Three questions, not one
Inverter sizing conflates three separate things, and separating them is what stops people buying twice the inverter they need.
Continuous is what it must sustain indefinitely: the loads that genuinely run together.
Surge is what it must survive for a few seconds while a motor starts. This sizes the inverter’s peak rating — it does not size the conductors, because a conductor’s thermal mass absorbs a few seconds easily.
DC input current is what it pulls from the battery. This is the output that feeds the rest of your design: the battery cable, the battery-side fuse, and the maximum discharge current your bank has to support.
Working out continuous load
Only what runs simultaneously:
| Load | Watts | Runs with others? |
|---|---|---|
| Fridge (running) | 150 | Yes, cycles all day |
| LED lighting | 60 | Yes |
| Laptop and chargers | 100 | Yes |
| Water pump | 400 | Briefly, occasionally |
| Microwave | 1200 | Not with the hob |
| Induction hob | 1800 | Not with the microwave |
A realistic worst case here is the fridge, lights, laptop and the hob together — around 2100 W, not the 3710 W the column adds up to. Add 25 % headroom and a 3000 W inverter covers it comfortably.
The DC side is where the money goes
I(DC) = continuous AC watts ÷ efficiency ÷ bus voltage under load
The calculator uses roughly 96 % of nominal as the default bus voltage, because current peaks when the battery is lowest and that is the case the cable has to survive. You can enter your own figure if you know the sag on your bank.
The consequence of bank voltage is dramatic:
| Inverter | 12 V | 24 V | 48 V |
|---|---|---|---|
| 2000 W | 185 A | 93 A | 46 A |
| 3000 W | 278 A | 139 A | 69 A |
| 5000 W | 463 A | 231 A | 116 A |
At 12 V a 5000 W inverter needs over 460 A, which is beyond any single conductor in the NEC tables up to 500 kcmil. If you are still choosing a bank voltage, this table is the argument.
Surge, and when to solve it differently
Most inverters offer roughly 2× continuous for a few seconds. If your surge demand is much beyond that, a larger inverter is one answer and often the worst one — you pay for continuous capacity you never use, plus its idle draw, to cover a two-second event.
A soft starter on the offending load is frequently cheaper. Fitted to an air conditioner or a well pump, it limits inrush to a fraction of what the motor would otherwise demand, and it lets a correctly-sized inverter handle a load that would otherwise have forced the next size up.
What comes next
The DC input current from this calculator is the input to two other decisions: the battery cable, and the overcurrent device that protects it at the battery end. Both are sized from that number, not from the inverter’s AC rating.