How to size an inverter and battery for your home, with a worked example
Most people buy an inverter by asking a friend what they have. That works until the lights go dim after an hour or the inverter trips with the mixer running. A few minutes of arithmetic avoids both problems. Here is the method, with an example you can copy.
Step 1: list what you want to run
Write down each appliance you want during a power cut and its wattage (printed on the label). A sample list:
- 3 ceiling fans at 75 W = 225 W
- 4 LED lights at 10 W = 40 W
- 1 television at 100 W = 100 W
- 1 Wi-Fi router at 15 W = 15 W
The total load is 380 W.
Step 2: choose the inverter rating
Inverters are rated in VA, not watts. As a rough guide, divide the load by about 0.8 to allow for power factor: 380 ÷ 0.8 is about 475 VA. Leave headroom for motors that draw more when they start, so an inverter comfortably above that figure is wise. Do not add heavy loads such as geysers or air conditioners to a normal home inverter.
Step 3: decide the backup time
Say you want about 3 hours of backup. Energy needed is 380 W × 3 h = 1,140 Wh.
Step 4: convert to battery capacity
A lead-acid battery should not be drained fully every time, and the inverter loses some energy as heat. A practical formula is:
Battery Ah = energy (Wh) ÷ (battery voltage × usable fraction × efficiency)
With a 12 V battery, 50 % usable capacity and 85 % efficiency: 1,140 ÷ (12 × 0.5 × 0.85) is about 224 Ah. So a single 12 V battery in the region of 200 Ah, or two smaller batteries wired as the inverter specifies, is the right order of size. Real runtime also depends on battery age and temperature.
Two mistakes to avoid
- Undersized battery: the inverter works, but backup is short and the battery ages quickly from deep discharge.
- Overloaded inverter: it trips or runs hot. Keep the total load well below the rated VA.
Not sure what your home needs? Compare battery types in our guide to tubular, flat plate and SMF batteries, then tell us your requirement and we will help you confirm the match.
