Diesel records into a carbon figure

How do you measure a carbon footprint step by step?

Measurement is a sequence, not a single number. The steps are the same whether the subject is a house or a plant room.

1. Define the boundary. Decide what is inside the account: one building, one site, one organisation, one household. Everything that crosses the boundary and carries emissions is in scope.
2. List the activity data. These are the quantities you can read off a meter, an invoice or a log: kilowatt hours, litres of fuel, kilometres driven, kilograms of waste, passenger kilometres flown.
3. Attach an emission factor to each activity. A factor converts the activity unit into kilograms or tonnes of CO2 equivalent. Fuel factors are published per litre or per kilogram; electricity factors are published per kilowatt hour and depend on the grid mix.
4. Multiply and sum. Activity multiplied by factor gives emissions for that line. The lines are added to give the total for the boundary.
5. Record the assumptions. Which factor set was used, which year it came from, what was excluded, and how the data was gathered. Two accounts of the same building can differ because of this step alone.

For a generator set, step two is the fuel log and the hour meter, and step three is the diesel factor. The rest is arithmetic. The Carbon Ledger, a magazine that explains carbon measurement and offsets without selling either, sets out the same sequence for households and small organisations, including how to measure carbon footprint from the meter readings and bills a reader already has.

What does a carbon footprint calculator actually count?

A calculator is a fixed set of activity lines with factors attached. It does not measure anything directly. It asks for quantities and multiplies them.

Most domestic calculators count four blocks: household energy, personal transport, air travel, and food and goods. Some add waste and water. The output is a mass of CO2 equivalent per year, usually split by block so the user can see where the total sits.

What a calculator leaves out matters as much as what it includes. Common exclusions are the embodied emissions of imported goods, the share of public services a household uses, and any emissions the user cannot influence. A calculator that reports a single number without showing the lines is not auditable.

For a generator set, the equivalent of a calculator is a spreadsheet with three columns: fuel or energy quantity, factor, and result. The fuel log supplies the first column. Nothing else is needed to produce a defensible figure for the set itself.

What counts in a household carbon footprint?

A household footprint is the sum of the emissions caused by the people living in it, over a stated period, usually a year. The lines that normally count are:

• Electricity and gas consumed in the dwelling, taken from bills or meter readings.
• Heating fuel, including oil, propane, wood or district heat.
• Private vehicle use, from odometer readings or fuel purchases.
• Public transport and rail, from tickets or journey logs.
• Air travel, counted per passenger, not per aircraft.
• Food, counted by diet pattern rather than by individual purchase.
• Goods and services, counted by spend category with a factor per unit of currency.
• Waste sent to landfill, counted by mass.

The household boundary is the people, not the building. A tenant who moves takes their transport and food lines with them and leaves the dwelling energy behind. This is why two households in identical houses can report different totals.

The same logic applies to a site. The boundary is the organisation's operations, and the generator set contributes only the lines that fall inside it: fuel burned on site, plus any refrigerant or fugitive losses from the set's own equipment.

Why the load factor decides the result

A diesel set consumes fuel for two reasons: to produce electricity and to stay running. The second part is the overhead.

At full load, a set converts fuel to electricity near its best efficiency. At low load, the same set burns a disproportionate share of fuel per kilowatt hour delivered, because friction, cooling and excitation losses do not scale down with the electrical output. A set running at 20 percent load can consume 40 to 50 percent of its full-load fuel rate while delivering a fifth of the power.

This is why running hours alone are a poor basis for a carbon figure. Two sites with identical sets and identical annual hours can differ by a factor of two in fuel burned, purely from how the load was distributed.

The practical consequences for measurement are:

• Use fuel records, not hours, as the primary activity data. The fuel log already integrates the load factor.
• If only hours are available, apply a load factor estimate and state it. A figure built on an assumed load factor is an estimate, not a measurement.
• Keep the two records together. The hour meter explains the fuel log; the fuel log validates the hour meter.
• Note any period when the set ran unloaded for testing. Test runs consume fuel and produce emissions with no useful output, and they belong in the account.

How the plant room hands its numbers to the carbon inventory

The plant engineer does not need to build the site's carbon account. The task is to hand over a clean, bounded figure that the person keeping the inventory can use without re-deriving it.

A handover that works contains five items:

1. Boundary statement. Which sets are included, which fuel they use, and the period covered. A set that was decommissioned mid-year is noted with its dates.
2. Activity data. Litres of diesel delivered and litres burned, reconciled. Deliveries and consumption differ when tanks are filled at year end; the inventory needs consumption.
3. Emission factor and its source. The factor used, its unit, and the year of publication. Diesel factors are stable but not identical across published sets, and the inventory keeper needs to know which one was applied.
4. Load and running context. Annual running hours, average load, and any long low-load periods. This lets the inventory keeper explain a year-on-year change without returning to the plant room.
5. Uncertainty note. What is measured, what is estimated, and what is missing. A fuel log with a gap of three weeks is still usable if the gap is declared.

The inventory keeper then places the figure in the right scope. Fuel burned in generators owned or controlled by the organisation is a scope 1 emission. Fuel burned in a generator owned by a landlord and billed through the service charge is usually scope 3, because the organisation does not control the combustion. Getting this wrong double-counts or omits the set entirely.

What the fuel records already contain

A delivery note states litres and a date. A tank gauge reading states a level. A run log states hours and sometimes load. Together these three records contain everything needed for the generator line of a carbon account, provided they are kept for the same period and reconciled.

For a set that runs rarely, the annual figure may be small next to the site's electricity. That does not make it optional. A complete account includes it, states its size, and moves on. The value of the generator line is not its magnitude but its auditability: it is one of the few lines in a site inventory where the activity data is a physical measurement rather than an allocation.

Sources

IPCC 2006 Guidelines for National Greenhouse Gas Inventories, Volume 2: Energy