Inverter size:
matching watts, VA and surge to your loads

Watts, VA and why both appear on the label

Appliances are labelled in watts, the real power they use. Inverters are usually sold in volt-amperes, VA, which is the product of voltage and current. The two differ because many household loads draw current that is partly out of step with the voltage, and the ratio between them is called the power factor.

A common planning value for a home inverter is a power factor of 0.8. That means a 1,000 VA inverter can supply about 800 W of real load. The number is not universal, because the actual power factor depends on the loads and the inverter's design, so read the datasheet's watt rating whenever it is available.

The first job in sizing is therefore a list. Write down every appliance that you want to run at the same time during a power cut, with its running wattage, and ignore the things that will not be switched on.

The sizing formula step by step

Sizing has three parts: total the simultaneous running load, convert to VA, and then add headroom so the inverter does not run flat out. Next, test the largest motor load against the inverter's surge allowance.

Do not forget that a calculator is only as good as the wattage you give it. Use the label or a plug-in power meter rather than guessing.

Worked example: fans, a pump and lights

A flat needs to run three ceiling fans, a small water pump and some lights with a TV during a cut. The label figures are: fans 120 W combined, pump 300 W, lights and TV 80 W combined.

Now test the surge. If the pump draws three times its running power when it starts, the momentary demand is 120 + 80 + 3 × 300 = 1,100 W. That is about 1,375 VA at 0.8 and the inverter must tolerate it briefly. Check the datasheet's peak or surge rating rather than assuming, because designs differ.

Sizing the battery bank to go with it

The inverter only converts power. The battery bank decides how long it lasts. Battery energy is rated in ampere-hours at a nominal voltage, so multiply the two to get watt-hours. Then work backwards from the backup time you want.

Using the 500 W load and three hours of backup: 500 × 3 = 1,500 Wh. Divide by 0.85 to cover inverter losses, which gives 1,765 Wh from the battery. At 50 percent usable depth you need 3,529 Wh of nominal storage. On a 12 V system that is about 294 Ah, which in practice means two 150 Ah batteries, and on a 24 V system the capacity falls to roughly 147 Ah.

This is why larger inverters are paired with 24 V or 48 V systems. Doubling the voltage halves the current in the cables, which keeps heat and losses down.

Which loads are easy and which need care

Heating appliances can appear harmless because they have no surge, yet a 2,000 W geyser needs a very large inverter and drains the battery within minutes. Plan to leave heaters off the backup circuit.

Air-conditioners deserve their own check. Inverter ACs ramp up gently, while fixed-speed ACs have high start-up current. Confirm with the AC's specifications and the inverter's manual before connecting.

Battery current, cables and fuses

The battery side of an inverter carries much more current than the mains side, and this is where undersized wiring causes trouble. The DC current is the load divided by the battery voltage and the inverter efficiency. For the 500 W example with 85 percent efficiency, a 12 V system draws 500 ÷ (12 × 0.85) = 49 A, while a 24 V system needs only 24.5 A for the same load.

Currents like these demand thick, short copper cables and a fuse or breaker placed close to the battery. Long thin leads get hot and waste energy as voltage drop, and a loose terminal can overheat. The inverter's manual normally lists the minimum cable cross-section and fuse rating, and those are worth following exactly.

Another quiet cost is idle consumption. Many inverters draw a few tens of watts just by being switched on with no load, which drains the battery overnight if left on. Look for the no-load figure on the datasheet, and use an eco or sleep mode if the unit provides one.

Final checks before buying

• Pure sine wave output is safer for motors, fridges and sensitive electronics than a modified sine wave.
• Make sure the inverter's input voltage matches the battery bank and that the charger current suits the battery's Ah.
• Leave ventilation space and place batteries away from living areas.
• Check the manufacturer's rated continuous watts, not just peak.
• Allow for future additions; going one size up often costs little.

Electrical installation should be done by a qualified electrician, and cable sizes and fuses must match the current involved.