Betelgeuse is far away, so it wouldn’t look very bright to us, would it? Well, it’s not all that far away. Merely 600 light years. Our sun is 500 light seconds. So, dividing 500 seconds into 600 years and squaring, it’s just about a factor of one quadrillion. Which would make supernova Betelgeuse only 1/10,000th as bright as the sun. Whew, no chance of excessive sunburn! (You’ll get zapped by possibly 1000 neutrinos, no big deal. Happily, the main radiation jets will be pointed away from the earth. But no predictions on what happens when the charged particles arrive; I’d have to do more research.)
But, that’s bright! The full moon is 1/300,000th as bright as the sun. So this arbitrary calculation says supernova Betelgeuse would be 30 times as bright as the full moon. Huge uncertainties in that number, but it seems safe to say it’s somewhere between one and several hundred times as bright as a full moon.