One of the most frequent questions our engineers receive is: “How long will my UPS run my appliances during a power outage?”
Calculating your UPS requirements does not have to be confusing guesswork. With two simple engineering formulas, you can accurately determine the exact KVA rating and battery Ah capacity you need.
Step 1: Calculate Your Total Power Consumption (Watts)
List all the appliances you intend to run simultaneously during a blackout and sum their rated wattage.
Typical Household & Office Wattage Table:
- LED Tube Light / Bulb: 20W – 40W each
- Ceiling Fan: 70W – 80W each
- Wi-Fi Router / Modem: 15W
- Laptop / Ultrabook: 50W – 90W
- Desktop Computer + Monitor: 150W – 250W
- 55-inch LED TV & Set-Top Box: 100W – 150W
- Inverter Refrigerator (250L – 350L): 150W – 250W
- 1.5 Ton Inverter AC (Running Power): 1,200W – 1,600W
Example Calculation for a Typical 2 BHK Home:
- 4× LED Lights = 80W
- 3× Ceiling Fans = 225W
- 1× 55“ Smart TV = 120W
- 1× Wi-Fi Router = 15W
- 1× Inverter Refrigerator = 200W
- Total Load = 640 Watts
Step 2: Determine the Required UPS VA Rating
UPS systems are rated in Volt-Amperes (VA). Because AC circuits have a Power Factor (PF) (typically 0.7 to 0.8 for home appliances), the formula to convert Watts to VA is:
$$\text{Required VA} = \frac{\text{Total Load in Watts}}{\text{Power Factor (0.8)}} \times 1.25 \text{ (Safety Margin)}$$
For our 640W example: $$\text{Required VA} = \frac{640}{0.8} \times 1.25 = 1,000 \text{ VA} = 1\text{ KVA}$$
Conclusion: An ESSMA 1KVA / 12V UPS or ESSMA 2KVA / 24V UPS is the ideal choice.
Step 3: Calculate Battery Backup Time (in Hours)
The standard formula for estimating battery backup duration is:
$$\text{Backup Time (Hours)} = \frac{\text{Battery Voltage (V)} \times \text{Battery Capacity (Ah)} \times \text{Efficiency (0.9)}}{\text{Total Load (Watts)}}$$
Example 1: 1KVA UPS with 1× 12V 150Ah Tubular Battery
- Battery Voltage = 12V
- Battery Capacity = 150Ah
- Efficiency = 90% (0.9)
- Total Load = 400 Watts
$$\text{Backup Time} = \frac{12 \times 150 \times 0.9}{400} = \frac{1,620}{400} \approx \mathbf{4.05 \text{ Hours}}$$
Example 2: 2KVA UPS with 2× 12V 150Ah Batteries (24V System)
- Battery Voltage = 24V (2× 12V in series)
- Battery Capacity = 150Ah
- Total Load = 600 Watts
$$\text{Backup Time} = \frac{24 \times 150 \times 0.9}{600} = \frac{3,240}{600} \approx \mathbf{5.4 \text{ Hours}}$$
Backup Estimation Reference Chart
| Load (Watts) | 12V 150Ah (1 Battery) | 24V 150Ah (2 Batteries) | 48V 150Ah (4 Batteries) |
|---|---|---|---|
| 200 Watts (Lights + Fans + Wi-Fi) | ~7.5 Hours | ~15.5 Hours | ~31 Hours |
| 400 Watts (+ TV + Laptop) | ~3.8 Hours | ~7.8 Hours | ~16 Hours |
| 600 Watts (+ Refrigerator) | ~2.5 Hours | ~5.2 Hours | ~10.5 Hours |
| 1,000 Watts (+ Heavy Electronics) | Not Recommended | ~3.0 Hours | ~6.2 Hours |
| 1,500 Watts (+ Inverter AC) | Incompatible | ~1.8 Hours | ~4.0 Hours |
Interactive Load Calculation
Rather than calculating by hand, you can use the interactive ESSMA Load Calculator directly on our homepage.
Simply select the appliances you wish to power, and our tool will automatically suggest the best inverter model and battery combination for your home or business.
