How to Measure Diesel Fuel Consumption on a Generator (Step by Step)

In this article

• What is a diesel flow meter and why use one on a generator?
• Step 1: Choose the right flow meter for your generator
• Step 2: Install the flow meter in the fuel line
• Step 3: Read and record your fuel consumption
• How accurate are oval gear flow meters?
• Mechanical vs pulse output: which is better for generators?
• Frequently asked questions

The simplest way to measure diesel fuel consumption on a generator is to install an oval gear flow meter in the fuel supply line. The meter sits between your fuel tank and the engine, counting every litre that passes through. Depending on the model, you either read the total on a built-in mechanical register or capture pulse signals for logging and remote monitoring. No complicated wiring, no calibration lab. You can have one fitted and reading in under an hour.

Below, we walk through the full process: picking the right meter, installing it, and getting reliable consumption readings from your genset.

What is a diesel flow meter and why use one on a generator?

A diesel flow meter is a device that measures the volume of fuel flowing through a line. The most common type for generators is the positive displacement oval gear meter. Inside the housing, two oval-shaped gears rotate as fuel pushes through them. Each rotation displaces a precise, known volume of liquid, so by counting rotations the meter tallies the total flow.

Why does this matter for generator owners? Because guessing your fuel consumption costs money. Without a meter, you are estimating burn rates from tank dip measurements or manufacturer spec sheets, both of which can be off by 10 to 20 percent in real-world conditions. A flow meter gives you hard numbers so you can budget fuel accurately and spot problems (like a dirty injector or a fuel leak) before they get expensive.

Common reasons people fit a flow meter to a generator include billing tenants or job sites for actual fuel used, tracking consumption trends over time, detecting fuel theft, and meeting compliance or reporting requirements on remote sites.

Step 1: Choose the right flow meter for your generator

You have two main options: a mechanical register meter and a pulse output meter. Both use the same oval gear measurement principle. The difference is how you read the data.

A mechanical meter has a built-in odometer-style counter on the front. You read the total litres (or gallons) directly off the dial, the same way you read a car's trip meter. No power supply needed, no screens to configure. It is the simplest, most rugged option.

A pulse output meter sends a square-wave electrical pulse for every unit of flow. You connect those pulses to a PLC, data logger, flow computer, or even an Arduino/Raspberry Pi to record consumption over time. This is the better choice if you want automated logging, remote monitoring, or integration with a building management system.

Size matters too. Match the meter's port size to your generator's fuel line. Most small to mid-range generators (up to about 100 kVA) use 1/2 inch or 1 inch fuel lines. Larger gensets may need 1.5 inch or 2 inch connections. Getting the size wrong restricts flow and can starve the engine, so check your fuel line diameter before ordering.

Step 2: Install the flow meter in the fuel line

Installation is straightforward. You are fitting the meter inline between the fuel tank and the generator's fuel inlet. Here is what you need and how to do it.

What you will need: your flow meter, two BSP fittings or hose barb adapters (matched to your fuel line), thread seal tape (PTFE), two adjustable spanners, and a short length of fuel-rated hose if you are adapting between sizes.

Steps:

1. Shut the generator down and close the fuel tank valve (or pinch the fuel line) so diesel does not flow while you work.
2. Cut or disconnect the fuel supply line at a convenient point between the tank and the engine. Pick a spot that is accessible for future readings.
3. Check the flow direction arrow on the meter body. Every oval gear meter has one. The arrow must point from the tank toward the engine.
4. Thread the fittings into both ports of the meter. Wrap two to three turns of PTFE tape on the male threads. Hand-tighten, then snug with a spanner. Do not overtighten aluminium threads.
5. Connect the meter to the fuel line on both sides. If you are using hose barbs, slide the fuel hose on and secure with hose clamps.
6. Open the fuel valve and check every joint for leaks. Wipe each fitting with a clean rag and watch for weeping over a few minutes.
7. Start the generator and confirm the meter's register (or pulse output) begins counting.

A few tips: mount the meter so the register face is easy to read (horizontal pipe runs work well). If your fuel line runs vertically, some meters can handle that, but check the spec sheet. Keep the meter downstream of any fuel filter so debris does not damage the gears.

Step 3: Read and record your fuel consumption

With a mechanical meter: note the reading before you start the generator and again when you stop it. The difference is your fuel consumption for that run. To get a litres-per-hour rate, divide the volume by the number of hours the generator ran. For example, if the meter shows 12 litres used over a 4-hour run, your burn rate is 3 litres per hour.

With a pulse output meter: connect the pulse wire to your logging device. Each pulse represents a known volume (for example, 1 pulse per 0.01 litres on the OGM-15P). Your logger counts pulses over time and converts them to a flow rate and cumulative total. This gives you a continuous consumption profile rather than just a start/stop snapshot.

Recording your data over weeks or months reveals patterns. You will see how load affects consumption, whether fuel use is creeping up (a sign of a maintenance issue), and exactly how much fuel each job or tenant is using.

How accurate are oval gear flow meters?

Oval gear (positive displacement) meters are among the most accurate mechanical flow measurement technologies for viscous liquids like diesel. Most quality units deliver +/- 0.5% to 1% accuracy across their rated flow range. That means on a 100-litre fill, the reading will be within 0.5 to 1 litre of the true volume.

Accuracy can drop if the meter is undersized (flow rate too high causes pressure loss) or oversized (very low flow rates may not fully engage the gears). This is why matching the meter to your generator's actual fuel consumption rate matters. For most diesel gensets in the 10 to 200 kVA range, a 1/2 inch or 1 inch meter covers the typical 2 to 60 litres-per-hour consumption window comfortably.

Temperature, viscosity, and the presence of air bubbles can also affect readings slightly. Keeping a fuel filter upstream and bleeding air from the line after installation helps you get the best results from day one.

Mechanical vs pulse output: which is better for generators?

It depends on what you are doing with the data.

Feature	Mechanical (OGM-25)	Pulse Output (OGM-15P)
How you read it	Dial on the meter	PLC, logger, or controller
Power required	None	Signal power from logger
Data logging	Manual (write it down)	Automatic and continuous
Remote monitoring	No	Yes (with the right setup)
Best for	Simple sites, manual checks	Automated systems, fleet tracking
Price	$379 AUD	$309 AUD

If you just need to check how many litres your genset burns per shift and write it in a logbook, a mechanical meter is the fastest, cheapest path to an answer. If you want a system that records consumption 24/7, alerts you to unusual spikes, or feeds data into a building management system, go with a pulse output meter and a compatible logger.

Frequently asked questions

How many litres per hour does a diesel generator use?

It varies widely by generator size and load. As a rough guide, a small 10 kVA genset at 75% load burns about 2 to 3 litres per hour, while a 100 kVA unit at the same load uses around 18 to 22 litres per hour. Real-world consumption depends on altitude, ambient temperature, engine condition, and the actual electrical load. That is exactly why measuring with a flow meter is more reliable than trusting spec sheets alone.

Can I use a flow meter with biodiesel or heating oil?

Yes. Oval gear meters work well with biodiesel blends (B5 through B100), heating oil, kerosene, and other light to medium viscosity fuels. Check the meter's chemical compatibility and viscosity range in the spec sheet. Aluminium-bodied meters like the OGM-25 and OGM-15P are compatible with standard petroleum-based fuels and most biodiesel blends.

Do I need to calibrate the flow meter after installation?

Oval gear meters come factory-calibrated and are ready to use out of the box. Over time (typically years of continuous use), you may want to verify accuracy by running a known volume through the meter and comparing the reading. If it drifts beyond the rated accuracy, most meters can be adjusted or serviced.

Will a flow meter restrict fuel flow to my generator?

Not if you size it correctly. Choose a meter whose rated flow range covers your generator's maximum fuel demand with room to spare. An undersized meter can create a pressure drop that starves the engine. When in doubt, go one size up. The manufacturer's flow range chart (litres per hour or litres per minute) is the key spec to match.

Can I use a flow meter to detect fuel theft?

Absolutely. If your meter readings show fuel disappearing when the generator is not running (or consumption jumping well above normal), that is a clear flag. A pulse output meter connected to a logger makes this even easier, because it timestamps every drop of fuel that moves through the line.