For a long time my wife has been suggesting we should get a garden waste shredder - we end up taking at least a dozen car loads of green waste to the tip a year, and often end up with piles of stuff waiting for a trip. I’ve been lukewarm, reckoning that we live close enough to the rubbish tip that buying a shredder is not justified. However, just for the hell of it, and inspired by King of the Road, I thought I’d do the calculation.

If you can’t be bothered reading on, Anne was right: we’re going to buy a shredder!

The Car

My car normally gets about 55 m.p.g. (that’s a real gallon, not one of those American “lite” gallons), but his is a short journey, invariably done with a cold engine, today on a dump trip it it got 45 mpg, so we’ll use that as typical.

The distance to the dump is about 2.3 miles, so a 4.6 mile round trip uses about 0.1 of a gallon or about 0.45 l of diesel.

It seems (see below) that my car will produce about 2.7 kg/l of CO2, so that corresponds to about 1.2 kg of CO2 (blimey, more than I thought).

Conclusion: Using my car for this trip produces 1.2 kg of CO2!

The shredder

Mains powered shredders, looks like they’re in the 2.5 kW department (for example: see this 2500W 40mm shredder).

A car load of shredding? Here we’re in complete guesstimate land, but I’ll guess, say 15 minutes, so we assume we have to use about 0.625 kWh.

Assuming 60% transmission efficiency (from King of the Road, which I think may be a big overestimate), then we need to scale our mains power estimates to get an estimate of fossil fuel power needed: we go from .625 to about 1 kWh.

What’s the CO2 emission for that power? Let’s do the worst case: in the UK, it would appear that the average CO2 emission per kWh for coal is 891 g/kWh.

Conclusion: Using mains power from a coal powered plant to do my shredding would produce about 0.9 kg of CO2.

(However, that’s a worse case, the UK power mix includes stuff that is much less polluting in a CO2 sense, it could be much less than that).


1.2 kg of CO2 using the car, and worst case 0.9 kg of CO2 using a shredder, so I’m going to go out and buy a shredder and hope that it doesn’t take longer than about 15 minutes to shred a car load (and if it does, console myself with the power mix argument).

Caveats: I’ve ignored the proportional production and disposal costs of the car (infinitesimal compared to the mileage on other trips) and the shredder (100%), but I hope that those become small in the limit of lots of loads of shredding. Similarly, I’ve ignored the benefit of having the shredded garden waste for compost, because I think that’ll be small in comparison to the fossil fuel costs.

Interesting numbers

  • US Gallon = 3.784l
  • Real gallon = 4.546l
  • 1.2 us gallons to a real gallon.
  • Diesel carbon content per US gallon (2778g - source EPA).
    • Resulting CO2 emission per US gallon of diesel (including an inefficiency oxidation factor and allowing for the O2 mass as well is: 10.1 kg per us gallon or 2.7 kg/l.