Even a 1000W inverter can only support a few hundred watts if it is connected to batteries by a small diameter or too long cables. This is because of the so called "voltage drop". At 220VAC mains, you can use 30-40 meters length extension cord without worry about voltage drop, but when talking about low voltages (12V, 24V or 48V), you will have to calculate this. The smaller cable size is, the larger voltage drop gets. Also, the longer cable you use, the more voltage will drop between the two ends of cord. So here is a simple rule: just use thick and short cable to connect inverter/UPS with battery terminals. For your reference, you may find exact length and diameter values in users' manual.