Inverter Losses and Standby Draw

Add up your appliances and you get a neat daily total. Then you watch the battery and it gives up more than that. Nothing's broken. The gap is real, and you should plan for it.

Why the battery gives more than the appliance takes

An inverter turns the battery's direct current into the alternating current that normal plug-in appliances use. In a stand-alone system, it's the part that generates AC low voltage from the battery¹. Smaller inverters typically run from a rechargeable 12 V lead acid battery².

The inverter makes no power of its own. The power comes from the DC source². So whatever the inverter wastes as it works has to come out of the battery too. Your appliance gets what's left.

That means your battery supplies the appliance's energy plus the inverter's own losses. Think of it as a toll.

Three sources of extra draw

Conversion loss. Every inverter loses some energy turning DC into AC. The maker's spec sheet should give an efficiency figure. Use the one for your model, not a number from a web page.

The inverter's own idle use. An inverter that's switched on may draw power even with nothing plugged in. Look for a "no-load" or "idle" figure in the manual.

Standby draw from appliances. Some devices keep using energy when they're not in use. In stand-alone systems, the stand-by mode of some loads wastes energy¹. Each watt of continuous standby uses about 9 kWh a year³. That's roughly 25 Wh a day for each watt. The figure comes from dividing the yearly one by 365.

Bigger isn't more efficient

It's tempting to buy a big inverter "just in case". The catch: when a higher rated inverter is used for a lower capacity load, overall efficiency is reduced¹. Size the inverter for your real peak load, as measured on the page about measuring real use, with some room for motor starts.

The waveform matters too

A modified sine wave inverter may also make some loads hum, according to the same source. Check what your appliances need and what your inverter's manual says.

Ways to cut the extra draw

• Switch the inverter off when nothing needs AC.
• Run DC appliances straight from the battery where they exist. Power drawn directly from the battery is DC, and it's used especially for lighting and DC appliances, with no inverter in the way¹.
• Use a power strip with a switch. It removes the need to unplug each device³.
• Match the inverter to the load, as above.

Putting it in your budget

In the budget you built earlier, add a line for the inverter's own use over the hours it's on. Then take the appliance watt-hours that go through the inverter and divide by the efficiency from the manual. The result is the battery-side figure. The page on sizing the battery and panels picks up from there.

A battery-side reading is a good check, because it catches losses a plug-in meter on the appliance can't.

Safety and licensed work

Inverters connect to batteries that can deliver a lot of current. Any job that connects or disconnects a battery, shunt, fuse or inverter cable starts with isolating the battery. Follow the maker's manual for your model, and use a licensed electrician if you're unsure. It's not routine DIY.

Fusing and cable choices for your system need the same care. And wiring an inverter into a building's fixed mains circuits, or into a switchboard, is licensed electrical work in Australia. Never treat that as a do-it-yourself job.

If your inverter or battery has a fault, don't open it up. Take it to someone qualified.