Sizing an inverter wrong in either direction causes problems: too small and it can't start your appliances; too big and you're paying for capacity (and standby drain) you'll never use. Here's the step-by-step for US van and RV builds.
Size your inverter automatically
Enter your 120V appliances and we'll calculate the inverter size, cable gauge, and fuse — free.
Step 1: List your AC (120V) loads
Only count things that need a wall-style 120V outlet — most laptops, phones, and small electronics charge fine over 12V/USB and don't need to go through an inverter at all.
| Appliance | Running watts | Surge watts |
|---|---|---|
| Laptop charger | 65W | ~65W |
| Blender | 300W | ~600W |
| Microwave (1,000W rated) | ~1,200W | ~1,800W |
| CPAP (no humidifier) | 30-60W | ~60W |
| Induction cooktop (single burner) | 1,500-1,800W | ~1,800W |
Step 2: Find your largest simultaneous load
Most people don't run a microwave and a blender at the same time, so size for the single largest device (or realistic combination) you'd actually run together — not the sum of everything you own.
Step 3: Add ~25% headroom and check surge
Inverter size ≈ largest load × 1.25, then confirm the inverter's surge rating covers the startup spike of motors and compressors (typically 2x running watts).
- Laptop only → 300-800W inverter is plenty.
- Blender or CPAP → 800-1,000W.
- Microwave → 1,000-1,500W, with surge rating of at least 1,800-2,000W.
- Induction cooktop → 2,000W+, and check the battery side carefully (next step).
Step 4: Check the battery-side current
This is the step people skip — and it's the one that determines your wiring and fusing, not just the inverter:
Battery amps ≈ inverter watts ÷ (12V × 0.87 efficiency)
A 2,000W inverter at full load pulls about 190A. That needs:
- 2/0 AWG cable (or larger) for the inverter-to-bus-bar run
- A Class T fuse rated for that current, mounted within ~7 inches of the battery
- A battery bank rated to deliver 190A continuously without excessive voltage sag — most 200Ah+ LiFePO4 banks handle this comfortably
Full wiring detail is in the wiring & safety guide.
Don't undersize the cable to save cost
A 2,000W inverter run on undersized cable will trip on voltage drop under load, or worse, overheat the cable. The cable and fuse must match the inverter's full-load current — not a "typical" estimate.
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Jack Kennedy is a self-builder who has built two vans. He created Roam Wired to simplify conversion planning and share free tools, supported by affiliate links.
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