When the light goes, an inverter is the difference between a minor inconvenience and a dead fridge and a dark house. It is also one of the easiest things to get wrong, because the people selling inverters and the people buying them often use different words for the same thing.
This guide explains how a basic inverter system works and how to size one for your own home. The maths is simple, and it protects you from paying for too much, or buying too little.
How it works
A basic inverter system has three parts. A battery stores energy. A charger fills the battery whenever mains power or solar is available. The inverter turns the battery’s direct current into the alternating current your appliances use. When the power goes out, the inverter switches over and keeps your chosen appliances running until the battery is empty.
Step 1: List what you want to power
Write down every appliance you want running during an outage, and find its power rating in watts. It is usually printed on a label at the back or the bottom. If it only lists amps, multiply by 230 volts. These figures are typical, so check your own appliances:
- LED bulb: 5 to 12 watts
- Ceiling fan: 50 to 75 watts
- 32-inch LED television: 40 to 60 watts
- Decoder: 15 to 25 watts
- Wi-Fi router: 10 to 20 watts
- Laptop charger: 45 to 65 watts
- Phone charger: 10 to 20 watts
- Fridge or small freezer: 100 to 200 watts while running, and much more for a moment when the compressor starts
Air conditioners, electric cookers, pressing irons and water heaters are the big power hogs. A typical one-horsepower air conditioner can need 800 watts or more. A small home inverter system cannot run them, so leave them off the list.
Step 2: Add it up
Here is an example. Six LED bulbs at 10 watts each is 60 watts. Two fans at 60 watts is 120. Add a television at 50, a decoder at 20, a router at 15, a laptop at 60 and a couple of phone chargers at 20. The total is 345 watts.
Add a safety margin of about 25 percent, because appliances do not always behave as the label says. That brings the example to roughly 430 watts.
Step 3: Size the inverter
Inverters are usually sold in VA or kVA, not watts. As a rule of thumb, divide your watts by 0.8 to get the VA you need. For 430 watts, that is about 540 VA, so a 1 kVA inverter is the sensible choice. If you plan to add a fridge, work it into the total first. Fridges are why many people end up needing a bigger inverter than they expected.
Pick a pure sine wave inverter. It costs more than a modified sine wave model, but it runs sensitive electronics, fridges and anything with a motor more smoothly and with less heat and noise.
Step 4: Size the battery
A battery’s capacity is given in amp hours (Ah) at a certain voltage. Multiply the two to get watt hours (Wh). A 12 volt, 200Ah battery holds about 2,400Wh.
You cannot use all of it. Lead-acid batteries, including the common tubular type, last much longer if you only use about half their capacity, so the usable energy is about 1,200Wh. Inverters also waste around 15 percent as heat. So for our 345 watt load, the maths looks like this:
- Load including inverter losses: 345 × 1.15 ≈ 397 watts
- Runtime: 1,200 ÷ 397 ≈ 3 hours
If three hours is not enough, you need a bigger battery bank, or a lighter load. Two 12 volt, 200Ah batteries wired in parallel double the runtime. Check that the inverter supports the battery voltage you choose, because larger inverters often run on 24 volts.
Lithium iron phosphate (LiFePO4) batteries cost more up front, but you can use 80 to 90 percent of their capacity and they usually last far more charge cycles. Over several years that can work out cheaper, so compare the total cost and not just the sticker price.
Adding solar
Solar panels charge the battery during the day, which cuts your dependence on the grid. You will need a charge controller, and an MPPT type gets more out of your panels than a basic PWM one. A simple way to start is to size the panels so they can roughly replace what you use in a day, then check the numbers with an installer.
Safety and buying tips
- Use a qualified electrician for the installation. Thin wires, loose connections and bad earthing are common causes of fires.
- Keep batteries in a ventilated spot, away from children, water and direct sun.
- Do not use a car battery as a long-term inverter battery. It is built for short bursts, not repeated deep discharge.
- Never run a petrol generator indoors or in a closed garage. The fumes contain carbon monoxide, which is deadly.
- Buy from a reputable seller, get a written warranty, and ask what exactly it covers.
Get two or three quotes, ask each seller to show you how they arrived at their sizing, and compare the numbers against your own. If a quote is far bigger than the maths above suggests, ask why.
