Choosing the right inverter size is the most important step in designing a reliable power backup system. An undersized inverter will struggle to handle your load, while an oversized one wastes money. Here is how to get it right.
Step 1: List Your Appliances
Start by listing every device you want running during a power cut. Common household appliances and their typical wattages include:
- LED Bulb: 9W
- Ceiling Fan: 75W
- Television: 120W
- WiFi Router: 10W
- Refrigerator: 150W
- Laptop: 60W
- Desktop PC: 200W
- Air Conditioner (1 Ton): 1000W
Step 2: Calculate Total Load
Add up the wattage of all devices you plan to run simultaneously. For example, if you need 4 LED bulbs (36W), 2 ceiling fans (150W), 1 TV (120W), and 1 WiFi router (10W), your total load is 316W.
Step 3: Apply the Safety Factor
Multiply your total wattage by 1.3 to get the required inverter VA rating. This 30% buffer accounts for power conversion losses and startup surges.
Formula: Inverter VA = Total Watts x 1.3
For our example: 316W x 1.3 = 411VA. An 600VA inverter would be the right choice.
Step 4: Choose the Right Inverter
Based on your calculated VA requirement:
- Up to 600VA: 600VA Pure Sine Wave Inverter
- 601–800VA: 800VA Pure Sine Wave Inverter
- 801–1000VA: 1kVA Pure Sine Wave Inverter
- 1001–1500VA: 1.5kVA Pure Sine Wave Inverter
- Above 1500VA: 2kVA+ Pure Sine Wave Inverter
Step 5: Size Your Battery
Battery capacity determines how long your backup lasts. Use this formula:
Battery Ah = (Total Watts x Backup Hours) / (12V x 0.8)
The 0.8 factor accounts for battery efficiency losses. For 316W load with 4 hours backup: (316 x 4) / (12 x 0.8) = 132Ah. A 150Ah tall tubular battery would be ideal.
Always Get a Professional Assessment
While these calculations give you a solid starting point, every home is different. Factors like voltage fluctuation in your area, cable run lengths, and appliance startup surges all affect system sizing. That is why we offer a free site inspection: our team will visit your home, assess your exact requirements, and recommend the perfect system.
Use our online power calculator for a quick estimate, or book your free inspection today.
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