Windup toy cars are also E=mc^2.

E = mc^2 implies that mass (m) is being converted to energy (E), and a windup toy car is definitely not converting mass to energy. Windup cars get their energy from torsional springs, which are E = (1/2) k theta^2.

>and a windup toy car is definitely not converting mass to energy

Ah, but it is. E=mc^2 is always true in the reference frame where the object is stationary. (We assume that we are co-moving with the windup car.) A spring under torsion is heavier than an unwound spring.

> A spring under torsion is heavier than an unwound spring

Does it have more (invariant) mass?

Yes.

No, it's E=(0.001M)C^2

That is absurd. It is E=MC^2 where M is 0.1% of the total mass of the reactants.

100% of the mass difference is converted into energy.

The GP was talking in relation to W/Kg, and the output per Kg is definitely not e=mc^2. It helps reading things in the context.

You are correct, though it would help to clarify that you're correcting the GP on the "per kilo" part, and not the "energy" part.

I agree, if you're deploying 1 kg of fissile material, only 0.1% of that will be converted to energy. We should talk about energy per mass of fissile material, not energy per mass of "mass difference".