Yawning leopard

Dear Evil Engineer: How many leopards do I need to heat my home?

Image credit: Dreamstime

The Evil Engineer weighs up the environmental credentials of carnivorous domestic heating systems.

Dear Evil Engineer,

I am frustrated by the lack of leadership on climate action, particularly regarding low-hanging fruit such as boosting the energy efficiency of housing stock. I ask myself: what can I do to prevent the Earth becoming uninhabitable through climate change?

Before anyone makes any snide comments; it goes without saying that I want the Earth to become a hostile wasteland scattered with the remnants of the humanity. But I want to do it myself and not let the fossil fuel industry claim the greatest honour in villainy since someone threw a meteorite at Mexico 66 million years ago. I want to be evil and green, like the Grinch before his heart grew three sizes.

This leads me to my query: I have two fearsome pet leopards and I was already considering buying more. How many would I need if I wanted to heat my pleasant two-bed semi entirely with leopards?

A conscientious villain


Dear villain,

Thank you for your timely letter. I too have been taking steps to lower my energy bills; it’s remarkable how much money you can save when you remember to carpet-bomb the headquarters of your energy provider before your bill is due.

Homes contribute 22 per cent of UK carbon emissions, of which almost half is accounted for by heating. As well as patching up the draughty fabric of your house, you can lower your heating needs by making the most of passive heat sources, such as sunlight and warm-blooded inhabitants.

How much heat can we draw from a leopard? Heat emission from resting warm-blooded animals can be estimated with the formula H=6.6m0.75, where H is BTU/hour and m is mass in pounds. For an adult leopard weighing 30kg – and converting to sensible units – that gives us power of 0.0448kW.

Now, to the house in question. You have not indicated how energy-efficient your house is at present, so we shall consider a couple of different possibilities, beginning with the most energy-efficient. Passive houses are so well designed and insulated that they do not generally require conventional heating systems. A certified passive house has specific heating demand no higher than 15kWh/m2/year. For an average UK house with floor space of 96m2, that gives us 1,440kWh/year in heating. A single leopard can generate 392kWh/year in heating, so 3.67 leopards – all right, four leopards – would provide all your heating needs in the most efficient house.

As of November 2020, there were just 1,300 certified passive houses in the UK. Most houses, of course, have much higher space heating demand. Research puts the average at 145kWh/m2/year for the typical house (or 50kWh/m2/year for new builds). This would call for 36 leopards (13 for a new build). How much space does a pet leopard need? Somehow, I suspect heating your home with leopards would require something akin to a leopard battery farm.

While people and pets are useful sources of passive heat, I’m less convinced that leopards could be considered ‘passive’ if bought and maintained primarily for your heating needs. Let’s take a look at how they measure up regarding their environmental impact.

Unless your leopards enjoy a jet-setting, consumption-heavy lifestyle rare among members of their species, their CO2 emissions will be mostly accounted for by their diet. Captive leopards are fed around 1-1.2kg of meat daily; given the limited space for your leopards to exercise, let’s use the lower bound. According to figures published in Science in 2018, beef produces 71kg CO2 equivalent (CO2e) when accounting for the warming effect of methane as well as CO2. In a day, then, a leopard generates 1.08kWh of thermal energy with emissions of at least 71kg CO2e: 66kg CO2e/kWh. UK government figures put the comparable carbon intensity for a domestic fossil-fuel boiler at 0.31kg CO2e/kWh.

Heating your house with leopards, then, is more than 200 times worse for the environment than using the boiler you probably already have. What you would be doing is effectively pumping beef into a furry, inefficient boiler. As 1kg of beef contains 2,500kcal (2.9kWh), a leopard has a thermal efficiency of just 37 per cent. That’s even lower than firewood, which is rated at around 50 per cent for fuel efficiency.

Eating less meat is one of the most effective steps individuals can take to minimise their personal CO2 emissions. But there are few herbivores that would make suitable pets for one such as yourself who wishes to be both evil and green. No one fears a villain who lives with pandas or rabbits for companions. Hippos are fearsome, yes, but they are essentially tempestuous swamp cows that release huge amounts of CO2 via their poo. The best suggestion I have would be geese: vegetarians armed to the (tongue with) teeth.

A large goose such as a greylag goose weighs 3.5kg and eats 1kg of plant matter (approximately 2kg CO2e/kg) in a day. Using those figures, I estimate a single goose could provide 0.215kWh on 2kg CO2e daily. That gives us a carbon intensity of 9.3kg CO2e/kWh: seven times better for the environment than leopards, but still considerably worse than the conventional fossil fuel boiler.

Look, heating your home with animals is always going to be worse for the environment than sticking with your boiler, no matter how old and dirty it is. But I am only the Evil Engineer; you alone can decide whether it is worth compromising the means by which you assert your villainy for the sake of being green. If you decide it is, consider installing a heat pump and covering it with offensive slogans.

The Evil Engineer

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