Solar Inverter Size Calculator
This solar inverter size calculator finds the inverter rating for a solar system. For a grid-tied system it sizes the inverter from the array and a DC to AC ratio. For an off-grid or backup system it sizes the inverter from your running load and the starting surge of motors and compressors. The solar inverter size calculator also gives the next standard size up.
Results
The Solar Inverter Size Calculator is part of our Solar collection. Browse more solar to size, convert, and check your work alongside this tool.
What the solar inverter size calculator does
A grid-tied inverter turns the DC power of the panels into AC for the home and the grid. Panels rarely reach their full rating, so inverters are usually sized a little below the array, a DC to AC ratio of 1.1 to 1.3. An off-grid inverter has a different job: it must power every load running at once and handle the brief surge when a motor starts. The calculator handles both cases.
How to size an inverter
For a grid-tied system:
For an off-grid inverter, the continuous rating must cover the running load with a 25% margin, and the surge rating, usually about twice the continuous rating for a few seconds, must cover motor starts:
The calculator then picks the next standard size: 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, or 15 kW.
Solar inverter calculator example
A grid-tied array of eighteen 400 W panels, 7.2 kW DC, at a ratio of 1.2:
Any inverter from 5.5 to 6.5 kW keeps the ratio between 1.1 and 1.3. For an off-grid cabin with 2,400 W of running load and a well pump that surges to 8,000 W, the inverter needs 4,000 W continuous to cover the surge, so choose a 4 kW unit.
How to use the inverter size calculator
Choose grid-tied or off-grid. For grid-tied, enter the array size in kW DC and the DC to AC ratio, 1.2 if you are unsure. For off-grid, enter the total watts of everything that can run at the same time and the starting surge of the largest motor load. Press Calculate to see the size and the nearest standard rating.
Reading your inverter sizing result
On a grid-tied system a ratio above 1.0 means the inverter will clip a little power on the sunniest hours, but it runs closer to full load the rest of the day, which raises yearly output per dollar. Ratios above about 1.3 clip enough to matter. Off-grid, an inverter that is too small trips when a large motor starts, so the surge figure often decides the size.
When to use an inverter size for solar system check
- Matching an inverter to a new solar array
- Checking an installer design
- Sizing an off-grid cabin inverter
- Planning a backup inverter for essential loads
- Adding panels to an existing inverter
Common inverter size calculator mistakes
- Matching the inverter to the panel label exactly. A 1.1 to 1.3 ratio is normal.
- Ignoring motor surge. Off-grid inverters must start the largest motor.
- Exceeding the inverter DC input limit. Check the maximum array size on the spec sheet.
- Forgetting voltage limits. String voltage must suit the inverter.
Tips for solar inverter sizing
- Leave room to add panels later.
- Pick a pure sine wave inverter for motors and electronics.
- Check the inverter warranty against the panel warranty.
Solar inverter sizing for grid-tied systems
Panel ratings are measured in lab conditions that rarely happen on a roof: heat, angle, and dust keep real output 10 to 25% below the label. Sizing the inverter to match the label would leave it working at part load most of the day. That is why designers oversize the array relative to the inverter. Start with the array from the solar panel calculator, then apply a ratio of about 1.2. Microinverters and optimizers follow the same idea, panel by panel.
Off grid inverter size and inverter load calculation
For off-grid and backup systems, list every load that can run at the same moment: fridge, lights, pump, and so on. Motors surge three to seven times their running watts for a second or two. An inverter that cannot handle that surge will shut down. Pair the inverter with enough battery capacity from the battery bank calculator, and for long cloudy spells, a backup generator sized with the generator size calculator.
DC to AC ratio table for common arrays
| Panels | Array DC | Inverter range (1.3 to 1.1) | At ratio 1.2 |
|---|---|---|---|
| 10 × 400 W | 4.0 kW | 3.08 to 3.64 kW | 3.33 kW |
| 15 × 400 W | 6.0 kW | 4.62 to 5.45 kW | 5.00 kW |
| 20 × 400 W | 8.0 kW | 6.15 to 7.27 kW | 6.67 kW |
| 25 × 400 W | 10.0 kW | 7.69 to 9.09 kW | 8.33 kW |
| 30 × 400 W | 12.0 kW | 9.23 to 10.91 kW | 10.00 kW |
Solar Inverter Size Calculator FAQ
What size inverter do I need for a 5 kW solar system?
At a DC to AC ratio of 1.2, about 4.2 kW AC. Anything from about 3.8 to 4.5 kW keeps the ratio between 1.1 and 1.3.
What is a good DC to AC ratio?
1.1 to 1.3 for most grid-tied systems. Sunny, hot sites lean lower; cloudy or cool sites can go higher.
What is inverter clipping?
When the array produces more DC than the inverter can convert, the extra is lost. A small amount at midday is normal and economical.
How do I size an off-grid inverter?
Add the watts of all loads that run together, add 25%, and check the surge rating covers the largest motor start.