A compressor's price tag is only the first of several bills a workshop pays for it. Over the years a machine is in service, the purchase price is joined by the electricity to run it, the servicing to keep it running, and the smaller costs that come from how well the system is looked after once it's installed. Which of these ends up mattering most depends less on the make of compressor than on how many hours a year it works under load.

Purchase price: paid once

The capital cost is fixed at the point of sale and does not move again, whatever happens to electricity prices or how the workshop's tool use changes over the following years. Because it is paid once and the running costs are paid every year the compressor works, a machine bought slightly larger or slightly smaller than the job needs has its price difference repaid, or repeated, many times over through the electricity bill.

Electricity: paid every hour under load

The motor draws power for every hour it spends compressing air, and that draw is set by its rated power in kilowatts, the hours it runs under load each year, and the price per kilowatt-hour on the business tariff. Because a compressor is typically in service for years rather than months, that per-hour cost accumulates well past the figure on the original invoice, which is why the working pressure and duty cycle matter as much to the bill as the model chosen.

Servicing: a cost that scales with hours.

Oil changes, filter replacement and the routine checks a compressor needs are priced against running hours. Skipping a service to save money tends to work the other way: a clogged filter or worn valve makes the motor work harder for the same output, and that extra effort shows up on the electricity bill long before it shows up as a breakdown.

The cost that doesn't appear as its own line

Air lost to leaks is paid for through the same electricity bill as everything else, so it rarely gets noticed as a separate cost until someone goes looking for it. A compressor with a receiver full of unrepaired leaks can spend a meaningful share of its running time simply making up for air that never reaches a tool. The Leak Cost Calculator turns a leak survey, or even a rough estimate of leak flow, into an annual figure by working through the same power, hours and price arithmetic that applies to the compressor itself; it can only work from the numbers entered, so a wrong estimate of leak flow or running hours produces a wrong estimate of cost.

Every extra bar carried on the pressure switch means the compressor is pushing the same air into a smaller space, and compressing air further takes more electrical power to do it, whether or not that extra pressure was ever needed at the tool.

Why the extra bar matters

A compressor's motor works harder as the cut-out pressure rises, because it is squeezing the same volume of air into a smaller final space than it would at a lower setting. Most workshop tools, an angle grinder or a DA sander among them, need a fixed working pressure to run properly, so any bar carried above that figure is being paid for on the electricity bill without doing anything useful at the tool end. Checking what your tools actually need is covered in How Much Air Your Tools Need: Sizing in Litres per Minute.

Turning the pressure down

Lowering the cut-out setting so the compressor stops filling the receiver sooner is one of the few changes to running cost that costs nothing to make. It also slows how fast a leak empties the receiver, because a hole of a given size passes more air at a higher pressure than at a lower one, and it starts cycling the compressor again sooner as a result.

Where leaks are already adding to the bill, the Leak Cost Calculator works out what a leak costs to run from the compressor's free air delivery, the hours the system is pressurised and the pressure it runs at, using the figures you enter for your own system.

Variable-speed drives

A fixed-speed compressor, piston or rotary screw, runs its motor at one speed and either stops, idles or unloads once the receiver reaches cut-out. A variable-speed drive (VSD) instead slows the motor as demand drops, so the compressor tracks what the workshop is drawing. That suits a workshop whose air demand rises and falls through the day; whether it actually beats a fixed-speed machine sized correctly for the job depends on how steady or how spiky that demand turns out to be.

The annual electricity bill for either type of compressor comes down to the same three numbers multiplied together: the power the motor draws while working, in kilowatts; the hours it actually runs under load each year; and the price charged per kilowatt-hour on the business electricity tariff. Where a piston compressor and a rotary screw diverge is in how that middle number, the hours under load, actually behaves once the machine is installed.

How a piston compressor draws its power

A piston compressor is built to run flat out or not at all. It fills the receiver until the pressure switch trips at the cut-out setting, then sits idle until demand pulls the pressure back down to the cut-in setting and the motor starts again. The share of each hour spent running under load rather than sitting idle, its duty cycle, is set by how much air the workshop actually draws off, not by the motor's rated output. Two workshops running the same model can see quite different bills depending on how steadily their tools are in use through the day.

How a rotary screw compressor draws its power

A rotary screw compressor is more often left running for the length of a shift, either unloading to idle between demands or, on a variable-speed model, slowing the screw element to track demand. That tends to spread the power draw across the day, but it does not automatically mean a lower bill. A screw compressor sized well above what the workshop actually needs will still burn power idling that a smaller piston unit, cycling on and off, would not.

Comparing the two on cost is really a comparison of two different duty cycles applied to two different rated powers. Reading a manufacturer's rated power and free air delivery (FAD) alongside your own hours of use, as set out in Piston, Rotary Screw, Scroll or Vane: Matching the Compressor to the Work, gives a steadier starting point than treating one type as cheaper than the other by default.

Leakage adds to the load regardless of type

A leak draws air, and therefore power, at every hour the system is pressurised, whether a tool is in use or not. On a piston compressor that shows up as more frequent cycling. Either way it adds to the same duty cycle that decides the electricity bill. The Leak Cost Calculator takes a leak flow, or a compressor's free air delivery and the share of its running time lost to leakage, and turns it into an annual cost at your own electricity price, so the effect can be weighed against the compressor's own running cost.