Estimate the inverter size you need from your running load in watts, with a typical 25% margin and optional startup surge, or check battery runtime.
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Inverter Size Formula
Choose required rating, battery runtime or supported continuous load. Rating mode checks real watts for running and startup loads separately. Optional true power factors add the corresponding VA requirements. The planning margin starts at a typical 25%; change it to suit your equipment, as it is not a universal safety rule.
Continuous demand:
Adding appliance running loads:
- W = total running load in watts
- Input Load = the total running-load value you enter
- Unit Multiplier = 1 for watts, 1000 for kilowatts
- Quantity_i = number of each appliance
- Watts_i = running watts for each appliance
- PF = true power factor at the specified running or startup condition
- Margin factor = 1 + the entered planning margin divided by 100
- Required VA = continuous apparent-power requirement when running power factor is supplied
Add simultaneous appliance running watts before entering the total. There is no appliance-list mode or automatic load-type preset. Enter the worst total real power during startup, including other running loads, or leave it blank if no load needs a higher startup; it must not be below running load. Optional power factors must match each operating condition. Compare continuous W and VA and startup W and VA for the required duration separately. The next listed W rating is only an illustrative continuous-rating comparison, not equipment approval. Runtime uses usable nominal battery Wh divided by efficiency-adjusted DC power; supported-load mode reverses the margin and can also apply a VA limit.
Illustrative Input Assumptions
| Load type | Examples | Power factor | Example margin factor |
|---|---|---|---|
| Resistive | Heaters, kettles, toasters, incandescent lights | 1.0 | 1.25 |
| Mixed home loads | TVs, laptops, lights, chargers, small electronics | 0.9 | 1.30 |
| Motor-driven | Refrigerators, pumps, fans, air conditioners, drills | 0.7 | 2.00 |
The preceding factors are hypothetical examples, not recommended defaults or verified properties of every load in a category. Use measured or manufacturer data; startup true power factor can differ from running power factor.
Illustrative Inverter Size Ranges
| Illustrative VA rating | Typical use |
|---|---|
| 150 to 300 VA | Small chargers, modem/router, laptop, a few LED lights |
| 500 to 1,000 VA | TV, lights, fan, small electronics |
| 1,500 to 3,000 VA | Refrigerator plus small loads, microwave, limited kitchen loads |
| 4,000 to 6,000 VA | Partial home backup, small cabin, several larger appliances |
| 8,000 VA and above | Whole-home backup, larger off-grid systems, heavy workshop loads |
These broad use examples do not guarantee appliance compatibility. Check actual simultaneous W/VA, startup duration, waveform, temperature derating and battery/BMS limits against the selected equipment.
Example Inverter Size Calculations
Example 1: Mixed home load
For a hypothetical 800 W running load, enter a 30% planning margin and a measured running power factor of 0.9. This illustrates continuous demand only; enter actual simultaneous startup power separately.
A candidate must meet at least 1,040 W and approximately 1,155.6 VA continuously, as well as the separately calculated startup demand for its required duration.
Example 2: Motor-driven load
For a hypothetical 900 W running load with measured running power factor 0.7 and an explicitly chosen 100% planning margin, compare these continuous requirements. These assumptions do not determine actual motor startup demand.
A candidate must meet at least 1,800 W and approximately 2,571.4 VA continuously. Obtain and enter actual startup demand and verify its duration separately.
FAQ
Is inverter size the same as battery size?
No. Inverter size tells you the maximum load the inverter can supply at one time, usually shown in VA, kVA, or watts. Battery size tells you how long the system can run that load. A larger inverter lets you power more devices at once, but it does not automatically give you longer runtime.
Why check both watts and VA?
Watts measure real power; VA measures apparent power. True power factor is watts divided by VA, not inverter conversion efficiency. Rating mode can calculate VA when a matching power factor is entered; supported-load mode can enforce an entered VA limit. Leaving those optional values blank does not verify VA capacity.
Should you choose a pure sine wave inverter?
Check the appliance and inverter manufacturers' waveform-compatibility requirements. Some loads can overheat, malfunction or fail with an unsuitable waveform. This arithmetic tool does not establish suitability for medical or other critical equipment.
