Parabolic Cooker Calculator
Calculate focal length, cooking power, concentration ratio, and boil time for a parabolic solar cooker based on dish size and reflectivity.
About this calculator
A parabolic solar cooker concentrates sunlight from a wide dish onto a small pot at the focal point, reaching much higher temperatures than a box cooker because the same collected energy lands on a far smaller area. This calculator derives focal length from the paraboloid geometry (diameter squared, divided by 16 times the dish depth) — shallower dishes have longer focal lengths, deeper dishes focus closer to the vertex — and reports the f/D ratio, where 0.25–0.5 is the typical sweet spot for cookers. Collected power is dish area times solar irradiance times reflectivity; an intercept factor (a simplified model based on how the pot size compares to dish diameter) accounts for the fraction of reflected light that actually lands on the pot rather than missing it, and a fixed 92% absorptance represents a blackened pot surface.
The reported maximum pot temperature comes from a Stefan-Boltzmann radiative equilibrium calculation — where absorbed power balances radiated heat loss — capped at 500°C for realism, since uninsulated metal pots lose significant heat to convection that this simplified radiative-only model doesn't capture. Because the calculator doesn't account for wind, imperfect solar tracking, or convective losses, real cooking power runs somewhat below the theoretical number, and boil times in practice tend to run longer. Use the concentration ratio and focal length primarily to size and aim your build; treat the temperature and boil-time outputs as optimistic upper bounds.
Inputs
Results
Cooking power at pot
933 W
≈ 16 laptops
How to Use This Calculator
- Enter the dish diameter and depth of your parabolic reflector.
- Set the reflector reflectivity (0–1) and current solar irradiance (W/m²).
- Enter the diameter of your cooking pot.
- The calculator outputs cooking power at the pot, focal length, concentration ratio, estimated max temperature, and time to boil 2 liters.
- Adjust dish diameter or reflectivity to explore trade-offs between power and construction complexity.
How the result changes with Dish diameter
| Dish diameter | Cooking power at pot |
|---|---|
| 0.75 | 249 W |
| 1.13 | 541 W |
| 2.25 | 2,052 W |
| 3.75 | 5,597 W |
What each input means
- Dish diameter
- Outer diameter of the parabolic reflector. 1.0–1.8 m is common for cooking.
- Dish depth
- Depth of the parabolic dish from rim to vertex. Deeper = shorter focal length.
- Reflector reflectivity
- Reflectivity of the dish surface. Polished aluminum ≈ 0.85, mirror film ≈ 0.90, anodized ≈ 0.70.
- Solar irradiance
- Direct normal solar irradiance. Clear sky at noon ≈ 800–1000 W/m².
- Pot diameter
- Diameter of the dark/black cooking pot placed at the focal point.
What each result means
- Cooking power at pot
- Net thermal power concentrated on the cooking pot.
- Focal length
- Distance from dish vertex to focal point where the pot should be placed.
- Concentration ratio
- Ratio of dish area to pot area. Higher = more intense heat focus.
- Max pot temperature
- Theoretical maximum equilibrium temperature at the pot (radiative balance).
- Time to boil 2 L water
- Approximate time to bring 2 liters of water from 25 °C to boiling.
- Dish area
- Total reflective area of the parabolic dish.
- f/D ratio
- Focal-length-to-diameter ratio. 0.25–0.5 is typical for solar cookers.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersDish diameter = 1.5, Dish depth = 30, Reflector reflectivity = 0.85, Solar irradiance = 900 = 5 input(s) provided
- Calculate Cooking power at potCooking power at pot = collectedPowerW * interceptFactor * potAbsorptance933 = 933
- Calculate Focal lengthFocal length = focalLengthM * 10046.9 = 46.9
- Calculate Concentration ratioConcentration ratio = dishAreaM2 / potAreaM236 = 36
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does making the dish deeper shorten the focal length?
Focal length is calculated as diameter squared divided by 16 times the dish depth, so for a fixed diameter, increasing depth in the denominator directly shrinks focal length. A shallow, wide dish focuses light far from the vertex, while a deep, curved dish concentrates it much closer in — which is why the pot needs to sit at a different height depending on how deep your dish is formed.
Why is the reported max pot temperature capped at 500°C?
That temperature comes from a Stefan-Boltzmann radiative equilibrium calculation, where absorbed power is set equal to radiated heat loss, and this simplified model doesn't separately account for convective heat loss from an uninsulated metal pot sitting in open air. The cap keeps the output realistic, since without it the pure-radiation formula can predict temperatures well beyond what any uninsulated pot actually reaches.
Does a bigger pot always increase cooking power at the pot?
Not directly — collected power depends only on dish area, irradiance, and reflectivity. A larger pot diameter raises the intercept factor slightly (since more of the reflected light lands on a bigger target), but it also lowers the concentration ratio, meaning the same power gets spread over more surface area rather than being intensified in one spot.
What does the f/D ratio actually tell me about my dish design?
The f/D ratio (focal length divided by dish diameter) describes how tightly the dish focuses light: 0.25–0.5 is the typical sweet spot for cooking dishes, giving a good balance between concentration and a focal point that's a practical distance from the dish surface to place a pot. A very low f/D means an aggressively curved dish with a very tight, close focus, while a high f/D behaves more like a shallow, gently focusing mirror.
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