Inverter backup time calculator

With two fans, three LED bulbs, a tube light, a TV and the Wi-Fi router (275 W), a 150Ah tubular battery runs for about 4 h 11 min. With just one fan and two LED bulbs (95 W) it runs about 12 h 8 min.

Yes. A BLDC fan uses about 30 W against 75 W for a regular fan. Swapping two fans in the example above takes the load to 185 W and the backup on a 150Ah tubular battery from 4 h 11 min to about 6 h 14 min.

Compare batteries by the energy they hold (kWh), not by Ah, because lithium batteries come at higher voltages. A 150Ah tubular battery holds 1.8 kWh and about 80% of it is usable; lithium batteries are sold as 100Ah modules of 12.8 V (1.28 kWh), 25.6 V (2.56 kWh) or 51.2 V (5.12 kWh), about 90% usable. On the same 275 W load: 150Ah tubular about 4 h 11 min, 12.8 V lithium about 3 h 21 min, 51.2 V lithium about 13 h 24 min. The 51.2 V ones need a 48 V solar or hybrid inverter.

Add up the watts of everything that will run together and divide by 0.8 to get the VA rating, then choose the next size up. 275 W comes to about 350 VA, which any standard home inverter covers. Above about 1,100 VA, home inverters usually need two batteries.

Battery volts × Ah × the share of the battery that can be used × inverter efficiency, divided by the load in watts. We use 80% for tubular, 90% for lithium and 80% inverter efficiency. For 150Ah tubular and 300 W: 12 × 150 × 0.8 × 0.8 ÷ 300 = about 3 h 50 min.