It basically depends on the load you are going to use and the capacity of the inverter you are using. I will categorize by the load in watt-capacity of Inverter-recommended battery ah. Less than 200 W-600 VA-100Ah Less than 400 W-600 VA-120 Ah and above Less than 400 W-800 VA-150 Ah and above Less than 600 W-800 VA-180 Ah and above With this combination, you will get optimum backup and the best battery life. In general, if you are technically sound enough and can calculate charging and discharging ampere then The charging amp should not be more than the Ah of the battery divided by 20. Discharging ampere should not be more than Ah of the battery divided by 5. You can have a 20-30% variance in practical usage. If you are choosing the right combination for your application then you will have peace of mind as it will give you optimum performance over the life span. Also, keep the inverter on Normal mode rather than UPS mode if you are not using any voltage-sensitive device like PC.