Buy a generator or build you a battery pack + ips.
I spent 1800$ last year on just parts alone to build me a 500AH lithium backup supply with inverter, it holds 1000w for about 4/5 hours and has already saved my ass during thunderstorms/windstorms ect.
Cheapest way is probably stacking SLA/FLA batteries in parallel on an IPS. 400w for 2 hours would need a 160ah lead acid, just don't use lead acid in enclosed areas.