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Whether you are on or off the grid, it is important to minimise peak electricity demand. Off-grid, this determines inverter cost (and internal energy losses), wiring capacity, battery capacity and other factors.
Most grid-connected homes have a connection capable of delivering 20 kilowatts or more. But even for grid-connected homes, trends in electricity pricing suggest we will pay much more for using electricity at times of peak network demand.
Even now, NSW time of use tariff consumers pay over 50 cents/kilowatt-hour during afternoons and early evenings. And, for existing gas connected homes, wiring from the street or in the home may have inadequate capacity to support a shift to electric cooking.
With an energy-efficient building, it is possible to reduce total peak electricity demand for heating, cooling, hot water, lighting and IT/entertainment to less than 2 kilowatts, with some scope for load shifting for hot water.
Cooking and major kitchen and laundry appliances are a different matter. But they need not be.
A typical induction cooktop with conventional electric oven is rated at 9 to 12 kilowatts peak demand, with at least two-thirds of that for the cooktop. And for many households, timing of cooking activities is difficult to manage.
Traditionally, off-grid homes have managed this problem by using LPG cookers, maybe combined with wood. But gas cooking is very inefficient and uses a non-renewable fuel, and LPG can be expensive, with managing tanks being an extra hassle.
Once you have shifted from gas for hot water and heating, keeping gas cooking incurs ever-increasing fixed gas supply charges, so it is very expensive.
I’m confronting this issue myself, as the capacity of my (historically) gas-connected home unit’s power cable from the street is only 10 kilowatts, and the wiring within my home to the kitchen has even more limited capacity. Yet I want to ‘go off gas’ and, eventually, take control of my interaction with the electricity network.
My analysis of the energy requirements for cooking tells me that it is feasible to provide family cooking capacity with peak electricity demand under 4 kilowatts (and maybe even lower), but it will require integration of micro-storage and smart management into the cooker. It will also require much better insulated cookware, and some changes in cooking behaviour, mainly avoiding boiling/simmering with no lid on the pot.
The critical peak loads for cooking relate to the heating up of cold food and cooking equipment to cooking temperature. For well-insulated equipment (with lids on), steady state electricity requirements are much lower, in the hundreds of watts. A typical Australian household with electric cooking uses less than 2 kilowatt-hours per day (on average) for cooking, so it is not a large load – but it is very peaky.
For example, to boil 1.5 litres of water on an induction electric cooktop in 4 minutes involves a peak demand of about 3 kilowatts – but only consumes just over 0.2 kilowatt-hours of electricity. Heating an oven to cooking temperature could require 3 kW peak demand, but only about 0.4 kWh of electricity.
So building-in around 0.5 kWh of electricity storage and some peak management ‘smarts’ could dramatically reduce the peak demand of an electric cooker. This would offer some important savings for consumers, including:
All we need is for a smart cooking appliance manufacturer to focus on limiting peak demand as well as optimising cooking efficiency.
This ‘built-in micro-storage’ approach also offers a way of reducing peak electricity demand requirements for dishwashers, clothes washers and even electric kettles. It would also allow them to heat up even faster while reducing peak demand…..
Alan Pears is an energy expert and associate professor at RMIT. For those interested in a broader discussion of energy efficient cooking, his article in ReNew magazine (www.renew.org.au) provides some useful insights.
This post was published on March 17, 2016 3:04 pm
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At the current commercially available best Lithium battery energy density of 140 Wh/L, 0.5kWh would require 3.6L of battery, too big to be incorporated into anything smaller than a stove. Since these appliances will be supplied at 240V, the wiring is no heavier than at present so why not just use some of the storage capacity of a standard PV-battery-inverter system in the garage or elsewhere?
For about 20 years now we have been using a convection microwave as our oven to cook everything that needs to go in an oven.
I'm not sure how this stacks up on power consumption but I'm sure it must be less than a conventional electric oven as it simply doesn't take as long to cook things.
The Sharp microwave we currently have is rated as a 950 watt unit.
Might be worth investigating both in terms of power consumption and also to alleviate the need to upgrade the wiring etc.
I agree microwave is efficient. The battery storage and inverter would still need to be able to provide that 950W.
My supplier (NSW Essential Energy area) charges 25.25 c/kWh including GST for peak and shoulder usage and I suspect charges in large cities would be even less. Our meters don't provide time-of-use data so we pay 20.72 c/kWh. Nowadays, connecting to the gas grid is unlikely to be cost-effective, but the rental on an LPG bottle might be worth considering for off-grid households. I somehow doubt that peak demand charges will increase to the extent that it becomes worthwhile to switch from electric induction to gas, but it's always worth knowing the tariff structure to make it easy to determine when it will be worthwhile to add storage to a solar system.
Some people get fooled by some utility's billing terminology of peak. In your case peak doesn't mean true peak time other wise you would be paying 50 cents.
Most time of use meters in NSW provide info on peak (7 - 9 am & 5 - 8 pm), shoulder (9 am to 5 pm & 8 -10 pm) and off-peak (weekends & 10 pm to 7 am). Most electricity suppliers have very similar charges for peak and shoulder, with off peak (i.e. weekends & 10 pm to 7 am) at 13.17 c/kW. Despite having solar, my own house doesn't have the metering for this tariff, but I do have some rented properties on a net feed-in tariff that do; surprisingly 51% (winter) to 58% (summer) of the electricity usage is at the weekend/overnight rate. I've asked Powershop about smart meters, but they still haven't worked out the details about any offers they might make for customers who will be looking to change their metering when the gross feed-in tariff runs out.
Not surprised about the last bit. I think all utility's plan is to stop us from making a decision on whether to go off grid or stay and if we do clout us.
For cooking we use 9kg LPG bottled gas, cost, $18 gives us 2-3months use.
$18/quarter is much cheaper than (e.g.) AGL's $51/quarter fixed charge!
Yes it sure is and it's amazing how many people aren't awake up to the savings or can't be told.
yep I have fork lift cylinders that are mine and is propane filled @ 60c. Conversely install more panel if u can then cook during the day. There's more than one way to skin a cat
We are an off grid community in Canada for the last 15 years. We have 12 kw PV and 40 kwh batteries. Our inverters can handle 9kw. What we do is use microwaves, insulated electric water boilers (Zojirushi are the best) induction electric single burners, electric rice cookers, toasters, etc. We also have two propane ovens, one 100% non electric. The other has a 500 watt "glowbar" in the oven to back up the propane. We virtually never exceed 4 kw draw. The single plate induction units are really cool, as you can add them one at a time and put them in convenient places. Remember, every day we cook full meals for up to 25 people all on solar/battery and it is every bit as convenient as on grid.
Right now we vastly overproduce electricity about 9 months per year so I am planning on increasing our induction electric cooking to zero the propane. Propane is always handy as back up and the amount used for cooking is trivial. One thing though: the chemical side effects of open flame gas are not healthy and need full venting at all times during cooking.
We sorted this out years ago. We are off grid with 2.4kw inverter sitting on a 25kwh battery.
Our kitchen has a gas cooker (just in case) but it has never been used. All our cooking is done electrically. No induction appliance is installed as they are too big on power. Everything is done with microwave oven, frypan, slow cooker, deep fryer, steamer, kettle, toaster etc.
No problem, no cost, no emissions.
Great article, Alan, as usual.
Fwiw, we have used various convection microwave cookers since 1979 (currently LG Solardom), and induction cooktops since 1998. I hate the idea of gas in the house (various reasons) so that has never been an option. My partner has become a great cook, and is energy conscious. She read your article and really emphasised the value of quality cookware - as you go induction, as usual, you pay for what you get. She also bought a Thermomix two years ago, and it gets used daily, again, within our self-imposed peak kW limit. We are looking at the Sunny Boy Storage unit, with a 2.5 kW limit for after dark use, and are confident that we won't over stress it.