentives; state incentive and net-metering data are verified against DSIRE. The state table is reviewed at least annually and after major federal or state policy changes.
Limitations: this uses a single flat retail rate rather than time-of-use, seasonal, or tiered pricing, escalates the export credit alongside retail even though avoided-cost rates often move independently, excludes financing interest, and excludes any resale value the system adds to the home. It also assumes you have enough tax liability to use any credit entered. Verify incentive eligibility with a tax professional.
Solar Feed-in Tariff Calculator (U.S. Net Metering)
Annual production is estimated as system size (kW) × peak sun hours × 365 × a 0.77 PVWatts-style derate factor. That production is split into a self-consumed share (valued at your retail rate) and an exported share (valued at your export or feed-in rate), then summed for a total annual value.
System: 6 kW · Sun hours: 4.5 · Self-consumption: 30%
Production: ~7,588 kWh · Retail: $0.16/kWh · Export: $0.05/kWh
Self-consumption savings: ~$364 · Export revenue: ~$266 · Total: ~$630
Limitations: this is a simplified annual estimate using a standard derate factor and flat rates; real net metering and net billing programs often use hourly or seasonal rates, rollover credits, and fees not modeled here.
Where defaults come from
Default electricity rates reflect a general U.S. residential average; default charging and system losses reflect typical figures cited for their respective equipment types. Every default is a starting point meant to be replaced with your own numbers for the most accurate result.
General limitations
Every calculator on this site produces a planning estimate, not a guarantee, a bill, or a substitute for professional engineering, electrical, or financial advice. All inputs and calculations run directly in your browser and are never sent to a server or saved in an account.
Review schedule
Each calculator's page carries its own publication and last-updated date in the byline at the top. We re-check rate, incentive, and code-reference data at least annually, and sooner when a federal or state policy change affects a result — the last-updated date on a page moves only when its method or data actually changes, not when we make cosmetic edits.