You really want to get a power specialist involved here, as the solution you need is composed of at least three components: the internal (interior) UPS, a PDU (Power Distribution Unit) and the generator itself.
I have used solutions from Leibert for the latter two items, which were tied into a massive diesel generator system owned, maintained and controlled by our facilities management department. The generated power fed the Leibert PDU, which fed the datacenter UPS racks. Leibert (now Emerson Network Power) has solutions for a variety of sizes and loads.
I was, at the time, located in Orlando, FL and we survived several near and direct hits from several hurricanes (Ivan was the biggest we dealt with) and were still able to maintain local/LAN access, WAN access and our Internet presence.
APC also has complete solutions available, but I emphasize the need to get a specialist in power disaster recovery involved.