The champions have power from their total stats (including bonus from items), the team power is the addition of the power of all 4 champs.

I don't know the exact formula. You could find out by equipping and unequipping different items and calculate the power per attack/def/hp and so on. 

But what is known about the champion power is, that points in resistance have a very big and points in speed a rather small influence. The power doesn't show the real strength of a team. An 90k speednuking team with no defensive stats can wipe out a 200k team in round 1 of the fight. The nuking team has a low power rating, because they neither need nore have any defense. If you see a team with speedbuffer (Arbiter, Apo, High Khatun or similar), debuffer (Warmaiden, Uugo, Tayrel or similar) and damagedealer (Foli, Trunda, Magnarr, Skullcrown or champs like that) that has a power rating that seems absurd low compared to your other opponents, you should avoid that fight.

Team power is a simple sum of champion power. Champ power is based off each individual stat, multiplied by a static multiplier.

All that said - please do yourself a favour and ignore team power (and, by extension, champ power) completely. Especially if you are new, it will serve absolutely zero purpose for you - in fact, it will likely do the opposite, and lead you towards making incorrect decisions.

It does help later in the game in arena specifically, to try to ascertain the type of team you're facing, but you need to have a pretty good understanding of the mechanics to make use of it.

Here is a screenshot from one of the CC's videos, don't remember which but it was exactly about power. 

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As you can see, a team with high power might have high CR/CD (usually the case with go first teams) or high resistance (go second defence teams), or both because why not. Which of this is the case depends on team composition and Arena rank.

(Edited to make image smaller)

That's a bit misleading. You're correct in your analysis of the raw data, but there's a big difference in how much of those values you can actually get.

As an example - my high RES/DEF champs are typically running 300-350 RES, and around 5K DEF, yielding about 95K team power, plus the extra amount coming from the other stats. On the other hand, my nukers will get to maybe 250 crit damage and 100% crit rate, and around 5K ATK, yielding around 50K team power.

The tl;dr is - if your opponent has team powers above around 180K, there's a very good chance they're running either RES/DEF or pure DEF. On the other hand, if they're below 150K, there's a very good chance they're running speed nuke teams. 

I heard these numbers were debunked so I ran them in the spreadsheet for one each HP, DEF, ATT, and Support champ.  These numbers are rubish!!!

80000 * Resistance

80000 * Crit Rate

1 * Everything Else.

Basically it's as above....lol /s