Inverter Sizing Guide: How to Choose the Right Wattage
Last updated: April 2026
To size an inverter correctly, add up the wattage of all appliances you will run simultaneously, then add a 20% buffer. A household running a fridge (200W), microwave (1,700W), and lights (50W) at the same time needs at least 2,340W continuous -- so a 3,000W inverter is appropriate. You must also check the inverter's surge rating to ensure it can handle startup spikes from compressors and motors, which can draw 2-4x their continuous wattage for a few seconds. Finally, match the inverter's DC input voltage (12V, 24V, or 48V) to your battery bank configuration.
Step 1: Calculate Your Continuous Load
List every appliance you might run at the same time and add up their wattage. This is your peak simultaneous continuous load -- the number your inverter must handle without breaking a sweat.
Do not average your loads across the day. Inverters must handle the worst-case moment when the fridge compressor kicks on, the microwave is heating dinner, and the well pump is running -- all at once.
| Appliance | Continuous | Surge | Notes |
|---|---|---|---|
| LED lighting (whole house) | 20-50W | None | Resistive load, no surge |
| Laptop charger | 50-100W | None | Resistive/electronic load |
| Full-size refrigerator | 100-200W | 400-800W | Compressor startup spike 3-4x |
| Microwave (1,000W rated) | 1,500-1,700W | 1,700-2,000W | Draws more than its cooking wattage |
| Induction cooktop (single) | 1,200-1,800W | 2,000W | Moderate startup spike |
| Mini-split heat pump | 500-1,200W | 1,500-3,600W | Compressor surge 2-3x continuous |
| Well pump (1/2 HP) | 500-750W | 1,500-2,250W | Motor startup spike 3x |
| Washing machine | 300-500W | 800-1,200W | Motor surge during spin cycle |
| CPAP machine | 30-60W | None | Requires pure sine wave |
| Power tools (circular saw) | 1,200-1,800W | 2,400-3,600W | Motor surge 2x at startup |
Step 2: Account for Surge Wattage
Motors and compressors draw 2-4x their continuous wattage for the first 1-3 seconds when starting up. This is called surge, startup, or inrush current. Your inverter's surge rating must exceed the highest single-appliance startup spike that could occur while other loads are already running.
Surge Sizing Example
Scenario: Fridge compressor starts (800W surge) while lights (50W) and laptop (100W) are on.
- Running loads: 50W + 100W = 150W
- Fridge surge: 800W
- Total instantaneous demand: 950W
Your inverter needs a surge rating of at least 950W. Now imagine the well pump (2,250W surge) starts at the same moment -- that is 2,400W instantaneous. Always plan for the worst overlap.
Step 3: Apply the 20% Buffer
Running an inverter at 100% capacity continuously causes excess heat, reduces efficiency, and shortens lifespan. The industry-standard recommendation is to oversize your inverter by 20% above your calculated peak simultaneous load.
The Formula
Recommended Inverter Size = Peak Simultaneous Load x 1.2
Example: 2,500W peak load x 1.2 = 3,000W inverter. This keeps the inverter operating at ~83% capacity during peak moments, well within its thermal comfort zone.
Do not oversize beyond 50% of your peak load. As noted above, larger inverters have higher idle draw, wasting battery capacity when loads are light. A 3,000W inverter is right for a 2,500W peak load. A 5,000W inverter would waste energy at idle for no meaningful benefit.
Step 4: Choose Your Input Voltage (12V vs 24V vs 48V)
The inverter's DC input voltage must match your battery bank voltage. Higher voltages mean lower current for the same power, which allows thinner (cheaper) cables and reduces voltage drop over long wire runs. Here is how the three common voltages compare:
12V Systems
24V Systems
48V Systems
Common Inverter Sizing Mistakes
These are the errors we see most frequently in off-grid system designs. Each one can lead to equipment failure, wasted money, or both.
1. Sizing to average load instead of peak simultaneous load
What happens: Inverter overloads when multiple appliances run at once (fridge compressor kicks on while microwave is running)
Fix: Add up the maximum wattage of all appliances that could reasonably run simultaneously, then add 20%
2. Ignoring surge/startup requirements
What happens: Compressor-based appliances (fridges, AC, pumps) fail to start because their 2-4x startup spike exceeds inverter surge rating
Fix: Check both continuous AND surge ratings. Ensure surge rating exceeds your highest single-appliance startup spike
3. Forgetting inverter efficiency losses
What happens: Battery bank depletes 10-15% faster than calculated because inverters are only 85-93% efficient
Fix: Factor in 90% efficiency when sizing your battery bank. A 2,000W load actually draws ~2,220W from batteries
4. Using 12V for high-power systems
What happens: Extremely high current draw requires expensive, thick cabling and causes significant voltage drop over distance
Fix: Switch to 24V for systems above 2,000W or 48V for systems above 3,000W
5. Choosing modified sine wave to save money
What happens: Sensitive electronics malfunction, motors run hot and wear faster, audio equipment buzzes, CPAP machines may not work
Fix: Always use pure sine wave for permanent off-grid installations. The price difference is now minimal