How do I use electricity throughout the day – the load curve

The previous article, how to save energy in your home, told you how on aggregate you use electricity in the home – that is, what percentage of your power is used by certain appliances. But how do you use electricity during the day, hour by hour? What appliances are on and when, and how does your consumption look in the morning, evening, afternoon and in the middle of the night?

This information is of great interest to households with a solar photovoltaic (PV) system, as they would like to know how to maximise the amount of energy they export to the grid, particularly when their state has a Solar Feed-in Tariff.

This answer to this question is in what electrical engineers and energy analysts call the “load curve” or “load profile”. It’s a graph that shows us what electricity consumption is at different times of the day, and what the components that make up that total are at different times. The load curve for a typical home is actually different in each state of Australia due to climatic variations, the usage of gas for heating and hot water, and different home appliance penetration rates.

The summer load curve

There are two very distinct load curves – one for summer and one for winter. In actual fact the load curve for an extremely hot summer day is quite different from a mild summer day (basically because of air conditioning) as it is for a very cold and mild winters day (due to heating). As an interesting little fact, engineers can tell when a really important football match is on in England because everyone turns on their kettle at half time, and you can see this on the national load curve! Nevertheless, we can generalise to get a typical load curve for summer and winters days.

The graph below show us how we use power on a summers day in NSW. It is only the load curve for households across NSW (commercial and industrial load curves, once again, look quite different). It is broken down into a rainbow of slices that depict what appliances are using power and when.

The graph starts at midnight, and surprisingly, at this point in time it is at its zenith. Why is this? Won’t most people be in bed, or winding down for sleep then? The answer is hot water. As the cream coloured section of the graph shows, all the off peak electric hot water heaters are programmed to come on in full force at midnight and 1am (to take advantage of cheaper electricity at night), and this consumes a massive amount of power.

As you would expect, the load curve goes down a lot in the wee hours of the morning when everyone is asleep and using little energy. However, it doesn’t go anywhere near zero, because a whole lot of appliances like fridges, freezers, hot water heaters and standby equipment are still on. Notice that they make up a very uniform stripe of consumption – they are always on, and hence take lots of energy over the course of the day.

The load curve then builds in the morning and has two little spikes, one for morning tea and one for lunch. You’ll notice that those two little bumps are mainly caused by people using hot water on demand, and hot water tanks needing to replenish used up water. A lot of dishwashers, kettles and cooking appliances tend to come on at this time too.

After about 1pm the airconditioners start to come on and the load profile really starts to climb. By 5pm they are on at full steam. At about this time people come home from work and school and turn on all of their lifestyle appliances such as TV’s, cooking equipment as well as lights throughout the home, adding to the peak.

This starts to settle down at around 9pm as people begin winding down, but then 9:30pm the hot water off peak ramp up begins, and demand soars back up.

As you can see, air conditioners, hot water heaters and refrigerators are the dominant parts of this diagram. And this is reflected in the previous article how to save energy in your home, where they weighed in at 23%, 37% and 12% respectively.

The summer electricity load curve for households in NSW
Source: EMET Consultants Pty Ltd

The Winter load curve

The winter load curve is markedly different from the summer one, basically because of the effect of temperature. It is characterised by the twin peaks or morning and evening, whilst summer is a big hump in the afternoon. Winter starts off at midnight in much the same way as summer, with off-peak hot water heaters on at full steam. However, you’ll notice in comparison that it’s much higher in total, almost by 1000 mega watts for the whole state. This is because in winter, hot water takes even more energy to heat up then it does in summer (the water starts of colder).

As the hot water heaters ramp down the wee hours of the morning look much the same as they do in summer. Notice, however, that the stripe of consumption for refrigerators is comparatively lower, as they now have to work less hard as the air outside is cold.

Now the differences really begin. People begin waking up and straight away turn on heaters, lights and take hot showers. This causes the morning spike, which gradually eases off throughout the day as people stop using hot water and turn down heaters. In winter the middle of the day is quite quiet, but in summer, as we saw before, things are on the move.

At 5pm the explosion in demand starts again, and the hot water, heating and lighting load is coupled with cooking and entertainment, making it peak at about 7pm as people watch the news and cook dinner. Eventually this eases down before the same 9:30pm spike that happens as in summer for hot water heating.

 The winter electricity load curve for households in NSW
Source: EMET Consultants Pty Ltd

These graphs show how electricity consumption is primarily driven by temperature. A lot of the little things we do, that we don’t quite realise, are what govern our energy consumption.

Now to the next issue: How does my consumption of electricity compare to my generation via solar PV cells, and how can I optimise my electricity use timing to extract the most benefit?

Kobad Bhavnagri
Solar Energy Consultant