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Can someone please explain some aspects of E=MC^2?

Sorry in advance if these are dumb questions, just trying to understand Einstein’s equation better. 

I understand E represents energy, usually potential energy, but what unit is E measured in? Is it dependent on what units are used in the case of M and C (i.e. using grams or kilograms etc, meters per second or kilometer per second etc) or are all of the variables associated constant units? If it’s constant, what is the correct unit to use for each variable?

For example, take a 5 kg object. Multiply by the speed of light (defined in this case as 299,792,458 m/s), squared, (8.987551787368e16 m/s), which gives us 4.493775893684e17. What unit is this number measured in? 

What makes the speed of light relavent to measuring energy in objects with mass? If light is made up of photons, and photons have a mass of 0, that would make the energy of photons equal to 0 according to this equation. Wouldn’t that mean light contains no energy, and therefore isn’t a form of energy like we know it to be? If it’s not a form of energy, why is light warm? How would solar panels work? How would photosynthesis work? How are lasers powerful enough to cut through metal? These are just examples to explain my thought process, not necessarily other questions that need to be answered. 

Thanks y’all, I’ve tried googling (albeit not as hard as I could’ve) but I found nothing that answers these questions, and figured it would be easier to ask.
gpt
> For example, take a 5 kg object. Multiply by the speed of light (defined in this case as 299,792,458 m/s), squared, (8.987551787368e16 m/s), which gives us 4.493775893684e17. What unit is this number measured in? 

Joule.

>  If light is made up of photons, and photons have a mass of 0, that would make the energy of photons equal to 0 according to this equation.

E = m c^2 relates the mass to the energy of a particle at rest. A photon is never at rest and therefore has 0 rest energy.

The equation is a special case of a more comprehensive relation between energy, mass and momentum. This relation is expressed by the following equation:

E^2 = (p c)^2 + (m c^(2))^2

When a particle is at rest, p = 0 and this equation simplifies to E = m c^(2).

But for photons, which are massless particles, the second part of the equation disappears and you're left with E = p c, which expresses a relation between the energy and momentum of a photon.