Part 15 reads almost the same for 900 MHz as for 2.4 GHz: You can use 1W if you are doing one of three things.
Thing 1 is to send a direct-sequenced spread spectrum signal (not in this case)
Thing 2 is to send a frequency-hopping spread spectrum signal, with a maximum dwell time of 400ms and a minimum of 50 channels in your spreading sequence (again, not the case here)
Thing 3 is to send a digital signal of at least 500 kHz RF bandwidth (which is likely to be the case here)
Additionally, there are bonuses for using good antennas (the FCC seems to want to encourage this). You do need to reduce your transmit power if you have a gainful antenna, but you only have to reduce power by 1dB for every 3dBi of antenna gain. For example, using a 3dBi antenna (for instance, a 5/8 wave) would double the strength of your transmitted signal, but would require you to turn down your transmitter power by 1 dB, making it roughly 800 mW rather than 1W. Put this together, and you get an effective radiated power (ERP) of ~ 1.6W.
In a more extreme case, imagine using a 24dBi directional antenna: You get a 2^8 boost in your signal from the antenna, and only have to cut your power by 8dB. Actual transmitter power ends up at 160mW, but the boost from the antenna gives you an ERP of ~40W.