Newsletter Archive

E-News Issue #30

5 April 2000

Your views

Don't forget to keep us informed about any ideas, problems, product information etc that you'd like to share with us.

Cooking with solar

This month's article is about using your excess PV power for cooking. Sorry if it is a bit irrelevant to you folks with a small system.

COOKING WITH SOLAR (PV)

I recently upgraded my solar system under the AGO rebate program.

The system (24V) is made up of eight 80W PhotoWatt and four 55W Arco solar modules together with a Selectronic SE12 inverter. On 24V this inverter is rated at 700W continuous and 900W for half an hour. Using Rainbow Power Company's software I chose to design the system to meet all my loads, even in the 'worst' month.

This means that my regulator will be reducing my available solar charge current during the remaining eleven months
(based on average weather conditions affecting actual daily solar insolation). My Plasmatronic PL40 regulator has the ability to divert the charge current or turn on other loads when the battery is fully charged.

Water pumping is sometimes a good choice, however I have an abundant supply of gravity feed water. The design software suggest that most months I'd have the power from 1 to 2 of my 80W solar modules to spare.

When the price of LP gas went up recently, I decided to look into electric cooking appliances.
A summary of the smallest (lowest wattage) appliances is listed below.
I found K-Mart to be the best place to find small, low wattage gadgets.

ApplianceCost (AUD)Watts (on label)Watts DC*
Slow cooker/crock $ 100 285 -
Bread oven $ 279 470 480
Snack oven $ 53 650 660
Coffee maker (5 cups) $ 38 685 704
Frypan (230mm) $ 74 700 752
Sandwich maker $ 40 750 803
Toaster $ 19 900 982
Hot plate $ 35 1100 1099
Microwave oven - 1200 -
Electric jug - 1400 -

* All measured wattages were taken on the DC input side of the inverter and measured by the Plasmatronic regulator.
The actual battery voltage was used to calculate the figure. This was usually about 25V.

  • Slow Cooker:
    I haven't bought this appliance but I hear that they are great for cooking some foods.
  • Bread Oven:
    This appliance was discussed in detail last month. They use between 250 and 650 Wh to make a loaf of bread. The average was 330Wh. A great way to use your excess solar power!
  • Snack Oven:
    The Ronson snack oven is designed to heat pre cooked food like grilled cheese sandwiches.
    The top griller used 365W, the bottom one used less at 285W and together they used 660W on my inverter.
    It takes about 8 minutes for a sandwich (88Wh). Larger oven grillers are available to cook meat etc however they are rated at 1500W.
  • Coffee Maker:
    My Mistral coffee maker is perhaps the best bargain! It is described as a "5 cup" unit - three 500ml mugs by my reckoning! It makes a mug full of great coffee in about 3 minutes using a miserly 59Wh of power. A bargain buy at $38!
  • Frypan:
    This Black & Decker 230mm skillet is a bit small for anything larger than 2 medium sized chops or 3 - 4 eggs. However, it worked well. It takes about 12 minutes to cook two thick steaks or fish cutlets or 150Wh. Because the element is bonded to the base of an electric frypan, I think they are more efficient than using an ordinary one on a hot plate. The frypan was not supplied with a cover though later I saw a similar sized Sunbeam one with a tin lid. Haven't been able to find a glass lid to fit it as yet. Larger "family sized" frypans were rated at between 1200 - 1600W.
  • Sandwich Maker:
    These are similar to waffle makers and you can prepare fruit or savoury hot sealed sandwiches in it. Takes 5 minutes or 67Wh to prepare two pieces.
  • Toaster:
    Doesn't need much description - it takes 2 to 3 minutes or 41Wh for 2 pieces of toast in the morning plus 59Wh for the mug of coffee!
  • Hot Plate:
    I initially chose not to purchase this appliance because its 1100W rating was in excess of the 900W thirty minute rating of my inverter. Interestingly, Selectronic give a second 45A thirty minute rating on this inverter which is actually 1100W (on the DC side) if one's battery voltage is 25V. My other cooking appliances were still not using my surplus power which infuriated me each evening when I turned on my LP gas!
    I took a punt and purchased a Tiffing brand model.

    I was pleasantly surprised when it only drew 46A @ 25V and my inverter could cook the evening meal. On the lower heat settings it cycles on and off for about a minute at a time. It takes about 7 minutes to bring 750ml of water with broccoli, brussel sprouts and corn on the cob to boil and about 3 minutes of simmering thereafter (about 156Wh).

    Fish and meat takes 6 - 12 minutes to fry depending on how thick it is. In all, a meal for two consisting of boiled vegies and fried meat/fish used an average of about 325Wh. However, meals with foods like rice, dried beans etc would take a lot longer to cook and use up more power. Cooking times are also affected by variables such as ambient temperature, type of pot etc.
  • Microwave Ovens:
    I have not purchased one as even the smaller units would use 1200W or more. Be aware that the glossy labels on the front of these units refer to the wattage of the output cooking power. This is around two thirds of the electrical power which you'll find on the compliance label! However, they are a very efficient form of cooking. According to Choice magazine around 75% of the heat produced during conventional cooking ends up warming water, air or oil rather than cooking your food.
  • Electric Jugs:
    Again I didn't purchase these as the smallest one was rated at 1400W. The old fashioned immersion type elements were 1600W! They're made to boil that water very quickly.

Conclusions:
All the appliances mentioned (except the micro wave oven) are known as resistive loads with a unity power factor. This means that they are relatively easy loads for your inverter. If one uses foods and recipes that are relatively quick to cook, a couple of 80W solar modules could supply 1 - 2 people with enough power to cook their meals.

However, before buying such appliances, you should speak to your solar supplier for accurate advice about your personal cooking needs, the average monthly solar insolation to expect in your area, the limitations of your inverter etc. If you need to run a generator to charge your batteries, then you should cook with wood or LP gas!

If you live on the electric utility grid powered by non renewable sources such as coal, then you'd contribute more carbon dioxide greenhouse gases using electricity. According to Choice magazine electric cook tops are more energy efficient than gas because only 40% of the energy produced goes to heat the saucepan and contents versus 60 - 70% for electricity. However, some two thirds of the energy in coal is lost through its conversion and power transmission to your home. So gas cooking is recommended for those of you on coal fired electricity.

So if your solar system is producing surplus power (and your inverter is big enough) then buying a few cheap 230V cooking appliances will use up your excess power and lower your fuel bill!

Cheers from Dave and all the RPC crew.

Dave Lambert (Director)

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We install solar systems in Northern NSW and Southern QLD.


QLD:
Gold Coast (from Coolangatta to Southport), Nerang and Hinterland (Beaudesert) and out West (Warwick, Stanthorpe, Killarney)


NSW:
Northern NSW (Tweed Heads to Yamba, including Evans Head, Byron Bay and Ballina); the Far North Coast Hinterland (Grafton via Lismore to Murwillumbah) and out West (Casino to Tenterfield, including Drake and Tabulam, as well as Woodenbong and Bonalbo)

For larger system we also go up to Brisbane or down to Coffs Harbour and even Glen Innes. Other places by arrangement.