Regarding the 20TW from the Wikipedia page, that's referring to the EIRP - effective isotropic radiated power. This is the amount of power that you would need

Regarding the 20TW from the Wikipedia page, that's referring to the EIRP - effective isotropic radiated power. This is the amount of power that you would need to radiate from a theoretical antenna with a perfectly spherically symmetric power distribution. The Arecibo antenna has a fairly narrow beam. The 20TW EIRP transmitter appears to operate at 2.38GHz. Plugging that frequency and the 270m diameter, a beamwidth calculator gives me a gain of about 75dBi. So, yes, it's like a 20TW transmitter, but only one tiny narrow little slice of it. At 75dBi gain, that's one (10^7.5)th of the total sphere, about 1 part in 30 million. Or, you could say, if the antenna were radiating the same power in every direction as it does in it's peak direction, that would be 30 million times as much power. So, dividing to find the power in that narrow beam, we get "only" about 600kW radiated in the direction it is pointing. Add in cable losses and other inefficiencies, and the power output at the amplifier is probably close to a megawatt.