Solar / Wind Hybrid System Sizing Tool: Array, Rotor & Battery Bank Sizing

Solar / Wind Hybrid System Sizing Tool

Your own solar:wind split · array kW · rotor size · battery bank

Demand & Source Split

The energy the loads actually consume per day, not the peak power rating of the appliances.
Solar 60% Wind 40%
There is no universally correct split — it depends entirely on what your site offers. Drag it and watch how the two component sizes trade against each other.

Solar Resource

Daily irradiation expressed as equivalent hours at 1000 W/m². Use the annual worst month if the system must never fall short.
Typically 0.75–0.85 — inverter, wiring, soiling, temperature and mismatch losses combined.

Wind Resource

Must be at hub height, not ground level — wind speed rises considerably with height.
1.225 at sea level, 15 °C. Thinner at altitude and in heat — a site at 2000 m loses roughly a fifth of its output for the same wind.
Betz limit is 0.593 — no turbine can exceed it. Good modern machines reach 0.35–0.45; small turbines often less.
Corrects the cube-of-the-mean error. About 1.91 for a Rayleigh wind distribution; 1.0 disables the correction entirely.
Used only to suggest a nameplate turbine rating for the rotor size calculated.

Battery Backup

Rough Cost Estimate (optional)

Reference prices only — set all three to zero to skip the cost block entirely.

Result

Set your daily demand and choose how much of it each source should carry — the tool sizes the solar array against your sun hours and derate, solves the rotor diameter the wind share needs with a proper correction for wind speed distribution, and works out the battery bank from your autonomy and depth-of-discharge limits

This is a preliminary sizing aid, not a system design. It works from single average figures for sun and wind, so it cannot capture the thing that actually determines whether an off-grid system keeps the lights on: the sequence of bad days. A week of overcast still weather in the wrong season will empty a bank sized on annual averages, which is why serious designs use hourly time-series simulation against real local data rather than a daily average. Peak sun hours vary widely by month and the annual mean will undersize a system that must never fail; use the worst design month if reliability matters. The wind side is more uncertain still — output scales with the cube of speed, so a modest error in the wind estimate becomes a large error in energy, and wind speed measured at ground level or at a nearby weather station is often far from what a rotor at hub height on your site will see. Terrain, turbulence, obstacles and wake effects all reduce real output below the swept-area calculation, and small turbines in particular rarely achieve the power coefficients quoted for large machines. The battery figure is usable capacity converted to nominal and takes no account of temperature derating, ageing, charge acceptance limits or inverter surge requirements. Costs are illustrative reference figures that vary enormously by country, scale and time. Have any system that matters designed and verified by a qualified renewable energy engineer against measured site data.

Solar/Wind Hybrid System Sizing Tool: Your Split, Correct Wind Physics, Battery Bank

Most free hybrid sizing tools make one of two mistakes: they only handle solar or if they do include wind, they simply cube the average wind speed and stop there, which substantially understates real annual energy yield. This solar/wind hybrid calculator starts from your daily energy demand and lets you set your own solar-to-wind contribution split with a live slider, since the right balance depends entirely on what your specific site offers, a windy coastal location and a sunny inland one call for genuinely different systems. The solar side divides demand by usable sun hours and an adjustable derate factor covering inverter, wiring, dust and temperature losses. The wind side solves backward for rotor diameter from the swept-area power equation, caps the power coefficient at the physically-impossible-to-exceed Betz limit and applies an energy pattern factor correction for the cube-of-mean bias, since windy hours contribute far more energy than calm hours take away, a detail most simple wind calculators miss entirely. As a genuine battery bank calculator it then sizes storage from your desired autonomy, depth of discharge and round-trip efficiency and an optional cost block turns the whole sizing into an indicative budget.

How to Use

Step 1: Set your demand and source split
  • Enter your Daily Energy Demand in kWh/day, the energy your loads actually consume per day, not the peak power rating of your appliances.
  • Drag the Solar/Wind Contribution Split slider to set how much of that demand each source should carry. There’s no universally correct split, watch how the two component sizes trade against each other as you move it and choose based on what your actual site resource looks like.
Step 2: Set your solar resource details
  • Enter your Peak Sun Hours (h/day), your site’s daily irradiation expressed as equivalent hours at 1000 W/m². Use your annual worst month here, not the yearly average, if the system must never fall short.
  • Enter your System Derate Factor, typically 0.75 to 0.85, covering inverter, wiring, soiling, temperature and mismatch losses combined.
Step 3: Set your wind resource details
  • Enter your Mean Wind Speed at Hub in m/s. This must be measured or estimated at actual hub height, not ground level, since wind speed rises considerably with height.
  • Enter your Air Density, 1.225 kg/m³ is standard at sea level and 15°C, thinner at altitude, a site at 2000m loses roughly a fifth of its output for the same wind speed.
  • Enter your Power Coefficient Cp. The Betz limit of 0.593 is the absolute physical ceiling no turbine can exceed, good modern machines reach 0.35 to 0.45 and small turbines often achieve less.
  • Enter your Energy Pattern Factor, roughly 1.91 for a typical Rayleigh wind distribution, this corrects the cube-of-the-mean error that most simple wind calculators miss entirely. Set it to 1.0 to disable this correction if you prefer a simpler, less accurate estimate.
  • Enter your Turbine Availability (%) and Rated Wind Speed, used only to suggest a sensible nameplate turbine rating for the rotor size the tool calculates.
Step 4: Set your battery backup requirements
  • Enter your Days of Autonomy, how many days the battery bank alone should carry your full demand with zero generation.
  • Select a Battery Chemistry preset: Lithium (LFP), Lead-acid AGM or Flooded lead-acid, each pre-filling a typical depth of discharge and round-trip efficiency.
  • Check the Depth of Discharge and Round-Trip Efficiency fields and adjust them if your specific battery’s datasheet gives different figures.
Step 5: Add a rough cost estimate, if you want one
  • Under Rough Cost Estimate (optional), enter your Currency Symbol, Solar Cost per kW, Wind Cost per kW and Battery Cost per kWh. These are reference prices only, set all three to zero to skip the cost block entirely.
Step 6: Size the system and read your result
  • Tap Size the System. The result shows the solar array size in kW, the wind rotor diameter and suggested turbine rating, the battery bank size in kWh and if you entered prices, an indicative total system cost.
Step 7: Export
  • Use Print / PDF for a clean printable copy or Copy to paste the figures into your project notes.

Key Features

  • User-controlled solar/wind split with a live slider showing the two component sizes trading against each other in real time
  • Solar array sizing from daily demand, peak sun hours and an adjustable system derate factor
  • Physically correct wind rotor sizing solved backward from the swept-area power equation, capped at the Betz limit so the sizing never assumes an impossible turbine
  • Energy pattern factor correction fixing the cube-of-mean error that most free wind calculators simply ignore
  • Battery bank sizing from autonomy days, chemistry-specific depth of discharge and round-trip efficiency, with 3 chemistry presets
  • Optional cost block turning the sizing into an indicative budget across solar, wind and battery components together
  • Prominent, honest scope disclaimer on why real off-grid reliability depends on the sequence of bad days, not just averages.

Formula / Logic Used

Solar Array Size

Solar kW=Daily Demand×Solar SharePeak Sun Hours×DerateSolar\ kW = \frac{Daily\ Demand \times Solar\ Share}{Peak\ Sun\ Hours \times Derate}

Wind Power Equation (Solved for Rotor Area)

P=0.5×ρ×A×v3×Cp×EPF,A=πD24P = 0.5 \times \rho \times A \times v^3 \times C_p \times EPF, \qquad A = \frac{\pi D^2}{4}

Where CpC_p​ is capped at the Betz limit of 0.593 and the energy pattern factor (EPF, roughly 1.91 for a Rayleigh distribution) corrects for the fact that a straight cube of the mean wind speed significantly understates real annual energy, since gustier, windier hours contribute disproportionately more energy than an average alone captures.

Battery Bank Size

Battery kWh=Demand×Autonomy DaysDoD×Round–Trip EfficiencyBattery\ kWh = \frac{Demand \times Autonomy\ Days}{DoD \times Round\text{-}Trip\ Efficiency}

Indicative Cost (optional)

Total Cost=(Solar kW×Costsolar)+(Wind kW×Costwind)+(Battery kWh×Costbattery)Total\ Cost = (Solar\ kW \times Cost_{solar}) + (Wind\ kW \times Cost_{wind}) + (Battery\ kWh \times Cost_{battery})

Who Should Use This Tool

Renewable energy engineers and off-grid system designers doing a preliminary sizing pass on a combined solar and wind installation before detailed site analysis. Also useful for renewable energy engineering students learning the Betz limit, energy pattern factor and battery autonomy concepts for their coursework.

Frequently Asked Questions (FAQs)

1. Why does most free hybrid sizing software understate wind energy yield?

Wind power scales with the cube of wind speed, so simply cubing the average wind speed significantly understates real output, since windy hours contribute far more energy than calm hours take away, a statistical effect the raw average alone can’t capture. This tool applies an energy pattern factor to correct for exactly this bias.

2. What is the Betz limit and why does this tool cap the power coefficient at it?

The Betz limit, 0.593, is the absolute theoretical maximum fraction of wind energy any turbine can physically extract, derived from basic fluid momentum conservation. Capping Cp at this value prevents the tool from sizing a rotor around a physically impossible turbine efficiency.

3. Is there a “correct” solar to wind split for a hybrid system?


No, the right balance depends entirely on what your specific site’s solar and wind resources actually offer, a windy coastal site and a sunny inland site call for very different splits. This tool deliberately leaves the split to you rather than assuming a fixed ratio and shows how the two component sizes trade against each other as you adjust it.

4. Why does a genuinely hybrid solar-wind system often need a smaller battery bank than either source alone?


Solar and wind resources often complement each other over a day or season, cloudy, windy days can still generate power and calm, sunny days do too, so a well-balanced hybrid system experiences fewer simultaneous “bad” periods than a single-source system would for the same reliability target.

5. Why does the tool warn that using annual average figures can undersize the system?

A system sized purely on annual averages can still fail during an unusually calm, cloudy stretch in the wrong season, since real reliability depends on the sequence of bad days, not just the yearly mean. This tool’s disclaimer recommends using worst-month figures and, for anything that matters, proper hourly time-series simulation against real local data instead of daily averages alone.

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