Tiny Watts Registry Calculations

ASES Tiny Watts –Simple Default Watt Estimates for Solar Devices and Applications

Here is how the Tiny Watts Team developed the Tiny Watts registry estimates.

Watts (W) is the standard unit of electrical power, and is the unit used to show solar power used by tiny watts devices and applications. Watts (power) is equal to Amps (current) X Volts (force). P (W) = A * V. Multiplying power for a particular device by the time used gives the total energy used.

Calculating the watts used by tiny devices powered by mini solar panels, such as solar chargers, is straightforward since the amps and volts of the device are generally labeled on the product. The default A and V values for all of the mini solar panel type of devices listed on the Tiny Watts registry are based on typical versions of these devices being currently sold. Of course, a tiny watts user can customize their device and input the specific amps and volts for their device.

Solar Cooking

There are 3 major types of solar cookers: solar oven, reflective-panel cooker, and parabolic solar cooker. There are additional types of cookers such as the commercial GoSun Grill, but this type of cooker falls under parabolic solar cookers. These different cookers vary in the way they capture sunlight and convert the energy of light to heat energy, and these variations are based on size, shape, and functional design. Learn more about how each design works.

In the context of our Tiny Watts program, the watts used by solar thermal devices (that are not operated by mini solar PV panels), in particular solar cookers, is determined using an approach that is based on thermal measurements. Since there are hundreds of solar cooker designs, and because not all of them have been tested yet for their standardized cooking power (in Watts), we are determining solar cooking power values based on preliminary data from the Solar Cookers International (SCI) Performance Evaluation Process (PEP), which uses the ASAE S580.1 protocol for Testing and Reporting Solar Cooker Performance. The power uptake by water heated by a solar cooker during a 10-minute interval is determined through data acquisition during testing and by using the equations:

Pi = (T2 – T1) (M Cv /600)

where:

Pi = Cooking power (W) for Interval i

T1 = Initial water temperature  (°C)

T2 = Final water temperature (°C)

M = Water Mass (kg)

Cv = Heat Capacity for water (4186 J/(kg°C))

600 = Interval time of a 10 min in seconds (s)

The mass of the water is determined by using 7 liters for 1 m2 of cooker intercept area, where liters = kilograms. The initial and final temperatures are calculated at the start and end of each 10 minute interval.

SCI calculated Pi every 10 minutes over a 4 hour test, centered about solar noon. They then calculated the standardized cooking power from the equation:

Ps = Pi (700/Ii)

where:

Ps = Standardized cooking power (W)

Pi = Interval cooking power (W)

Ii = Interval average solar insolation (W/m2)

Insolation is how bright the day is which is found using a Pyranometer during the test. The standard powers from each main type of solar cooker (parabolic, panel, and solar oven) are then plotted on a graph and a best fit line is created. 

The team then developed a set of cooker sizes for tiny watts users to choose from based on popular solar cookers, and developed a range of interface areas describing these cookers. Based on the areas, the cooking powers were determined from the best fit line.  

Here are the team’s estimates of watt-equivalents for the standardized cooking power (Ps) of different solar cookers. 

Solar Oven Box:

Small: < 0.008 m2 (12 x 12 in.) → 11 W

Medium:  0.008 m2 – 0.012 m2 (12 x 12 in. – 24 x 24 in.) → 29 W

Large: > 0.012 m2 (24 x 24 in.)  → 46 W

Parabolic Cooker: 

Small: < 1 m2 176 W

Medium: 1 m2 – 1.5 m2286 W

Large: > 1.5 m2395 W 

Panel Cooker:

Small: < 0.008 m2 (12 x 12 in.) → 9 W

Medium:  0.008 m2 – 0.012 m2 (12 x 12 in. – 24 x 24 in.) → 23 W

Large: > 0.012 m2 (24 x 24 in.)  → 36 W

The ASAE S580.1 protocol is a part of the standards by the International Organization for Standardization (ISO) for testing clean cookstoves and clean cooking solutions, published during the past few years. Learn more on the testing protocol used and about SCI’s preliminary data in this webinar.

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