Inverter Capacity Calculator – Size Your Inverter
Calculate the required inverter capacity for your electrical loads. Enter your devices and their power consumption to determine the right inverter size.
Recommended: 20-25% for motor startup
Step 1: List all devices you want to power with their wattage ratings and daily usage hours.
Step 2: Add up the total watts and calculate daily energy consumption in watt-hours.
Step 3: Add surge margin for motor startup, then select inverter and battery based on results.
What Is an Inverter
An inverter converts DC battery power to AC power for household devices. Sizing matters - too small and the inverter shuts down under load. Too large and you waste money on capacity you'll never use. The key is matching both continuous wattage and surge capacity.
Why Surge Margin Matters
Motors and compressors need 2-3 times their running wattage to start:
Devices with Surge
Refrigerators, air conditioners, well pumps, power tools. These need 20-50% extra inverter capacity to handle startup without tripping.
Devices without Surge
LED lights, phone chargers, laptops, TVs. These draw steady power and don't need extra surge capacity.
Battery Sizing Explained
Battery capacity (Ah) depends on your daily energy use and system voltage. We multiply by 1.5 to avoid draining batteries below 50% - deep discharges kill lead-acid batteries fast. Lithium batteries can discharge deeper but cost more upfront.
| Device | Running Watts | Surge Watts |
|---|---|---|
| LED light bulb (10W) | 10 W | 10 W |
| Laptop charger | 65 W | 65 W |
| LED TV 55" | 120 W | 120 W |
| Refrigerator | 200 W | 600 W |
| Microwave | 1,000 W | 1,000 W |
| Well pump (1/2 HP) | 1,000 W | 2,000 W |
| Space heater | 1,500 W | 1,500 W |
| Window AC (5,000 BTU) | 500 W | 1,500 W |
Surge watts apply only during startup (1-3 seconds). Running watts are continuous draw.
Compare running vs surge watts. The highest surge determines minimum inverter surge capacity. Total running watts determines continuous inverter rating.
Battery Capacity Formula
Ah = (Daily Wh / Battery Voltage) × 1.5
The 1.5 multiplier keeps lead-acid batteries above 50% discharge. For lithium (LiFePO4), use 1.2 multiplier since they can discharge to 80%.
12V vs 24V Systems
Higher voltage means lower current for the same power:
- 12V systems: Simple, common, good for under 2,000W
- 24V systems: Lower current, thinner wires, better for 2,000-4,000W
- 48V systems: Professional installations, over 4,000W
Days of Autonomy
Want backup for cloudy days? Multiply battery capacity by days of autonomy. Three days autonomy means tripling battery capacity - expensive but useful for off-grid living.
What size inverter do I need?
Add up all device wattages, then add 20-25% surge margin. For example, a fridge (200W) + lights (50W) + TV (120W) = 370W. With 25% margin, you need a 463W inverter - round up to 500W or 600W.
How long will a battery last with an inverter?
Divide battery watt-hours by total load watts. A 100Ah 12V battery has 1,200Wh. Running a 100W load gives 12 hours, but limit to 6 hours to avoid deep discharge on lead-acid batteries.
Can I run a refrigerator on an inverter?
Yes, but size for surge. A 200W fridge might need 600W startup. Use a 1,000W inverter minimum. Also consider a soft starter to reduce surge to 2-3x instead of 5-6x running watts.
What is the difference between pure sine wave and modified sine wave?
Pure sine wave inverters produce clean power like the grid - required for sensitive electronics, motors, and medical devices. Modified sine wave is cheaper but can cause humming in motors and won't work with some devices.
How many batteries do I need for a 3,000W inverter?
At 12V, a 3,000W inverter draws 250A. You'd need 250Ah of battery per hour of runtime. For 4 hours at 50% discharge, that's 2,000Ah - about four 200Ah batteries in parallel. Consider 24V or 48V to reduce current.
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