Inverter Size Calculator
Estimate inverter wattage and approximate DC current from running load, startup surge, margin and battery voltage.
Estimate inverter wattage and approximate DC current from running load, startup surge, margin and battery voltage.
Inverter wattage and DC-current requirements must be calculated together.
Use this tool for planning, then verify the exact inverter, battery, cable and fuse specifications.
Add Running Watts, Startup Surge and 20% Margin
The calculator also estimates DC current so you can see why higher-voltage battery systems are useful at large power levels.
Calculator Guidance
Use the result as a minimum class, not a substitute for manufacturer surge data or electrical design.
Inverter Size Calculator
Approx. 204A DC at 12V
How It Works
The calculator adds simultaneous running watts, the largest additional startup surge and margin, then estimates DC current at 90% efficiency.
Examples
| Load | Running | Surge | Suggested Class |
|---|---|---|---|
| Laptop + TV | 200W | Low | 300–500W |
| Refrigerator | 400W | 1,200W | 2,000W |
| Microwave | 1,500W | Low | 2,000W |
| RV AC | 1,600W | 2,500W | 3,000W+ |
Limitations
Use Actual Nameplates
Generic figures are estimates.
Check Surge Duration
Not all inverters sustain peak equally.
Match Battery Voltage
12V, 24V and 48V are different products.
Check BMS Current
Lithium batteries can trip.
Size Cables and Fuse
Follow the manual.
Runtime Is Separate
Battery watt-hours control duration.
How to Use the Result in the Real World
Treat the calculated result as a planning baseline rather than an exact promise. Real systems lose energy through conversion, wiring, temperature effects and internal electronics, while appliances can draw differently from their nameplate ratings.
Leave practical headroom instead of selecting equipment that exactly matches the result. Extra margin helps with startup surges, battery aging, changing weather and future loads, and it reduces the chance that normal variation pushes the system beyond its limits.
When the result affects a major purchase, verify the important inputs. Measure appliance consumption when possible, check the exact inverter or charge-controller limits, and compare the calculation with the equipment manufacturer's specifications.
Frequently Asked Questions
What margin should I add?
About 20% is practical.
Why is DC current so high?
Battery voltage is much lower than AC output voltage.
Should I add every surge?
Only realistic overlapping surges.
What efficiency is used?
The calculator uses 90%.
Does this calculate runtime?
No.
Is 24V better than 12V?
For large systems, often yes.