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Calculate cost savings from solar cooking

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The following was devised as a means of estimating energy dollar savings and the environmental benefits of cooking outdoors during the summer months using a solar cooker. After giving an overview of the cost and environmental impact of cooking a series of formulas is then given in order to help one estimate the cost benefits of outdoor cooking in general and in particular solar cooking.

A basic introduction to the energy use and environmental issues surrounding cooking can be found at http://michaelbluejay.com/electricity/cooking.html. The numbers associated with cooking on this link are merely costs and ignore other environmental factors. Thus while use of an electric convection oven might cost less than a gas oven the carbon footprint of the electric oven is significantly higher if non-renewable energy sources are used. The efficiency of the electric generation system in the United States is approximately 30% and about 50% of the power is generated from coal. Thus the actual carbon footprint of using electricity to cook is on average about three times higher than that associated with natural gas. Also note that while natural gas use is much better from a carbon footprint standpoint there are associated health "nasties" such as carbon monoxide that are generated as a by-product of using natural gas for cooking. Indoor air is often an order of magnitude more polluted than outdoor air and cooking is a significant contributor to poor indoor air quality, a fact which can be simply verified any time one cooks fish indoors!

To complete this survey we need to make some basic assumptions with regards to cooking times. All times are based on averages as observed in Arizona. For the purpose of this survey a full meal is a main course (meat, fish, or poultry) plus rice, steamed vegetable or other side dish. We will assume that an average cooking time of two hours for the oven and one hour for the range is necessary to cook a full meal. Of course direct measurements of energy use during cooking would be the preferred method of estimating the cost, but this can be a rather complex exercise.

A main dish (meat, fish, or poultry) would thus be a partial meal and require only two hours cooking time for the oven.

A side dish (something cooked on the range) would equal one hour cooking time. Cookies or a snack would require half-an-hour of oven time.

With these definitions, fill out the section below based on your experience during months that require air conditioning in the home. Note that this number is going to depend upon how long the air conditioning season is where you live. If air conditioning is only used for three months than estimate the quantities for a three month period and if the air conditioning season is six months likewise estimate the quantities for six months:

How many full meals did you cook? ________
How many partial meals did you cook? ________
How many side dishes did you cook? ________
How many snacks did you cook? _______

The formulas that follow will be used to generate an estimate of the energy used while cooking during months in which the air conditioning is used.

Using the numbers previously assigned for cooking times for the four various types of meals, multiply the number of meals by the hours of oven or range time assigned that category. Make sure to keep the oven times separate from the range hours:

__ (number of full meals cooked) x 2 oven hours = ___ hours oven used
__ (number of full meals cooked) x 1 range hour = ___ hours range used
__ (number of partial meals . . . ) x 2 oven hours = ___ hours oven used
__ (number of side dishes . . . . ) x 1 range hour = ___ hours range used
__ (number of snacks . . . . . . . ) x .5 oven hour = ___ hours oven used

Because the elements in an electric oven/range heat up and then go on and off as electricity is needed to maintain certain temperatures, energy is not being consumed during the entire cooking time. Here again we have to make another assumption, and figure that the element is drawing electricity only half the time (other formulas that have been published do not include this step, and therefore the results are twice as high). Add all the hours the oven was not used together and divide this number in half. Do the same calculation for the range. Remember only divide the result by two if an electric oven or range is used:

___ total # hours oven used (full, partial and snacks) (divided by 2 if an electric oven) = _____
___ total # hours range used (full, side dishes) (divided by 2 if an electric range) = _____

Take the number of hours the heating element for oven and range were used and plug them into the following equation:

  1. of oven cooking hours x 3.2 kW + # of range cooking hours x 1.2 kW} = _____kWh

This equation is an estimate of the number of kilowatt hours used while cooking, which is a measure of the amount of energy used while cooking.

Based upon information in a 1999 report by the U.S. Energy Information Agency on carbon dioxide one can estimate the carbon impact of cooking. The carbon footprint of cooking will depend upon the energy source used and the following conversion factors roughly apply:

  • For gas cooking, multiply the number of kWh by 0.0002045 to obtain the tonnes of carbon dioxide released per year
  • For electric cooking fueled by coal fired plants, multiply the number of kWh by 0.000963 to obtain the tonnes of carbon dioxide released per year
  • For electric cooking fueled by natural gas fired plants, multiply the number of kWh by 0.000569 to obtain the tonnes of carbon dioxide released per year
  • For electric cooking fueled by renewable electricity the carbon footprint is negligible

There is an additional benefit to cooking outdoors. If one does not have air conditioning one knows that the house gets miserably hot while cooking during the summer. Folks who lived in hot climates in bygone eras were well aware of this and tended to keep the cooking area separate from the house. If one has air conditioning one might not notice this as much, but one's air conditioning is being used to remove the energy added to the house from cooking. There are estimates that state that about an additional 50% of the energy put into the system for cooking is used by the air conditioning system to remove the heat generated from cooking from the home. For a well-insulated home, however, this estimate is far too low. A well-insulated home, recall, is designed so that energy in the form of heat cannot enter the home. If heat cannot enter the home it is also the case that the heat generated from cooking cannot leave the home. Thus, the second law of thermodynamics--which basically states that one cannot design a perpetual motion machine--requires that at least all the heat generated by cooking be removed by the air conditioning system. Measurements were recently performed when an outdoor kitchen was installed in a well-insulated house with a 16 SEER air conditioning unit. SEER is a measure of air conditioning energy efficiency, the higher number being better. The average SEER rating for air conditioning units in the United States is about 10. These measurements confirmed that at least 110% of the energy used in cooking was required to remove the heat generated from the house. Thus, if your house is well-insulated and energy efficient, multiply the number of kWh used in cooking by a factor of 2 in order to estimate the total amount of energy used while cooking including the additional air conditioning duty.

Total energy used = Energy used for cooking * 2.1 = ______ kWh

This formula does not apply to cooking outdoors during the heating season because the heat from the oven and range actually lowers your heating bill. The above formula also assumes that one has an energy efficient home. The actual factor might change considerably from home to home. The additional air conditioning duty will also impact the carbon footprint of cooking. For the measurements cited above the following numbers can be used as an example to give a ball park figure for the magnitude of the numbers that can be expected. For the purposes of this example assume that the electricity is generated from a coal-fired plant (the actual home in question uses electricity from renewable sources):

  • Number of kWh used to cook during 6 months requiring air conditioning: 900 kWh/year
  • Minimum amount of additional air conditioning usage: 990 kWh/year
  • Carbon footprint of energy used to cook with gas: 900 kWh/year * 0.0002045 tonnes/kWh = 0.18 tonnes CO2/year
  • Carbon footprint of additional energy used by the air conditioning system: 990 kWh/year * 0.000963 tonnes/kWh = 0.95 tonnes CO2/year

Thus about one tonne of CO2 is released from the home in a year owing to cooking, most of it from the additional air conditioning duty. In the above example the kWh directly used for cooking estimated using the above formulas were cross checked against the actual gas usage. Since the house has solar hot water all of the natural gas usage during the summer is associated with cooking. The formulas above were within 5% of the actual usage.

To calculate how much money this is equal to you must now take this number and multiply it by the amount the utility company charges its customers per kilowatt hour. Electricity rates tend to average about 0.10/kWh. Thus, the next step would be to take the number of kilowatt hours used while cooking multiplied by the cost.

_____kWh x $0.10/kWh = $ _____

This is the estimated cost of cooking indoors per year.

For the above example, the actual cost of cooking indoors is thus estimated as follows. The price of electricity in this case was $0.139/kWh, so the actual figures were as follows:

  • Cost to cook during 6 months requiring air conditioning: 900 kWh/year * $0.139/kWh = $125/year
  • Additional air conditioning required during these 6 months: 990 kWh/year * $0.139/kWh = $138/year

Thus in the above example it costs about $260 dollars a year to cook indoors from the formulas. In reality the direct cost as measured of cooking using natural gas was much lower, namely, $60 a year. Thus the measured cost of cooking indoors for a year was actually abou $200 a year What can be done to lower this cost?

First of all, note that cooking using an oven is much less energy efficient than cooking using a range or a slow cooker. The simplest thing that can be done is to simply not use the oven! In the above example this would reduce the cost by about 50%, or save $100 per year. The disadvantage to this approach, of course, is that for those who like broiled meats a significant dietary behavioural change would be required!

What about cooking with a charcoal fired BBQ? This would actually cost more. Charcoal cooking is horribly inefficient as one must first pre-heat the coals and then after the cooking is done the embers burn for quite some time. This is also very unfriendly from an environmental standpoint since charcoal is practically pure carbon and as such has a relatively large carbon footprint. Charcoal cooking also releases significant quantities of environmental nasties such as carbon monoxide.

What about using a propane or natural gas fired outdoor grill? First of all, from both a cost and carbon footprint standpoint, natural gas is preferred over propane. In doing so one would eliminate the additional air conditioning burden caused by cooking, and it turns out that using a BBQ grill rather than an oven to cook meat will save about an additional 25% percent in direct cooking costs since ovens are not that energy efficient. So the actual savings in the above example would be about $150 a year. Assuming a good natural gas grill with side-burners would cost about $900 and that installing the proper natural gas equipment would cost about $600, the savings would be realized in about 10 years.

What about cooking with solar? A solar cooker costs about $200. In the above example the solar cooker would pay out within one year!

Solar cooking is by far the most cost efficient way to "go solar". The payout period for the solar hot water system in the above example is not good, on the order of 30 years. Solar electric generation without any subsidies would be worse, much worse. Solar cooking, on the other hand, makes complete economic sense.

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