Key takeaways
- Size for continuous watts and surge watts.
- A practical rule of thumb: add 25% headroom above your simultaneous running watts.
- Motors and compressors need roughly 2–3× their running watts to start; hard-start loads like well pumps can briefly demand 3–7×.
- Oversizing can increase idle losses and cost.
- Inverter choice affects battery draw and wiring requirements.
The quick answer
Add up the running watts of everything you’ll power at the same time, multiply by 1.25, and pick the next size up. Then check surge: the worst case is your biggest motor starting while everything else runs, so the inverter’s surge rating must cover that moment. Use the calculator below — it does both checks plus battery-side amps.
Inverter sizing calculator
Check the loads you plan to run at the same time. Defaults come from our solar load calculation table — edit any value to match your actual equipment.
Step 1: List your AC loads and peak watts
Add up the AC devices you may run at the same time. For each device, use nameplate watts or a measured value (many appliances vary during operation).
Peak watts ≈ sum of simultaneous AC watts
Related: How to size a solar system
Step 2: Account for surge (starting) power
Some loads require a high startup surge (motors, compressors). Inverter specs typically list a surge rating for a short time window.
Surge headroom = inverter surge rating − expected surge load
If you’re near the limit, the system may trip or fail to start the device reliably. A soft-start kit on a well pump or compressor can cut its startup demand by 50–70%, often removing the surge bottleneck entirely.
Step 3: Check battery-side current draw
Inverters draw significant current from the battery, especially at lower system voltages. A rough estimate:
Battery amps ≈ AC watts ÷ (battery volts × efficiency)
Example: 1,000W ÷ (12V × 0.9) ≈ 93A. High currents impact wiring size, fusing, and heat.
Battery capacity calculator Components overview
Step 4: Choose inverter type and waveform
For many off-grid and RV use cases, waveform matters for compatibility.
Pure sine vs modified sine wave Micro vs string inverters (grid-tied)
If your load list lands in the 1500–2000W continuous range, this is the honest default: pure sine for electronics and motors, remote switch, and cables in the box — no surge-headroom upsell.
Check price on Amazon Price & availability shown on Amazon.com — we may earn a commission.Worked example: small off-grid cabin
Loads running together: refrigerator (200W), LED lighting (300W), laptop + phone charging (100W), microwave (1,500W input — a “1,000W output” microwave draws ~1,400–1,600W from the bank).
- Simultaneous running load: 200 + 300 + 100 + 1,500 = 2,100W
- With 25% headroom: 2,100 × 1.25 = 2,625W → pick a 3,000W inverter (next common size)
- Worst-case startup: microwave starting while the rest runs = 600 + 1,500 = 2,100W; refrigerator starting while the rest runs = 1,900 + 1,200 = 3,100W ← the binding case. A 3,000W inverter with a typical 2× surge rating (6,000W) covers it.
- Battery-side at 12V (90% efficiency): 2,100 ÷ 10.8 ≈ 194A continuous, 3,100 ÷ 10.8 ≈ 287A during startup — heavy. At 24V both numbers halve; at 48V they quarter. This is why bigger inverter loads push systems toward higher bank voltage (see 12V vs 24V vs 48V).
Common sizing examples (quick ranges)
| Use case | Typical inverter size | Common notes |
|---|---|---|
| Charging + small appliances | 300–800W | Lower surge needs |
| Microwave / mixed RV loads | 1,000–2,000W | Surge and wiring matter |
| Heavy loads | 2,000–4,000W+ | Battery bank and voltage become critical |
FAQ
What happens if my inverter is too small?
Is a bigger inverter always better?
Does inverter size change battery size?
Should I choose pure sine wave for solar?
How much inverter surge rating do I need?
Next logical reads
Pure sine vs modified sine wave Inverter keeps shutting off (troubleshooting) RV solar sizing guide How to size a solar system Solar system cost breakdown