Energy Performance

Comparisons of Energy Performance Between SWH Systems

SWH system performance depends on a number of aspects in which collector performance is just one characteristic. More importantly is where and how the system is installed, how the hot water is being used on a daily basis, and how the supplementary heating controller is configured.

Most brands of system give similar performance in NZ - the aspects above are more important than any differences between the systems.

Potential purchasers of systems should check if the information provided by the different SWH retailers with whom they are seeking information relates to total system performance or is just relating to the efficiency of the collector. Much information provided to the public by some retailers is misleading as it only relates to the collector. Collector performance has little relevance as it is the performance of the whole system that affects the efficient use of solar energy to heat water. System performance can be significantly affected by pump flow rate, location of inlet and outlet ports on tanks, location of supplementary heating in a tank, and supplementary heating controller settings.

A standard AS/NZS 4234 is available so that system energy performance can be calculated. The standard sets out a methodology for calculating the energy performance of an SWH system under reference conditions in specific climate zones. New Zealand is covered by two climate zone, based on the climates of Auckland and Dunedin. The results of this testing provide a good guide on system performance in individual situations where the system may be installed.

If thermal performance of a system is to be claimed by a solar system retailer the method of calculation should be AS/NZS 4234. While the calculation is under specific reference conditions these have been chosen so as to closely resemble actual performance. However because every solar water heating application is different it is necessary to standardise variables such as hot water demand, irradiation, inclination and orientation.

It should be noted that system performance calculations in terms of the standard AS/NZS 4234 are based on packaged systems. Packaged systems are fully designed systems where all the individual components (eg collector, tank, supplementary heating controller and pump) have been tested to ensure that they each meet the standard and then put together to make a whole system. The design of a packaged system is based on optimal integration of the components. The performance characteristics of each element are combined in a computer simulation to calculate the likely energy performance of that particular arrangement of the system components and under the reference conditions.

Some suppliers are importing components and making what are called "custom built" systems eg they are not designed to work together as an integrated whole. A retrofit installation where the existing hot water cylinder is used is a custom built system. The energy performance of a custom built system can not be calculated until after it is built and it is likely that energy performance will be lower than for a similar packaged system which has been designed as an integrated whole for optimal energy performance.

If the system you buy is put together by your plumber where he buys a collector from one place and a tank from another then you will not be buying a packaged system, and therefore the plumber can not tell you what its performance may be, as he won't have been able to have had its performance modelled. However some suppliers provide packaged “retrofit” systems where your existing tank is used. In these cases you need to ensure that the tank that they have assumed for modelling is similar to your tank.

If a supplier can not provide you with certified energy performance results defined from AS/NZS 4234 then you should think seriously about the quality of what is being offered to you. To check the information provided you should refer to a list of all current complying systems for sale in New Zealand which are listed on the EECA website Complying SWH systems.

A retailer should be able to provide you with guidance.

Monitored Actual Energy Performance

A number of solar system suppliers undertake monitoring of actual systems that they have installed. You should refer to the system suppliers website to find this information. EECA has also undertaken independent monitoring of some systems and these are reported in the case studies available here.

The results of independent monitoring of some systems is also available from the SPLASH website Monitoring undertaken by SPLASH provides real time data on a number of applications and brands.

Indicative System Performance

When selling a SWH system the customer will want to know what its energy performance will be. A system’s actual energy performance can never be accurately stated as a system performance depending on how hot water is used, how the supplementary heating is delivered, and the specific orientation and inclination details of installation. However the retailer can give an indication of expected performance by reference to the energy performance calculation modelling (AS/NZS 4234:2008) that a system supplier will have had prepared to obtain listing of Complying SWH Systems. The energy performance is calculated according to the reference conditions set out in the standard AS/NZS4234:2008 to ensure that the performance of all brands and designs of systems are stated according to the same reference conditions.  

While it is useful to be able to refer to the modelled energy performance, under the Fair Trading Act where the system is installed significantly differently from the reference conditions in the standard, the Retailer should advise how the actual installation differs from the reference conditions used for modelling and what effect that could have on actual energy performance. The Table in clause 4.2.2 of G12/AS2 can assist the retailer on how different the performance could be as a result of the system being installed to different inclination and orientations to that set out in AS/NZS4234;2008.

Where the system energy performance is being referenced against a modelled system under the Family of Systems Criteria, then to comply with the Fair Trading Act the Retailer should advise the customer that the performance is expected to be similar.