Peak electricity demand and going off the grid: the cooking dilemma

One Step Off The Grid

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:

• Avoiding the need to upgrade wiring from the street, and within the house, when shifting from gas cooking to electric: this is potentially a substantial cost saving, and breaks down a potential market barrier to the wider adoption of induction cooking in existing homes with gas cooking: the electric cooker industry is missing out on a big opportunity!
• Reducing the peak system capacity requirements for off-grid households to use electric cooking
• Reducing evening and morning peak demand on electricity networks

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 article was first published in One Step Off The Grid, where you can read more stories about people’s experiences with solar, storage, energy efficiency and other technologies, both on and off the grid.