Sizing a pure sine inverter and battery bank
An inverter converts DC from a battery bank into household AC. Sizing it is two questions: what is the worst-case surge, and how much energy do you need between recharges? Get both wrong and the inverter shuts down the first time a refrigerator starts.
Pure sine vs modified sine
A modified sine inverter approximates household AC with a stepped waveform. It works for simple resistive loads and it is cheap. Pure sine produces a clean waveform — often cleaner than the grid — so motors run cooler and electronics behave.
We only sell pure sine. The devices that misbehave on modified sine include laser printers and copiers, clock radios that lose time, variable-speed sewing machines, transformerless chargers, dimmers, furnace controls and HID lighting. For audio and video work, pure sine is not optional.
Continuous vs surge watts
Inverters carry two ratings: continuous (runs all day) and surge (seconds). Motors can pull 3–4× their running load at startup. Size to the sum of running watts plus the largest single surge — not the sum of every surge at once, which almost never happens.
| Load | Surge factor |
|---|---|
| Refrigerator / freezer | 5× |
| Air compressor | 4× |
| Dishwasher | 3× |
| Automatic washer | 3× |
| Sump pump | 3× |
| Furnace fan | 3× |
| Industrial motors | 3× |
| Circular saw | 3× |
| Kerosene / diesel heater | 2× |
Watts to amps: sizing the bank
Two formulas cover almost every DC calculation you will do:
Inverters are 80–90% efficient, so add 10–20% to the DC side. A shortcut that folds this in: divide AC watts by 10 to get DC amps on a 12 volt system.
Example. 500 W AC load for 4 hours a day. 500 ÷ 10 = 50 DC amps. 50 A × 4 h = 200 Ah discharged. At a 50% depth of discharge you need a 400 Ah bank; for longer life, at 25% DoD, an 800 Ah bank (and in practice a little more than either, because capacity falls at high discharge rates).
Typical battery sizes
| BCI group | Voltage | Capacity |
|---|---|---|
| GC2 (golf cart) | 6 V | 220 Ah |
| 31 | 12 V | 105 Ah |
| 4D | 12 V | 200 Ah |
| 8D | 12 V | 245 Ah |
In vehicles, run the inverter from dedicated deep-cycle batteries, not the starting battery. Charge both sets through a battery isolator so the inverter can never strand you.
Parts mentioned in this guide
Common questions
Can I run a microwave on a 1000 W inverter?
Usually not. A 1000 W microwave draws more than its cooking rating, and startup surge pushes it higher. Size for the nameplate watts plus surge, then add inverter inefficiency.
Do I need pure sine for a laptop or phone charger?
No, but small switching supplies are exactly the loads that run hot or buzz on modified sine. Pure sine is the safer long-term choice and costs little at small wattages.
Will an inverter drain my battery when nothing is on?
Idle draw is real — typically a few watts. For long unattended periods use the inverter's remote switch or a transfer-switch model that shuts down cleanly.