DHW Tanks – W/K from kWh/day

4 August 2018 by Jason Quinn


The Passive House Planning Package (PHPP) requires that energy losses from Domestic Hot Water (DHW) Storage tanks be entered as Watts per degree Kelvin (W/K) rather than the more common KWh/day. This explains how to convert from kWh/day to W/K and some ways this can be more complicated than you would expect as you need to know the temperatures difference at which the kWh/day was measured.

NZ or AU test data

For a typical 180L tank NZ standards require less than 1.7 KWh/day standby losses at 55˚C. Note AU standards are tested the same but are of poorer quality. Note NZ/AU testing is performed at 55˚C temperature difference. If you have kWh/day from AS/NZS 4692 testing use 55˚C

1.7kWh/day -> 1700Wh/24h=1700/24W=70.8W

Simply divided by the temperature difference to obtain 70.8/55W/˚C=1.29W/K

European test data

European standby loss testing is performed at a 45˚C temperature difference.

For our example 180L tank with 1.7 kWh/day standby losses that would yield:

Similarly 1.7kWh/day -> 1700Wh/24h=1700/24W=70.8W

But divided by the smaller temperature difference to obtain 70.8/45W/˚C=1.57W/K

What other losses should be included in storage losses

PHPP Version 9.3 Manual recommends adding additional 0.5 to 3.0 W/K of storage losses

My personal opinion/guide is the following:

  • Excellent would have ALL exposed pipework and fittings insulated continuously under the clamps with insulation thickness twice the diameter of the pipework. Insulation on fittings carefully glued and fitted. Additional 0.5 W/K storage losses.
  • Medium would have all exposed pipework and fittings insulated continuously under the clamps (or non-metallic clamps over-insulated) with 13mm insulation. Insulation on fittings carefully glued and fitted or fitted and taped with designed for this application. Additional 1.5 W/K storage losses.
  • Normal would have all exposed pipework and fittings insulated reasonably with very few gaps with 13mm insulation. Additional 3.0 W/K storage losses.
  • Not acceptable: Completely exposed metal piping or fittings within 1.5 meters of tank on hot water lines/vent and cold water line completely uninsulated.


The below graph shows the NZ Minimum Energy Performance Standard (MEPS) plotted in terms of W/K as used in PHPP. The data points are for tanks tested to this standard and sold in AUS/NZ. Note that although AUS uses the same test standard they have a higher permitted MEPS so some tanks are shown that are not permitted (in theory) to be sold in NZ.

kWh/day test data is available from all NZ importers / manufacturers for electric storage DHW tanks as it is required in order to sell an electric storage tank. Note that currently Heat Pump and Solar storage systems sold as systems may not have this data as it is not a legal requirement.


Figure 1: European Efficiency Classes, NZ and AU 2005 standards and many tanks available in NZ/AUS plotted in terms of W/K.



Figure 2: MEPS covers: Electric storage displacement water heaters with a nominal capacity from 6.5 L to 630 L inclusive. Plotting the W/K for Water heaters from the website.



AS/NZS 4692.1:2005 Electric water heaters – Energy consumption, performance and general requirements

AS/NZS 4692.2:2005 Electric water heaters Minimum Energy Performance Standard (MEPS) requirements and energy labelling

Sanne van den Dungen with revisions by Gerry Carrington, “Minimum Energy Performance Standards – How does New Zealand compare with other countries?”, CSAFE, University of Otago, 2011.

Standing Heat Losses from household hot water cylinders, Climate Connect Project, Waterworks Community, Email: wwvalley@dodo.com.au http://waterworksvalley.com

Link updated 6June2024 to   https://reg.energyrating.gov.au/comparator/product_types/58/search/

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