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Step 7 Guide 2: How to calculate equipment electricity use

This is the seventh of 20 steps in the in the Energy Management Action Programme (Energy MAP).
Learn about Energy MAP and what it can do for your business.

 

In a nutshell

Quantifying energy use for each equipment category helps you identify the large energy users, and prioritise these for early action. If you are implementing capital investments to deliver savings it is often necessary to justify these in terms of cost-benefit, which means you must quantify overall energy use by a piece of equipment and likely savings.

This guide provides you with the formulae to calculate operating costs and savings, and discusses techniques to quantify energy use for different plant and equipment.
Formulae to calculate annual electricity use and cost
For each piece of equipment calculate annual electricity use and cost. You will need the three straightforward formulae provided below.

Annual electricity use, expressed in kWh, depends on electrical power when on and run hours:

  • Annual electricity [kWh] = Power [kW] x annual run hours [kWh].
  • Running cost depends on electricity use and the Average Unit Price (established in Step1):
  • Annual cost [€] = annual electricity [kWh] x AUP [€/kWh]

There are a number of methods for establishing the power of equipment. Their accuracy depends on the piece of equipment.

  • Use the nameplate rating [kW]
  • Take a current reading using a grip-on ammeter and calculate the power using the formula:
    Power [kW] = 0.184 x current [amps]*

Note that for a 3 phase load total power is the sum of the power for each phase (using above formula). If you don’t know what a 3-phase load is, have an electrician to take the current readings.

*Power [kW]  = voltage [V] x current [Amp] x powerfactor / 1000
   = 230 x current x 0.8 /1000
   = 0.184 x current

  • Log power use using a power logger (or electricity analyser) or submeter

Lights

Identify the different types of lamps in use and their wattage. Wattage is usually written on the lamps, or you can ask whoever maintains the lamps. Then quantify the number of lamps of each type. You may find it helpful to break these up into zones, e.g. ground floor reception, ground floor offices.

To establish annual run hours for each zone consider how they are controlled and the occupancy hours.

Note: for the purposes of simplicity, we have ignored control gear losses. If you know in what cases these apply, add 15% to electricity use.

TIP: if you work late one night you will see what time the lights are actually switched off. You will also develop a great insight into night-time operating characteristics: what plant and equipment is on and off. Compare this to your night profile, established in Step 6.

Office Equipment

The IT manager should be able to tell you the number of PCs in use. Alternatively, find out the number of occupants and estimate what proportion of these have PCs.

Power use by PCs varies depending on manufacturer, year, screen type, screen powersave settings and computer powersave settings. At this point, using an overall figure will be sufficiently accurate:

Annual electricity use for one value PC with monitor in an average office,  where the computer and monitor are switched off (using button) at night and both automatically go into standby mode after 20minutes 160kWh
Annual electricity use for one value PC with monitor in an average office,  where the computer and monitor are switched off (using button) at night, but do not go into standby (i.e. powersave not enabled) 480kWh

Source of the above is www.eu-energystar.org, and it. To do more accurate, site-specific calculations, go to this website and use the energy calculator.

For other office equipment – printers, photocopiers, fax, speakers, vending machines - either allow an additional 15% of the total annual electricity use for PCs, or count the number of each type and use the Energy Star online calculator.

IT Room

The simplest approach is to find the distribution board supplying power to the IT room and take current readings for each circuit.

Assume the load is broadly the same at night as during the day, annual electricity use = power x 8760 hours/yr.

Note that UPS systems draw a small quantity of power when in standby mode. This can be established either by interrogating the UPS controls or taking an ammeter reading. Assume this load is stable/continuous.

Pumps

Pumps are used for a variety of purposes: to circulate heating water, circulate chilled water for cooling, circulate hot water for washing, pressurise drinking water, and dispose of rainwater.

To establish power use either take an ammeter reading (preferable) or use the nameplate. An efficient approach is to take ammeter readings for the larger pumps and accept nameplate for the smaller ones.

Annual operating hours for each pump should be established by considering how it is controlled. Heating, Domestic Hot Water (DHW) and chillwater systems should all operate on time schedules or time switches. A view must be taken on seasonal variations in run hours.

 

Ventilation

Supply and extract fans, often contained in packaged air handling units, are required for respiration. They may also fulfil a heating/cooling role.

To establish power use either take an ammeter reading (preferable) or use the nameplate. An efficient approach is to take ammeter readings for the larger pumps and accept nameplate for the smaller ones.

Annual operating hours for each fan should be established by considering how it is controlled and what the associated time schedules are.

Cooling – Air-Conditioning Units (DX units/splits)

Small packaged air-conditioning units are proliferating. They are often referred to as DX (direct exchange) units, or split a/c units. Each unit has an indoor unit (evaporator) and outdoor unit (condenser). They are generally installed to provide cooling, but are generally capable of heating too.

There are so many variables that affect electricity use that establishing annual electricity use can only be done accurately by logging/submetering electricity use over a long period. However, this is not practical.

To establish power use take the nameplate rating. This is generally on the side of the outdoor unit. If available, note “electrical input”, “cooling output” and “heating output”.  If “electrical input” is not available, there may be some indication of Full Load Amps (FLA).

Use your own judgement, based on the application, to establish the likely number of operating hours per year. In an IT room it will be on continuously, in an office it might only be used during office hours from May to September.

The unit will always draw a small quantity of power when it is on. However, it is only when the compressor is running that the nameplate power is drawn. As a rough indicator, assume that the unit draws its nameplate power for 50% of the time that it is on.

Example:
FLA = 6amps per phase
=> Total Nameplate Power = 6 x .184 x 3 = 3.3 kW
On for 1000 hours per year
=> Draws Nameplate Power = 1000 x 50% hours
=> Estimated Annual Electricity = 3.3 x 500 = 1,650kWh

 

Cooling - Chillers

The other common source of cooling are chillers.  Again, there are so many variables that affect electricity use that establishing annual electricity use can only be done accurately by logging/submetering electricity use over a long period.

Our first preference is to log electricity use over a 1 or 2 week period and extrapolate this to annual use, making some allowance for seasonal variations.

If this is not possible, then a similar approach to that outlined for air-conditioning units should be used. Critical here is to evaluate if the chiller simply operates on timeschedule throughout its year, or if there are controls in place to inhibit its operation in cool weather.

Catering

Assessing energy use for catering is difficult to do accurately, particularly as electricity, gas, or a combination may be used for cooking.

Our first preference is to use submeter readings, or log electricity use over a 1 or 2 week period and extrapolate this to annual use, making some allowance for seasonal variations.

A rule-of-thumb approach is to allow 5kWh/meal. If gas is used for cooking, then 80% of this (4kWh/meal) will be gas, and the balance electricity.

 

Other

For other energy uses, use your judgement to establish which of the above techniques is most appropriate.

Create A Pie Chart

Get the total annual electricity use for each of the categories of equipment (lights, pumps, ventilation, etc.).

Subtract each of these categories from the figure for total annual electricity use, established in Step 1. This balance represents a combination of miscellaneous users not quantified (e.g. security cameras, fire monitoring system) and any errors in your calculation. Ideally it should be less than 25% of total.

Use these values to create a pie chart.

Heat Use

Fossil fuels, such as gas, are burned in boilers to generate heat. That heat is used for space heating and domestic hot water. The space heating component may in turn be split into heat loss through the building fabric and ventilation heat loss. Ventilation heat loss may be as a result of the controlled mechanical ventilation of a building and through uncontrolled ventilation including open windows, doors and unsealed fabric.

Calculations of heat use associated with these different elements is specialised and beyond the scope of this document. For more information on calculation methods refer to CIBSE Concise Handbook.

Return to Step 7

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