TER/TPER/TDER and Notional Building PV

Related Articles:

Foundation Area and Maximum Number Of Storeys

The Target CO2 and Energy Metrics for 2021 (England) and 2022 (Scotland, Wales) Building Regulation calculations now include a contribution of On Site (PV) Electricity generation. These reduce the targets, sometimes even down to zero. This article discusses what influences the size of this PV system, how it can be reduced and the potential impacts of reducing it.

The Target Emissions Rate (TER) and Target Primary Energy Rate TPER (Target Delivered Energy Rate TDER in Scotland) are derived from the Energy Consumed by fuels used for Heating, Cooling, Lighting, DHW and Aux (Fans & Pumps) in the Notional Building along with Notional Building PV contribution subtracted. 

The fuel consumptions are multiplied by the associated Carbon or Primary Energy Factor for the fuel (see NCM Modelling Guide) and these are all summed (with the PV subtracted) to generate Notional Emissions Rate and Notional Primary Energy Rate. 

The Notional Delivery Energy Rate is calculated purely by summing the fuel consumptions and subtracting the PV generation.

TER, TPER and TDER use the same figure as the corresponding Notional Rate but are limited to a minimum value of zero. This is necessary as the Notional Building PV can be very high, exceeding the energy consumption of the building services.

Reduction of the Notional Building PV can be achieved for English and Scottish buildings and this can be an effective measure for achieving Emissions/Primary Energy/Delivered Energy Compliance.

In producing BRUKL assessments it is important to understand how the Notional PV is derived and ensure the correct settings are made in the model to enable this to be calculated correctly. Please see Foundation Area and Maximum Number Of Storeys .

Notional PV: National Rules

The size of the Notional PV System is defined differently for England, Scotland and Wales, it can depend on:

    1. The proportion of building area that is heated

    2. The number of storeys in the building structure

    3. Floor area

    4. Roof Area

    5. The activity type, particularly whether it is classified as the type of activity which normally has roof-lights

    6.  The proportion of heating provided using heat pumps

The following graph shows how PV size is dependent on the number of storeys in the entire building structure (the BRUKL may only apply to part of the building structure). This graph is a simple case where all spaces are heated (no heat pumps) and each storey has the same area / footprint

This illustrates:

    • A strong relationship between PV size and number of storeys for England and Scotland, this is related to the availability of roofspace on the building compared with the gross building area.

    • In Wales the PV size is largely proportional to the building area, particularly if the building has no roof.

Building Type

A PV of 35kWh/m² is a very large array and for a building type with low energy needs may result in a TER of zero. You should not generally have such a large PV system unless:

    • The building is a single storey (or you have not specified the number of storeys / edited the Foundation Area) 

    • The building is dominated by activities classified as TOP-LIT

The following graph shows PV size is also influenced by top-lit activities (English assessments only).

If you compare this with the previous graph, you will see the size of the PV is double that generated for other activities.

There are 20 of these activities (out of a total just under 300):

Specifying Heat Pumps

For Scotland and England you can reduce the Notional PV by replacing the HVAC System heat source with a Heat Pump. These can be Electric, Gas (including Biogas) or Oil, Air Source or Ground Source. A heat pump for the DHW System will not reduce the Notional PV. However, note that the Notional Building HVAC System mimics the Actual Building HVAC so the energy consumed by HVAC will reduce in both Actual and Notional Building and whilst the PV may be significantly reduced, the targets: TER/TPER/TDER may reduce too. Heat Pumps will most benefit low rise buildings (1 or 2 storey) or buildings with low heat demand. However if the initial assessment, generates TER, TPER or TDER of zero then Heat Pumps may be of greater benefit.

Note that for Welsh assessments, the Notional PV cannot be reduced. The TER/TPER/TDER are largely fixed and are not dependent on the Actual Building HVAC spec. Thus improvements to the Actual Building spec will generally not result in a moving target (TER etc will generally not change) and thus adding heat pumps should generally help to achieve compliance.

Heat Demand and Heated Area (England and Scotland)

SBEM calculates a Heat Demand for all spaces. The Notional PV is based on

    • Foundation Area which is calculated only for the heated area

    • Total heat demand (ie whole building area)

Thus only if the whole building is heated AND the heat source is a heat pump will the Notional Building PV be zero.

For example. 

    1. A  building has all zones heated without the use of heat pumps, generating a Notional PV of 16kWh/m²

    2. When the HVAC is replaced with one with a heat pump as source the PV reduces to zero.  

    3. When the building has spaces which are not heated such that the heat demand from the heated spaces (non heat pump) is the same as that for unheated spaces the PV will be 8kWh/m².

    4. When the HVAC is replaced with one with a heat pump as source the PV reduces to 4kWh/m².