Cooking appliances on an Inverter

Cooking our food uses a lot of power. You could try solar cookers or ovens which work directly off the sun. These are a slow type of cooker, that retain high levels of food nutrients and work well. However, apart from sunlight you also need patience.

In general, we suggest that if you are on a solar, or other Renewable Energy system, you would do all of your cooking on gas or on a wood stove. However, in good weather, you may often have a surplus of power and some forms of electric cooking are surprisingly efficient.

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 Plasmatronics 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$100285----Bread oven$279470480Snack oven$53650660Coffee maker (5 cup)$38685704Frypan (230mm)$74700752Sandwich maker$40750803Toaster$19900982Hot plate$3511001099Microwave oven----1200----Electric jug----1400----

*All measured wattages were taken on the DC input side of the inverter and measured by the Plasmatronics 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: See 'Bread-makers' tab. 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 or 150 Wh to cook two thick steaks or fish cutlets. 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. It cycles on and off every few minutes when set to the lower temperature settings.

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 using a 2 slice model, a 4 slice model used about 16wh per slice, plus 59Wh for the mug of coffee! Our technician Ray later found a 700W model and tried it out - it drew 31.5 amps on the DC side of his inverter and took 2.2 minutes on setting 4 to make his wholemeal toast. That is about 30Wh per toasting.

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 Tiffany 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, Brussels 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 veggies 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: See 'Microwave Ovens' tab.

Boiling Water

An electric jug or kettle is not usually recommended on a solar system, as they require quite a lot of power.

Most electric jugs are 1.5 to 2 litres in size and are rated between 1600 and 2200 Watts. I managed to locate a small electric kettle at Kmart, which was rated at 1250 Watts. It holds one litre (4 coffee cups). The model was a Kambrook KE9.

Using my 1600 Watt Selectronic SA22 inverter it used 1355 Watts (54 Amps @ 25 Volts). With my water at 17C it took three minutes to boil 500ml or 5.5 minutes for one litre. Total power used to boil one litre was 5 Ah @ 25Volts.

Bread-makers

HOW I BAKED MY FIRST LOAF OF BREAD

I recently upgraded my own solar system, thanks in large part to the new Australian Greenhouse Office 75% rebate, so I decided to purchase a bread maker.

For those of you not familiar with this product, I'll briefly describe what it does. In simplistic terms, it bakes your bread with about 30 seconds of work - just measure your flour, water, yeast, salt and any other goodies you want to put in your bread. Select the loaf size, crust colour, flour type and the time you want your loaf to be baked by and the machine does the rest! Over the next 2 - 3 hours, the unit goes through several cycles: two types of kneading, a rise, first punch down, second rise, shape, third rise, bake and an optional keep warm cycle.

I did some research first. I had a look at a Choice magazine article (published by the Australian Consumers Association). They tested 11 models and reported that all but one baked an excellent loaf of bread. They calculated the cost (with grid electricity) at less than half that for a store purchased loaf. The article suggested that one look for the following features: adjustable loaf size, removable pan and lid for easy cleaning, timer, keep warm function, crust colour selection, capability to make jam, pasta and pizza dough, fruit/herb dispenser. We asked the author how much power they used and she reported that they used between 250Whrs and 650Whrs with an average of 330Whrs per kilo loaf. However, the Choice article didn't tell me how they'd run on my SE12 Selectronic Sine Wave inverter (700W continuous rating). I remembered some of our customers had given us some good feedback in this regard so I consulted the archived editions of the Rainbow Power Company's free email newsletter. Someone had earlier warned us that the Panasonic SD250 had problems even on a sine wave inverter.

In the December 99 newsletter, Stephen Hart had reported:

"I currently have a Breville BB400 bread-maker. It works perfectly on both my previous modified square wave inverter (Selectronic Silver Series 600) and my new sine wave inverter (SEA Boxer 1500). On the SS600 it was slightly noisy, as you would expect from any induction motor. On the sine wave it performs just like on the mains. This is a fairly expensive machine ($279 at Target stores) but has more features than any other machine, including 30 minute power failure memory protection. It also has the lowest power consumption at only 425W. My only complaint of this machine is that it is quite noisy when kneading. The pan fits fairly loosely in the machine, and the kneading paddle fits loosely on the shaft, so it tends to rattle and clang a bit.

I went through several other machines before getting this unit. (Thanks to Myers stores for their flexible returns policy). Details as follows - Sunbeam BM100 - not tried on inverter. During the heating cycles the 600W element is switched on and off several times per second as the chamber approaches the cut off temperature. You can hear the relay "fluttering" inside. My understanding is this is no way to treat a relay, and I doubt an inverter would appreciate it. I phoned the service department who said to return it to their service agent, they thought it didn't sound right, but when I took it to the service department, they said it was normal. I got two more under warranty but they did the same. Returned for refund... Breville BB350: Worked OK on modified square wave inverter (SS600) though quite noisy, more so than the BB400. This model proved quite unreliable. I had three replacements under warranty, they all died mid-cycle after a couple of weeks each (on the mains, not on inverter). My sister had one of these too, which lasted about four months, the warranty replacement has lasted three months so far... Panasonic SD250: This machine was streets ahead on quality. It was quiet but was completely useless on the modified square wave inverter. It wouldn't even start ...as soon as it was plugged in you could hear a relay clicking on and off about once a second. No other signs of life. I didn't own the sine wave inverter at the time so didn't try it on that."

Armed with this information, I visited my local appliance store. The prices ranged between AUD$99 and $279. The wattage on the compliance plate ranged between 450 and 700W. I found the Breville BB400 recommended by Stephen. Unfortunately it was the most expensive one at AUD$279. However, it had all the features recommended by Choice magazine and I knew it would work successfully off my inverter. The price included 6kg of flour and a colour book with about 100 bread recipes to tantalise me. So I lashed out and bought it, got home and a few minutes later I was baking my first 500g loaf of bread!

At a battery voltage of 25.0V the bread-maker drew about 100W while kneading. While baking, it drew a maximum of 480W, however this cycled on and off every few minutes. The power used for a 500g loaf was 210Wh (about 10Wh for kneading and 200Wh for baking). A one kilo loaf would use about 265Wh (all measurements taken on the DC side of the inverter using the Plasmatronics PL40  regulator with shunt).

The economics of the exercise are impressive. The unit is obviously very efficient. However, having designed and sold solar PV systems for some 15 years, my general advice has been that solar PV is not cost effective for cooking and heating. Times are changing! Appliances are becoming more efficient and the cost of solar modules is coming down (especially if one considers the AGO rebate. If I baked a loaf a day, I'd save about $400 per year compared with store bought bread (not to mention the convenience and taste sensation of home baked bread). Assuming one lived in a reasonably sunny location and baked during the day, an 80W solar module should be able to provide the power. At a cost of about AUD$700 for an 80W solar module I think my accountant would suggest that I made a wise decision (even if one was to consider the cost of the bread-maker and arguably a larger battery than might be needed!)

Oh, and before I forget, the bread was wonderful!

Note
If there are no other loads on your inverter at the time of baking, you may need to put your inverter in 'Run Mode' rather than on Standby. During certain cycles, the dough is just allowed to 'rest' and only the timer in on which is probably not enough to keep your inverter running. The bread oven then gets confused.

Microwave Ovens on an Inverter

Microwave ovens are an energy efficient method of cooking many food items compared with using an electric hotplate, stove or oven. However, they still use a lot of electricity on a Renewable Energy system. We normally recommend using LP gas for most of your cooking needs unless you have surplus power from your system.

The power used by a microwave oven is also a bit confusing for the novice to appreciate. When you look at microwave ovens in the shop, most will have a big glossy label advertising the output power. This label will typically read between 500 - 1000 watts. Many people will assume that this is the power that the appliance consumes. This is not the case! The actual power drawn from the power point is typically 40% greater. This higher figure will usually be written on the compliance/specification label on the back or bottom of the oven.

The situation is worse again when you run the microwave off an inverter. They have a fairly poor power factor and are not an efficient or easy load for an inverter to run.

We recently checked a typical microwave oven. The glossy label advertised the output power to be 800W. The compliance specification label read 1150W. When run off a Selectronic SA22 Sine Wave inverter, the current draw on the DC side was 70A @ 24V or 1680W - more than double the advertised 'output power'.

Depending on the size of your system and the current state of charge of your batteries, this may still be an acceptable load when you consider that a microwave oven is often only on for 3 - 5 minutes.

Rice Cooker

I recently bought a rice cooker. They come in a variety of sizes (generally referred to 3, 6 or 9 cups). The so-called 6-cup is the most common size.

I decided on a Tiffany brand 6-cup model. The compliance label rated it at 350 Watts. On the DC side of my Selectronic SE22 inverter it used 412W.

Using it to its full capacity required about 900 grams of wholegrain rice (about a one litre container). I added 1.3 litres of water and turned it on.

Some 1-¼ hours later I had three litres of beautifully cooked rice. Total power used was 20Ah@25.8 Volts (about 515 Wh).

I also tried it with 1 kilo of white rice and 1.3 litres of water. Interestingly it absorbed the water and cooked a lot quicker. It took about 45 minutes and only used 310 Wh of power.

A rice cooker works by having a thermostat touching the bottom of the inner pot. Once the water is all absorbed, the temperature of the pot bottom goes up and it shuts down. The element is also on the bottom. I feel a rice cooker is quite energy efficient as most of the heat from the small element is directed to the bottom of the inner pot. The 20mm air gap between the inner and outer pot is quite a good insulator, as the outer pot only gets a bit warm to touch.

For those of you who like a bit of spice in your life, try adding a few bay leaves, Garam Masala, Cardamon or Italian Herbs to the pot. If you like a lot of spice to help warm you up those winter nights, try adding a bit of Tandoori spice or chilli powder. For a yummy dessert, try using some coconut milk, currants and a little cinnamon.

While we don’t normally suggest using solar for your cooking requirements, you may find that you have excess power on sunny days. Why burn wood or gas if you have excess solar power?

Convection Oven

I recently bought a bench top Convection Oven from a supermarket chain-store for $59. This has a large round 12 litre glass/Pyrex oven. The glass lid has a fan forced element on it. The hot air circulates around the food which is placed on a rack on the bottom of the glass bowl.

I put a large 2.1kg chicken and later added 4 medium sized potatoes in it and set the temperature to 200ºC and the timer to 75 minutes. The oven is rated at 1200 watts and the whole cooking process used 1.28kWh. If you are connected to the grid, the cost for the power would have been about $0.20c which is pretty reasonable. If you were on a solar system, you would have needed 3 – 4 large 130W solar panels to generate this amount of power. As I said earlier, this may be an option in good sunny weather when you have surplus power to use, and do not want to heat the house unnecessarily.

BTW the chicken was perfectly roasted and my neighbour said the lamb shanks he cooked in his were perfect.

Cooking with Gas

Using a Westinghouse™ GXL540SLP gas oven.

This oven actually turns the gas OFF and re-ignites it every time the thermostat registers a fall in temperature. This is done with a device called a flat plate igniter. A quick check on the internet indicates that these are now commonly used.

The Rating plate on the oven says 730 Watts. The ONLY other electric device in stove is a small oven light ... maybe 20 or 30 Watts. There is not even an electric clock or timer (its mechanical).

The oven was running for 2 hours and 5 minutes; the meter registered 0.42 KWH in this time.

I made sure the owner actually cooked for the whole time (apart from warm up) so the oven had a real world heat loading.