Calculator inputs
Enter your connected load, simultaneity, surge behavior, growth allowance, and array ratio. The tool recommends a practical standard inverter rating.
Example data table
| Scenario | Total Load (W) | Simultaneous (%) | Largest Motor (W) | Array (kW) | Suggested Inverter | Comment |
|---|---|---|---|---|---|---|
| Small cabin hybrid | 2,400 | 75 | 450 | 3.2 | 3,000 W | Good for lighting, refrigeration, and modest surge loads. |
| Family home backup | 4,800 | 80 | 900 | 5.5 | 6,000 W | Leaves useful margin for pumps and future circuits. |
| Workshop off-grid | 7,200 | 70 | 1,800 | 8.0 | 10,000 W | Motor surge drives the selected standard size upward. |
| Three-phase farm hybrid | 14,000 | 65 | 2,200 | 18.0 | 15,000 W | Array ratio and running diversity both influence the result. |
Formula used
Simultaneous load = Total connected load × Simultaneous factor
Additional surge = Largest motor watts × (Startup multiplier − 1)
Startup demand = Simultaneous load + Additional surge
Design continuous load = Simultaneous load × (1 + Safety margin) × (1 + Future expansion)
Load-based inverter size = Design continuous load ÷ (Maximum loading × Derating factor)
PV-based inverter size = Solar array DC watts ÷ Target DC/AC ratio
Recommended minimum = Greater of load-based and PV-based checks, except pure off-grid systems prioritize load and surge first.
Single-phase AC current = Inverter watts ÷ (AC voltage × Power factor)
Three-phase AC current = Inverter watts ÷ (√3 × AC voltage × Power factor)
DC input current = Inverter watts ÷ (DC voltage × Efficiency)
How to use this calculator
- Choose the system type, phase, frequency, and AC/DC voltage combination that matches the inverter family you intend to buy.
- Enter the total connected load, then reduce it with a realistic simultaneous load factor to reflect what actually runs together.
- Add the largest motor or compressor and its startup multiplier so the calculator can test surge performance.
- Include safety margin, future growth, and derating to cover heat, dirty enclosures, altitude, and design uncertainty.
- Set maximum loading to keep the inverter below full stress during extended operation.
- Enter solar array size and a target DC/AC ratio when sizing hybrid or grid-tied equipment around the PV field.
- Press the calculate button to show the recommended inverter above the form, then export the result as CSV or PDF.
Frequently asked questions
1. Why does simultaneous load matter?
Most circuits do not run together at nameplate power. Simultaneous load avoids buying an oversized inverter based on unrealistic full-demand assumptions.
2. Why include the largest motor separately?
Motors, pumps, and compressors can pull several times their running watts during startup. That brief surge often determines the minimum inverter class.
3. What does maximum inverter loading mean?
It is the share of nameplate output you are willing to use continuously. Designers often stay below 100 percent to reduce heat and extend service life.
4. Should off-grid and grid-tied systems use the same method?
Not exactly. Off-grid sizing is driven mainly by running load and surge demand, while grid-tied and hybrid designs also consider PV array ratio.
5. Does inverter efficiency change the AC size recommendation?
Efficiency affects DC-side current and backup energy throughput more than AC nameplate power. AC sizing still depends mostly on load, headroom, and surge.
6. What is a good DC/AC ratio?
Many systems land between 1.1 and 1.35, but climate, clipping tolerance, orientation, and financing goals can justify values outside that range.
7. Why does a higher DC voltage help?
For the same power, higher DC voltage reduces current. Lower current can simplify cabling, improve efficiency, and limit conductor heating.
8. Is this enough for final procurement?
Use it as a design screen, not the final authority. Always confirm manufacturer datasheets, code requirements, protection devices, and site conditions.