Celsius is basically the same as Kelvin, it just puts 0 in a different place.

See Wikipedia:

By definition, the Celsius scale (symbol °C) and the Kelvin scale have the same magnitude; that is, a rise of 1 K is equal to a rise of 1 °C and vice versa, and any temperature in degrees Celsius can be converted to kelvin by adding 273.15.[1][5]

You seem to confuse metric with SI units. And prefixes for the base units (like km instead of 1000m) do exist in the SI system.

I'm not forcing anyone, I'm trying to influence them to use joules instead

Also packages are made for different country so they need to contain the specific info

Relevant ~~xkcd~~ what-if:

https://what-if.xkcd.com/11/

Celsius, l/100km, and kph are still all metric.

And in some cases, using metric would simply be worse in daily life.

No?

Like using Kelvin instead of Celcius

They are the same thing. They just use different starting points.

meters per second instead of kilometers per hour

Meters per second and kilometers per hour are both the same system, called the metric system.

seconds in general instead of hours and days

What are you on about? Seconds, hours and days are all part of the same system.

Fun fact: The fuel consumption of a car that needs 10 litres / 100 km (24 miles per gallon) could be simplified to 0.1 mm2

Actually yes. 10 L = 10 dm^3 = 10 000 cm^3. That means 10 L / 100 km = 10 000 cm^3 / 100 km.

To simplify further:

10 000 cm^3 / 100 km =

10 cm^3 / 100 m =

0.1 cm^3 / 100 cm =

0.001 cm^3 / 1 cm =

0.001 cm^2 =

0.1 mm^2

Is it metric? Yes. Is it practical? Not really.

It was already metric from the start before any mathematical simplification was done, so the metric system was definitely practical here.

Is the simplest mathematical form always the same as the simplest practical form? Definitely not, but that has nothing to do with the metric system.

And how do you convert that to mechanical or electrical energy?

From Wikipedia:

However, the calorie is not part of the International System of Units (SI), and is regarded as obsolete,[2] having been replaced by the SI derived unit of energy, the joule (J),[9] or the kilojoule (kJ) for 1000 joules.

Every food already have joules besides of calories.

Can you explain the maths on that?

I thought 1 Joule equals 1 Watt for 1 second. 8 MJ = 8000000 Ws. If you divide by 3600 (60 * 60), you get ~2222 Watt hours or 2.222 kWh.

Calories are metric. This is like wanting people to switch to Kelvin instead of Celsius.

I'd be down for base 10 time.

By more understandable you mean they're used to. They can get used to something else.

Actually, the second is the base unit.

Joules is the correct and accurate unit. Calories are used out of habit despite being a less optimal unit not endorsed by science. So it's weird to be called a redneck for stating that.

PS: and no, this is not imported from the US where no one has ever heard of joules. Joules are however commonly used in some countries already, so there's hope.