It needs to be at least 10MW. Maybe 12MW.
At 5MW, if more than 5 robots come to charge, they drain the battery. It eventually charges the robots at half rate.
The original roboport has 4MW of output and 5MW of input. It seems an n+1 philosophy. But 10MW should work, since there is always some time where robots have to approach the charger from where they were waiting. It should never discharge the battery, unless there's a brownout or blackout, and if power is restored, it should always have enough time to recharge the battery, eventually.
