The four weather modes

The calculator can size a system against four different views of the same site. Each answers a slightly different question.

Geometric (clear-sky): "What's the absolute maximum solar power you can generate at your site, with no clouds and no climatology baked in?" Pure physics & math: solar position, clear-sky atmospheric transmittance, plane-of-array transposition. The sky condition slider lets you scale the clear-sky beam by a representative clearness index. This is the theoretical ceiling, not a weather estimate. Does not take into account geographic variations in the atmosphere or site elevation.
PVGIS monthly TMY: "What does the average year actually produce at your site?" JRC's PVGIS service returns monthly plane-of-array irradiance derived from satellite-based climatology observations. This is the default fetch when you tap Fetch from PVGIS.
PVWatts hourly TMY: "What does an average year look like hour by hour?" NREL's PVWatts returns an 8,760-hour typical-meteorological-year time series that drives the hourly simulator based on real, local observations. Use this when you want to see the daily pattern, not just monthly totals.
Open-Meteo 7-day forecast: "What is the next week going to look like?" An hourly ensemble forecast (GFS + ECMWF blend) feeds the Scenario Planner so you can pressure-test an upcoming trip or weather window. Deliberately not cached in your browser, so you'll need to update it after leaving & returning to the website.

The other three modes (PVGIS, PVWatts, Open-Meteo) ignore the sky-condition slider, since it only affects the theoretical geometric calculation. If you switch modes, expect the numbers to change; those differences are meaningful and tell you something about the site rather than signaling an error.

Geometric (clear-sky) chain

Geometric mode runs a first-principles physics chain that takes only latitude, longitude, date, panel tilt and azimuth as input. No climate file is consulted. The calculations look at 15-minute steps across each representative day of the month:

Solar position — declination + equation-of-time + hour angle give the sun's zenith and azimuth at every step.
Extraterrestrial irradiance — Spencer (1971) eccentricity correction on the solar constant G_SC = 1361 W/m² (IEC 60904-3).
Clear-sky GHI — pure airmass attenuation using the Laue/Meinel beam and diffuse coefficients: τ_b = exp(−0.357·AM^0.678), τ_d = max(0, 0.271 − 0.294·τ_b). Two constants, no aerosol or altitude term since geometric mode answers the clear-sky-ceiling question, not the site-specific-weather question.
Kt modulation — the sky-condition slider multiplies GHI by a representative clearness index (SKC=0.75, FEW=0.60, SCT=0.475, BKN=0.325, OVC=0.15).
Erbs decomposition — Erbs (1982) diffuse-fraction correlation splits the Kt-scaled GHI into direct-normal (DNI) and diffuse-horizontal (DHI) components.
Hay–Davies POA transposition — anisotropic transposition projects DNI + DHI onto the tilted panel plane, accounting for the angle of incidence and the circumsolar brightening near the sun's disk (the area of the sky immediately around the actual sun).
kWh/m²/day — integrate plane-of-array power across the day to get daily energy density; multiply by panel watts, solar-access fraction, and the controller/wiring/RTE efficiency stack for delivered DC kWh.

The Show Math walkthrough in the calculator's Scenario Planner prints every step of this calculation with your actual numbers substituted.

PVGIS & PVWatts (real weather data)

For a real weather estimate, the calculator fetches plane-of-array irradiance from one of two free public services. PVGIS (Joint Research Centre, European Commission) returns monthly POA values derived from satellite climatology with global coverage. PVWatts (NREL) returns hourly POA from the U.S. National Solar Radiation Database (NSRDB); coverage is best in North America, somewhat spotty elsewhere.

Both services pre-bake the tilt and azimuth into the returned values, so changing those inputs after a fetch requires a re-fetch. (Geometric mode does its own transposition, so reorienting the panels recomputes instantly.)

NASA POWER is offered as a third source for sites where neither PVGIS nor PVWatts has coverage. POWER's solar product is global horizontal irradiance (GHI), so the calculator transposes it to plane-of-array using the same Hay–Davies model the geometric chain uses.

7-day operational forecast

The Scenario Planner can pull a 7-day hourly forecast from Open-Meteo (a GFS + ECMWF ensemble blend). This is the answer to "is my system going to make it through the next week?" which is useful before a sailing passage, a cold snap, or a multi-day cloudy window.

Forecast data is intentionally not cached on disk. A stale forecast is misleading in a way that stale climatology is not: a six-hour-old forecast can be wildly wrong, while a year-old TMY is still the best available estimate of an average year. The forecast window lives in memory only and refetches when you open the Scenario Planner.

Wind power

Wind energy is modeled with a Rayleigh wind-speed distribution and a three-zone power curve: zero below cut-in, ½ρAv³C_p between cut-in and rated, capped at the rated power between rated and cut-out, and zero above cut-out (the turbine furls its blades or stalls them to protect itself in strong winds).

The Rayleigh distribution is a one-parameter probability density for wind speed; knowing the long-term mean wind speed alone is enough to estimate how often each speed bin is encountered. The calculator integrates the power curve against the Rayleigh PDF in 1,000 bins per month and multiplies by hours per month to get monthly wind kWh.

Hub height matters because wind speed scales with height through a boundary layer: v(h) = v_ref · (h / h_ref)^α, where α (terrain roughness) ranges from ~0.10 for water through ~0.14 for grassland to ~0.30 for forest or suburban terrain. Since power scales as v³, a modest hub-height increase compounds; raising the tower usually buys more power than increasing the rated wattage.

Vertical-axis turbines (VAWT) use a rectangular swept area (diameter × rotor height) rather than the circular disk of a horizontal-axis (HAWT) turbine, and the effective hub height is shifted up by half the rotor height.

Battery balance

The simulator runs an hourly energy-balance loop. Each hour: solar + wind + (optional) supplemental charger + (optional) backup generator feed a common DC bus. Loads are then subtracted. AC loads pass through the inverter's partial-load efficiency curve first. Net surplus energy charges the battery, capped by the chemistry-appropriate maximum charge rate (a C/2 cap for lithium, C/10 for flooded lead-acid), as applicable. Net deficit draws from the battery, capped by the depth-of-discharge target.

Peukert's exponent (the chemistry's tolerance to high discharge rates) corrects the apparent capacity downward when the load is heavier than the battery's rated discharge. A 100 Ah AGM rated at the 20-hour rate delivers ~100 Ah at 5 A but only ~80 Ah at 50 A; the difference is that the chemistry can't move ions to the electrodes fast enough. Allow the battery to "rest" for a period of time, and it may be able to squeeze out some more charge.

Depth-of-discharge defaults reflect cycle-life behavior: 50% for lead-acid (a lead-acid bank drained to 80% repeatedly is a one-season investment), 80% for LiFePO4, 60% for NiMH/NiCd. The hard cutoff is the BMS-level emergency stop, not the design target.

Loads & duty cycle

Most appliance nameplates can be confusing. A 70 W mini-fridge with a 50% duty cycle averages 35 W, but the compressor still draws 70 W when it's running, and the transient surge on startup is ~3× that. The calculator separates these three numbers:

Effective W — average draw, used for the energy balance.
Duty cycle % — the fraction of each hour the appliance is actually drawing (compressor cycling, vent fan thermostat, water pump on demand). Defaults to 100%.
Inrush W — the startup spike (Advanced mode only). Used to warn if it exceeds the inverter's surge rating or a boosted duration cap (if spec'd in the inverter, common in many All in One systems).

Seasonal load profiles let you mark a load as inactive in some months, which is useful for heating, cooling, or anything you only run part of the year.

Data sources & further reading

PVGIS — Joint Research Centre, European Commission. re.jrc.ec.europa.eu/pvg_tools/
PVWatts — National Renewable Energy Laboratory. pvwatts.nrel.gov
NASA POWER — NASA Langley Research Center. power.larc.nasa.gov
Open-Meteo — open-source weather forecast API. open-meteo.com
Spencer, J.W. (1971). "Fourier series representation of the position of the sun." Search 2(5): 172.
Erbs, D.G., Klein, S.A., Duffie, J.A. (1982). "Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation." Solar Energy 28: 293–302.
Hay, J.E., Davies, J.A. (1980). "Calculation of the solar radiation incident on an inclined surface." Proc. First Canadian Solar Radiation Data Workshop.