UK utility Centrica provoked some debate last week when it proposed the introduction of “time-of-use” tariffs to the UK electricity market. Customers with smart meters could be offered deals giving them different power prices for different times of day, or even, as caught the eye of the general press, free electricity on Saturdays.

Centrica, which over recent years has become a major player in the US, is already testing such offerings with its Direct Energy customers in America.

Such deals would not likely be introduced in the UK until next year, and the majority of UK customers do not have smart meters yet. But that didn’t put people off wondering how to best make use of free power.

Saving laundry — and the use of energy-hungry tumble driers — to Saturday is perhaps the simplest possibility, though others suggested cooking meals (with an electric cooker) at the weekend and then freezing them for the week ahead.

Of course, appliances like freezers can’t just be turned on one day a week, and a “free electricity Saturday” tariff might also charge customers higher on other days of the week in return. So customers would need to weigh up the benefits carefully before switching.

But the proposal highlights an important aspect of the energy supply debate. Meeting demand for electricity or gas supplies is not simply a question of bringing in the right amount of power and gas over the year as a whole, but providing it at the right time.

That has implications for the size and flexibility of infrastructure that must be built. Smoothing out power demand over the course of a day, or week, could eliminate the need for an expensive power plant that would otherwise generate only a few hours per day, but need to be rewarded for standing in reserve.

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In the gas market, pipelines, such as the UK’s national transmission system, need to be able to operate whether demand is 450 million cubic meters/day in the middle of winter, or 140 million cu m/day as in the current hot summer. For much of the year capacity may be available, but unused.

The German power market, however, offers an interesting example of how to tackle peak demand. The growth of renewable energy capacity in the country over recent years could actually be removing the need for conventional gas, coal or oil plants to fire up for peak load.

German solar power generation can now on certain days meet the majority of extra demand seen during peak hours, allowing other plant to operate in constant baseload mode, without having to stop and start.

The graph, from EEX transparency data, shows expected power generation over the course of one day this week. Wind power is not particularly strong on the day in question, but the solar output can be seen to make a significant contribution. Of particular benefit is that the solar generation capacity provides most electricity during the daytime hours when power is most needed.

According to German renewables research group IWR, Germany generated a record 5.1 TWh of electricity from solar in July, up 37% from the year before, and meeting more than 10% of total power demand for the second month in a row.

Solar output hit a new record of almost 24 GW of production early in the afternoon of July 21. The country now has a total installed solar capacity of 34 GW, making it the country’s largest source of power by installed capacity.

Germany has on occasion this year faced the opposite of a peak power demand problem, and has seen weekends when wholesale power prices have hit zero, or negative levels, as peak solar and wind output flood the system.

Energy network operators will face new challenges over coming years as they learn to deal with the effects of major changes in patterns of both demand and supply.

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