Off-Grid Inverter Basics

An inverter converts the direct current your batteries store into the alternating current household appliances expect. It is the component that decides what your system can actually run, and the one most often bought on the wrong specification.

Pure sine wave versus modified sine wave

Grid power is a smooth sine wave. Inverters approximate it to varying degrees.

Modified sine wave is a stepped, blocky approximation. It is cheaper to produce and adequate for simple resistive loads such as incandescent bulbs, heating elements, and basic tools. It causes problems everywhere else: motors run hotter and less efficiently, audio equipment buzzes audibly, and a growing list of electronics either misbehave or refuse to run at all. Variable-speed motors, many medical devices, and some battery chargers are specifically incompatible.

Pure sine wave reproduces the waveform properly. Everything that runs on grid power runs on it.

The price gap has narrowed to the point where modified sine wave is difficult to justify for anything but a dedicated single-purpose load. Buy pure sine wave.

Sizing: continuous, surge, and the gap between them

Inverters carry two ratings, and both matter.

Continuous rating is the power it can supply indefinitely. Sum the running watts of everything that could plausibly run simultaneously, not everything you own but everything realistically on at once.

Surge rating is the brief overload it tolerates, usually for a few seconds. This exists because motors draw far more current starting than running. A fridge compressor rated 150 watts running may pull 400 to 900 watts for the fraction of a second it takes to start. Well pumps and air compressors are worse.

The sizing question is therefore not “what do my appliances add up to” but “what is the worst instant”: everything already running, and then the largest motor starts. That moment sets the surge requirement. Add up the running watts of everything realistically on at once, then add the startup surge of the largest motor load on top of that total.

An inverter that is too small does not fail gracefully. It either shuts down on overload or fails to start the appliance at all, which typically presents as a fridge that hums and clicks but never runs.

Do not oversize wildly to compensate. A larger inverter has a higher idle draw, and on a small system that penalty is paid every hour of every day.

Idle draw is the specification people forget

Every inverter consumes power simply being switched on: converting, running fans, keeping electronics alive. Depending on size and design this is roughly 5 to 30 watts.

That sounds trivial and is not. Twenty watts continuously is 480 watt-hours per day. On a cabin system budgeting 2,000 watt-hours daily, the inverter’s idle consumption alone is nearly a quarter of the entire budget, before a single appliance is switched on.

Two mitigations:

• Search or standby mode puts the inverter to sleep and wakes it when it detects a load. It is effective, but it will not reliably wake for very small loads such as a phone charger.
• Switch it off. The simplest answer for systems where AC is only needed some of the time. Many off-grid cabins run lighting and pumps on DC specifically to keep the inverter off.

Efficiency

Conversion is not free. Good inverters run around 85 to 93% efficient at reasonable loads, meaning 10 to 15% of the energy leaving the battery is lost as heat.

Efficiency also falls sharply at very low loads. Running a 3,000 watt inverter to power a 20 watt laptop is markedly inefficient. This is another argument against oversizing, and an argument for running small persistent loads on DC where practical.

This loss must appear in your sizing arithmetic. A 500 watt AC load draws roughly 570 watts from the battery once conversion losses are counted, and at 12 volts that is nearly 48 amps rather than the 42 the naive calculation suggests.

Inverter types, and why the difference is a safety matter

This is where terminology causes real trouble, because three quite different devices are all called inverters.

Off-grid (standalone) inverters take battery DC and produce AC. They have no connection to the utility and no ability to synchronise with it. This is what a cabin with no grid service uses.

Hybrid (inverter/chargers) do the same, plus charge the batteries from an AC source (a generator or the grid), and switch between sources automatically. This is the usual choice for a system with a backup generator.

Grid-tie and grid-interactive inverters synchronise their output with the utility. They include certified anti-islanding protection, which shuts the inverter down when the grid goes away. That protection exists to stop the inverter energizing a line that utility workers believe is dead.

The critical point: a plain off-grid inverter must never be connected to house wiring that is also connected to the utility. It cannot synchronise, and it cannot detect a grid outage. Connecting one to a circuit that can reach the utility risks backfeeding the line and killing a lineworker, and it can destroy the inverter when grid power returns out of phase.

Powering circuits from batteries during an outage requires either a transfer switch or interlock that physically isolates the house from the utility, or a properly certified grid-interactive system with a utility interconnection agreement. These are different builds with different requirements, and mixing them up is the most dangerous mistake in this entire subject area.

Choosing a system voltage

Inverters are built for specific battery voltages, commonly 12, 24, or 48. Higher battery voltage means less current for the same power, which means thinner cable and smaller fuses.

The difference is substantial. A 3,000 watt inverter at 12 volts draws roughly 250 amps at full output, needing very heavy and expensive cable. The same inverter at 48 volts draws about 63 amps. Above roughly 2,000 watts, low-voltage systems get impractical quickly, which is why larger installations are almost always 48 volt.

Decide battery voltage and inverter size together. Retrofitting a different system voltage later means replacing the batteries, the inverter, and the charge controller.

A short buying checklist

• Pure sine wave, unless you have a specific reason otherwise.
• Continuous rating covering realistic simultaneous loads.
• Surge rating covering your largest motor start on top of that.
• Idle draw. Check the number, especially on small systems.
• Correct battery voltage for the system you are building.
• Low-voltage disconnect, to stop the inverter flattening the bank.
• The right type for your situation. If the grid is involved, read the section above again.