We report a compact H-band power amplifier with high output power in 250nm InP HBT technology. Stacking and parallel power combining together provide the desired output power. Common-base stages with base capacitive degeneration act as stacked power cells. Four power cells are combined by a compact low-loss 4:1 transmission line network. At 270GHz, the four-stage amplifier has 16.8dBm saturated output power with 4% power-added efficiency (PAE). Over 266-285GHz, the amplifier's saturated output power is 14-16.7dBm with an associated 2.2-4%PAE. The 3-dB small-signal bandwidth extends from 233GHz to 281GHz with a peak gain of 20.5dB at 264GHz. The amplifier has a compact area of and /mm ² of 57.6mW/mm ² . To the authors' knowledge, these results demonstrate a record output power and for H-band amplifiers working around 270GHz.