#11 – How Energy Actually Works and Terminology

 About this Factsheet

Festival organisers, production managers, festival staff and power users commonly report being bamboozled by technical language about energy.  Smart approaches to energy management require baseline knowledge about how temporary power works between many stakeholders. This factsheet, produced with energy consultancy Entersys, provides an explanation of phrases and terms and some know-how to help everyone take part in conversations more confidently.

What is Efficiency?

Energy efficiency is simply the process of doing more with less. In the context of this guide efficiency can be considered and measured in several ways:

  • The amount of usable power (kilowatt hours / kWh) generated per litre of fuel consumed.
  • The overall ‘fuel per audience day’ (or litres of diesel used per person per day) at the event. *
  • Either of the above compared to figures from the previous year.
  • How well matched the generator size is to the load – see below

*You can check how your event compares with UK averages by using the Fuel Tool at www.powerful-thinking.org.uk/resources/fuel-tool/ 

Generators consume a baseline of fuel even if they are not employed at capacity, and the ‘fuel consumed’ versus ‘power generated’ relationship is not linear; efficiency is therefore largely determined by the load on the generator. A good rule of thumb is that around 75-80% load is perfect (optimum). Going downwards, anything between 75% and 50% load is still good (reductions in efficiency are marginal), but as you go down from 50% to 25% efficiency reduces significantly. Below 25% is low efficiency, and is wasteful of both fuel and costs.

Voltage, Current and Power 

These are basic electrical terms but can be quite confusing at times, so let’s take a brief look at each one:

Voltage: The force that makes electricity flow through a wire. Its unit of measurement is the Volt (V). For our purposes most items are designed to run on 230V AC. Occasionally we may come across items designed for 110V AC (using a yellow plug) and 400V AC (using a red plug). A higher voltage isn’t necessarily ‘better’ – the output voltage of the generator needs to match the working voltage of the appliances connected to it.

Current: The amount of energy that flows through a wire over a given time. The thickness of the wire restricts its flow. Plugs and sockets are rated up to a certain current carrying capacity – 13A domestic, 16A & 32A blue  etc. The symbol for current is (I) and the unit of measurement is the Amp (A).

Power: Rate at which energy is consumed by a system. We can consider it in two ways, either the amount of power a machine can produce — a generator for example; or the amount of power a system consumes — a kettle for instance. Power is measured in Watts (W).

The Relationship Between Voltage (V), Current (I) and Power (P)  

Power is simply the product of voltage across and current flowing through an item.

P(W) = I(A) × V(V)  

A light bulb is designed to run at a voltage of 230V and consumes 40W of power. What current will flow through it?

I = P / V        = 0.17 A 

 Practically, most items run at 230V. All items sold in the UK will have a rating plate on them somewhere, stating what voltage they are designed to operate at and what power they will consume. From this we can establish the power requirement for a system: we simply add up the power of each item. If the power isn’t given but the current is, we can convert to power using the above formula.

So, for a production office we can work out how much power we need:

2 x       Desk lamps @ 40 W each                                               =        80  W

1 x        Laptop @ 90 W                                                                   =       90  W

1 x       Laminator @ 6 A (Power (Watts) = 6 A x 230 V        = 1,380 W

1 x       Coffee Machine @ 3 kW                                                     = 3,000 W

1 x       Radio Charger at 460 W                                                     =     460 W

         TOTAL                                                                               5,010W   

We can convert the Laminator into Power (W) from Current (A) by using the above formula to find it is 1,380 W or 1.38 kW

We now know our office consumes 5.01 kW. So we need a 5 kW generator right? Well, yes we would, if we were to be running all the appliances at once. In catering that may be the case, especially over a show weekend where they are doing pre-cooks and long serving sessions. But we need to apply a little experience. Because we know our office appliances don’t all run at once, let’s allow for 75% at any one time, which reduces our demand to approx. 3.75 kW, or 16 A.

Generators, kW, kVA and kWh 

Due to the physics of AC power generator and consumption, generators are sized in kVA rather than kW. In most instances we can consider that:

    1 kVA = 0.8 kW 

Current measurements on a generator are instantaneous — a snapshot of the current supplied when we happen to look at the meter on the front panel; just as our car speedometer only shows the instantaneous speed we are travelling at when we happen to glance down at it. So, in order to show the cumulative energy produced we need to view it as energy produced over time. This is measured in kilowatt-hours (kWh).

KWh is the common unit used to bill electricity to consumers. For example, a 60-Watt lightbulb that burns for one hour uses 0.06 kWh (i.e. 60 Watts). Over 10 hours it would be 0.6 kWh.

Due to the nature of AC power generation, generators generally have 3 separate power outputs, called ‘phases’. For example if a 100-kVA generator could supply a total current of 420 Amps, there would be 140 Amp x 3 outputs. It is important to keep the load on each phase as equal as possible to allow the engine to run smoothly. Putting all the load on one phase causes engine wear and damages the machine.

Communicating with Your Power Company 

Power companies are run by engineers who tend to talk in engineering terms, which can seem both exclusive and confusing. So let’s run through a few of the more common ones to explain them:

Bunded tank: All generators have a fuel tank inside of them and this will allow them to run for a certain time at full load — the generator manufacturer will specify for how long. In order to increase the running time many suppliers will supply an external tank that holds more fuel, to increase running time and save on refuel time. These are double skinned or ‘bunded’ which is a failsafe if one of the skins becomes pierced. The tank should also contain the fuel if it is tipped over.

Distro Box:  Simply an electrical box used to distribute the generator outputs. This will contain the circuit breakers to protect the circuits in the event of a fault.  Modern units are generally black plastic cubes with sockets on one side and clear windows containing the breakers on the other.

Residual Current Device (RCDs):  Special circuit breakers designed to operate in the event of a fault. They are designed to protect people against electric shock. A correctly designed system may contain several RCDs set at differing sensitivities, so shock protection is maintained but if one trips it won’t turn off whole areas of an installation. Beware of the term ‘nuisance tripping’ — unless the RCD is at fault itself they don’t nuisance trip, they trip because they sense a fault, the cause of which should be investigated.

Uninterruptable Power Supply (UPS):  A device from the computer industry, which will provide power to a circuit should the generator fail. They are often used on ticketing cabins, CCTV and Internet infrastructure where a simple generator failure would cause disruption to site communications etc.

Fuel Filter: Before the diesel is fed into the engine it is filtered to remove any moisture and dirt to and protect the engine. Over time, these filters can become blocked and choke the flow of fuel, causing the engine to run erratically. They can also cause plumes of white smoke to emit from the machine. This is not a concern from a fire safety perspective but the machine does need immediate attention.

Download Factsheet #11 – How Energy Actually Works