The case for moving air conditioners off-peak

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There are two ways of dealing with the projected growth in peak power demand. There is the lazy, business-as-usual approach that simply upgrades the power supply system to meet projected demands. Then there are approaches that actively look for ways of reducing peak demand.

This article looks at the potential benefits and problems associated with reducing peak demand by switching enough air conditioners from “on demand” to off-peak power[1].

Switching air conditioners to off-peak would not be acceptable to the community if the result is a reduction in comfort levels. However, this comfort problem can be avoided by storing enough cold (or heat) to allow room temperatures to be controlled while the air conditioner heat pump is not running. A number of systems are commercially available which could be used to provide this storage.

Avoiding the power price required for major power-system upgrades is not the only potential benefit of this move to off-peak power. Consumers may benefit not only from the use of cheaper off-peak power, but also from lower power consumption. In addition, the switch to off-peak can be made in very small steps where and when required.


How much might peak demand be reduced by moving air conditioners to off-peak power?

The following graph from Energy Action shows that the average peak power consumption for the three hottest NSW days during 2011 was a massive 45% higher than the average daily peak for the year.

Most of this increased demand on very hot days will come from air conditioner heat pumps. However, heat pumps on refrigerators, cool rooms and freezers will also consume more power on hot days. The graph also shows the potential for using off-peak for reducing peak demand.

In winter, power required for heating will contribute to peak demand. For 2009 to 2011 NSW annual summer peaks ranged from 13,765 to 14,580 kW. Annual winter peaks ranged from 12,908 to 14, 274 kW.

Air conditioners have such a large effect on peak power because their average power consumption increases rapidly as the difference between inside and outside temperature rises.[2]

How might enough cold be stored to avoid loss of comfort when air conditioners go off-peak?

There is growing interest in the use of Phase Change Materials (PCMs)[3] to store both cold and heat. The commercial PCMs that might be used for air conditioning applications behave like “ice with a different melting point”. Like ice, they can store a large amount of cold in a small volume and will release this cold over a narrow temperature range. Unlike ice, a PCM may be selected with a melting point closer to the desired room temperature. For example, the PC25 product produced by Australian manufacturer PCP Australia melts at 25 deg C and requires only 15 litres to store one kWh. The heat required to melt PC25 is enough to raise the temperature of the melted PCM by 55 deg C. (Less energy is wasted the closer the storage temperature is to the desired room temperature[4].)

The simplest alternative avoids the need to change the air conditioning system by using PCM products such as Dupont Energain Thermal Mass Panels to store cold within the room. One disadvantage is that stored cold can be wasted because the panels will continue to cool the room when this cooling is not needed. The room may also need to be kept at uncomfortable temperatures while cold is being stored.

The preferred alternative places the PCM in an insulated storage tank outside of the room. The system is set up so that the air conditioner system cools the tank. Cold water from the tank is then pumped to the rooms for temperature control when required. This PCB Australia circuit shows one way that this could be done. Separating cold storage from the room avoids the waste of cold when the room doesn’t need to be kept cool as well as providing more flexibility in the choice of PCM melting point. The PCM and water in the storage tank have to be separated to avoid contamination of the PCM.

Power can be saved for both alternatives if cold storage can be scheduled for cooler times of the day when the difference between storage and outside temperature is lower.

Either of the above alternatives could be used to reduce peak power while keeping rooms warm during winter. PCM based cold storage can also be used to allow the heat pumps on cold stores, freezer rooms etc. to be switched to off-peak power.

Would all air conditioners need to be converted for off-peak?

No. Air conditioners would only need to be converted for off-peak when and where the need arises. Some consumers may decide to convert their air conditioners to off-peak early to take advantage of lower electricity bills.

Who should pay for the move to off-peak?

All power consumers should benefit from lower power prices because the need to spend billions on power system upgrades disappears. It doesn’t seem unreasonable that part or all of the cost of converting for off-peak should be shared by all consumers.

Are there better alternatives?

A number of alternatives have been suggested. These include:

1. Using market forces to increase the price of power during peak demand periods: Hard to imagine many people turning off air conditioners during a heat wave just because the power price is higher.

2. Using solar PV: Problem is that NSW summer peaks tended to occur mid to late afternoon when solar PV output is dropping. Winter peaks tended to occur after the sun has set. Solar PV and off-peak air conditioning with cold/heat storage may be a good combination.

3. Moving something else to off-peak first: It may be more cost effective to move things like dishwashers, clothes dryers and aluminum smelters to off-peak before starting on air conditioners.

Conclusion: Moving air conditioning to off-peak power as an alternative to power supply upgrades warrants serious consideration.

John Davidson is a retired process engineer with an interest in climate action. He has no association with PCP Australia, Dupont or any other company that is involved in heat or cold storage systems.


[1] Only the air conditioner heat pump would be on off-peak. Controls and some fans would still need to use on demand power.

[2] Both the flow of heat into a building and the power required to pump a unit of heat out of a building are proportional to the difference between inside and outside temperature. Doubling this temperature difference would increase average heat pump power consumption by a factor of four.

[3] For background notes on PCMs and their applications see here.

[4] Doubling the difference between storage and outside temperature doubles the energy required to store a unit of heat or cold.  

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  • Warwick

    The NSW demands are out by 3 orders of magnitude…it should be MW not kW.

  • Chris Mrakas

    “You cannot manage what you cannot measure.”

    The root of the problem is that we do not measure the important things in a timely and meaningful way. And more importantly you must normalize the data to compare year to year.

    To underscore the complexity of the issue. As an example if you spend $50k on aircon in 2008 and $60k on aircon in 2009 does this mean you are inefficient in 2009. What if I told you that in 2009 we had lots of heat waves (including four days of 40c in a row). At GreenBox our technology takes all this into account to provide real-time measurement and normalization.

    Most of the buildings that use air conditioning do not measure energy use of these climate systems. This applies to commercial and residential. If the question was asked how much energy is used for climate control – I doubt that anyone could answer this. It is certainly not available from meter level monitoring. Then knowing this is not enough. You need to know the outside temp and inside temp.

    Most occupants of buildings do not understand the building inherent climate signature. (building signature). This is the relationship between internal climate with power, climate systems and external climate. Significant wastage is the result.

    The internal climate of a building is related to many influences for example some are: how many occupants are in the building, duty cycles of equipment, thermal properties of the building and the amount of exposure of the skin to the sun and other elements. All this once understood, measured can be made more efficient. Many systems operate when they do not need to. Many systems have been designed and installed with inherent flaws.

    Amory Lovins discusses many of these in his papers.

    It is too simplistic to think that moving energy consumption to off-peak for air conditioning will solve such a complex problem.

  • Dave Smith


    This idea is certainly worth considering. A couple of points:

    (1) Get electricity prices right (ie with time-of-use distribution prices properly reflecting a distributor’s cost drivers) and I am confident that entrepreneurs will soon be developing and marketing the kind of system you describe. No need to introduce new cross-subsidies; just unwind the existing ones.

    (2) From an engineering point of view, it is unnecessarily complicated to pump cold water around the house to cool it. The cold storage can simply be used to cool the a/c coolant, which is then used to cool the house in the normal way.

  • Warwick: Quite right, the NSW peaks are in mW,not kW.

    For some reason none of the links came through. For those that are interested the links are:

    PCM Background notes:
    Energy Action:
    PCP Australia:
    PC25 product details:
    PCP Australia circuit:
    Dupont Energain thermal mass panels:
    NSW peak power data:

  • I haven’t got the data to determine how many (if any) householders could justify installing PCM storage on the basis of savings on their power bills. The real justification comes from the collective savings to all consumers that would come from reducing the need for power system upgrades. Various sources claim that the subsidy on air conditioning id about $7000 per air conditioner. This subsidy comes from the costs associated with the spending on power system upgrades required to handle the power demands of air conditioners on very hot days. I would be surprised if the cost of inserting the heat/cold storage tank would be as high as this. (And keep in mind that not all air conditioners would need conversion.)
    Also keep in mind that avoiding the need for power system upgrades requires the conversion of enough air conditioners in specific locations. This is unlikely to happen if we are depending on householders with money to invest taking the plunge. We need the certainty that comes from generous subsidies (or a reverse auction system which allows householders to say how much they are willing to commit.)

  • An even cheaper option before you start going down the route of storing coolth.

    Half of the air conditioners out there are C.O.P 2.0 (average of units installed prior to year 2000)

    Upgrade all the old C.O.P. 2.0 average air conditioners to C.O.P 5.5 (for example Mitsubishi Heavy Industries have a model SRK20ZJX-S see the government website.

    That’s a 63% saving. That will halve the residential air conditioning demand for about half of the units out there.

    A 25% overall saving should keep enough headroom on the network for the next decade or two.

  • Gary

    I don’t have a problem with your idea, except for one glaring omission, if a household has enough solar energy capacity installed to cover what the Air Con uses and also puts power back to the grid as well and I would assume this to be the case in most instances? then why should these people have to pay yet again for something that isn’t their problem?

    Encouraging more people to go solar would make more sense though, than what you suggest in my opinion.

  • Mathew: Replacing old air conditioners with more efficient ones will certainly reduce power consumption. However, its effect on peak power demand is less certain. Air conditioners tend to be run flat out during heat waves.
    To be sure of reducing peak power demand the more efficient air conditioners would have to be installed with smaller motors.
    Gary: What we are doing at the moment is allowing all air conditioner owners to be subsidized by those that don’t use them. If we wanted to be completely fair we would insist all air conditioners be moved to off-peak and leave it up to the owners to decide whether they wanted to install heat/cold storage (or what-ever) to maintain their comfort.
    Given that what is needed is a gradual conversion of air conditioners to off-peak and that the change benefits all of us, the most practical approach is for the conversion to be paid for by all power users.
    Solar doesn’t provide any power to cover winter peaks and a limited amount of power for summer peaks. I think solar power is a separate issue

  • Gary M

    I like it. Heat storage should be cheaper than electricity storage. Are there readily available commercial systems? Can I fit one to my airconditioner (or does it already have one)?