Tiny Watts Calculations

Small Scale Solar 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. Check here to 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. For more information on the testing protocol used, click here.  You can learn more about SCI’s Preliminary data in this webinar.

Utilize the Sun to Live a Sustainable Life!

The sun rises everyday and produces free power! Here are a few simple switches from using traditional powered appliances to utilizing the free energy from the sun. 

Lower your carbon footprint and blood pressure:

  • Hanging your clothes on a clothesline rather than using an electric or gas dryer
  • Drying your food from the sun rather than using an electric food dryer or oven
  • Plant a garden to grow your own food
  • Heat your shower water by the sun rather than from an electric or gas water heater

Hanging your clothes can save anywhere from 2,000 W-hr to 4,000 W-hr depending on the size of your load. To estimate watt-hours (W-hr) for hanging clothes to dry, the team did a simple calculation of the electrical energy avoided by drying outside. You could also use this simple calculator.

The team estimated 2,000 Watt-hour for a small load of laundry, 3,000 Watt-hour equivalent for a medium load of laundry, and 4,000 Watt-hour equivalent for a large load of laundry.

Drying your food from the sun rather than using an electric food dehydrator can save 464 Watt hour per pound of food dried, assuming a water content of 80%

It is assumed that drying grain takes 2000 BTU / lb. of water 

1 BTU / hr = 0.29 Watts (conversely, 1 Watt =3.14 BTU / hr)

therefore 0.29 W* 2000 BTU / lb = 580 Whr to dry food that has 1 lb. of water.  

Most common types of food for drying are fruits and vegetables, which contain a lot of water. Melons can be 90% water, blueberries contain about 80% water. A banana has about 74% water. Zucchini, radish, and celery are 95% water, tomatoes are 94% water, broccoli is 91% water, carrots are 87% water, and green peas are 79%.

The Tiny Watts Team assumed 80% water content, so 580 W⋅hr / lb * 0.8 = 464 W⋅hr to dry a pound of fruits/vegetables.

Growing your own food is a great way to cut down on energy used in the transportation of goods. Gardening can be done in the ground or in smaller spaces using containers.  Home-grown food is often fresher and more nutritious than the store-bought alternative. Some may even find gardening to be a relaxing and satisfying activity. 

Learn more on agriculture emissions from our ZEN program. 

Heating your shower with the sun can save 2,000 Watt-hours per 10 minute shower. 

See the label of your water heater to know it’s maximum wattage (1200 Watts for example). Multiply that by your shower time in hours (10 mins= 1/6 hr) to get the electricity usage (1200*1/6=2000 Watt-hours or 2 kWh).

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