What an inverter does and why the ratings matter
An inverter is one of the most important pieces of equipment in a solar energy system. The U.S. Department of Energy describes it as a device that converts direct current (DC) electricity, which is what a solar panel generates, to alternating current (AC) electricity, which the electrical grid and most household appliances use. In DC, electricity is maintained at constant voltage in one direction. In AC, electricity flows in both directions in the circuit as the voltage changes from positive to negative.
Fundamentally, an inverter accomplishes the DC-to-AC conversion by switching the direction of a DC input back and forth very rapidly, according to the Department of Energy. As a result, a DC input becomes an AC output. Filters and other electronics can be used to produce a voltage that varies as a clean, repeating sine wave that can be injected into the power grid. The sine wave is a shape or pattern the voltage makes over time, and it is the pattern of power that the grid can use without damaging electrical equipment, which is built to operate at certain frequencies and voltages.
When you are planning a small off-grid, backup or portable system, the specifications printed on an inverter data sheet can feel overwhelming. This guide walks through the key ratings you will see, explains what each one means in plain language, and shows you what to compare when you are at the inverter sizing stage. Every figure quoted from a manufacturer manual is one model’s data, used here only to illustrate how ratings are presented.
Continuous, maximum and surge output ratings
The most prominent number on an inverter is its continuous output power. This is the amount of AC power the unit can deliver indefinitely under normal conditions. The Victron Phoenix 750 VA manual, for example, lists a continuous output power of 750 W for that model. Alongside it, the manual gives a maximum output power of 800 W for three minutes and a surge rating of 1400 W.
These three numbers serve different purposes. Continuous output tells you what the inverter can run all day. The maximum output rating describes a short-duration capability, often a few minutes, that can cover brief load peaks. The surge rating is the highest power the inverter can deliver for a very short moment, typically to start motors or other devices that draw a large inrush current when they first switch on.
When you compare inverters, look at all three figures, not just the continuous wattage. A tool with an electric motor may need a surge capacity several times its running power for a fraction of a second. If the inverter cannot supply that brief peak, the tool may not start even though its running power is well below the continuous rating.
Pure sine wave and total harmonic distortion
The quality of the AC waveform matters for many loads. The Victron Phoenix 750 manual states that its output waveform is a pure sine wave with total harmonic distortion (THD) below 3%. The Hong Kong Electrical and Mechanical Services Department (EMSD) notes that the quality of the AC generated, including the harmonic contents, voltage and frequency stability, should be considered when choosing an inverter.
Harmonic distortion is a measure of how much the output waveform deviates from a perfect sine wave. A lower THD percentage means a cleaner waveform. The DOE notes that the clean sine wave is the pattern of power that electrical equipment, built for certain frequencies and voltages, can use without damage.
The EMSD also points out that protection features such as short-circuit protection, overload protection and deep discharge monitoring for standalone inverters should be evaluated. The Phoenix 750 manual lists its own protection set: input low voltage, overload, short circuit, low battery alarm, input over voltage and over temperature. These protections help prevent damage to the inverter and the connected equipment when conditions move outside safe limits.
Efficiency and why it varies with load
No inverter converts DC to AC without some loss. The EMSD states that the conversion efficiencies of inverters of different designs are generally in the range of 85% to 95%, and that an inverter of good conversion efficiency should be used to reduce energy loss. The Phoenix 750 manual shows how efficiency changes with DC input voltage and load. For the 12 V model, full-load efficiency is listed as 89%. The 24 V version reaches 91% at full load, and the 48 V version reaches 92%. At partial loads the numbers shift, and the manual gives a range of 88% to 94% across the family.
Efficiency is not a single fixed number, so check the figures for the model and voltage you plan to use. When you are calculating your energy needs, factor in inverter losses so your battery bank and solar array are sized to cover both the loads and the conversion overhead.
No-load current draw and why it matters for small batteries
Even when no appliance is connected, an inverter draws some current from the battery to keep its control circuits alive and to produce the AC output voltage. The Phoenix 750 manual lists the no-load current draw for each DC voltage variant. The 12 V model draws 1.25 A with no load, the 24 V model draws 0.64 A, and the 48 V model draws 0.31 A. The manual also describes an adjustable power saving mode to conserve energy.
This idle consumption matters a great deal in small systems with limited battery capacity. A continuous draw of 1.25 A at 12 V is roughly 15 W, which over 24 hours adds up to about 360 watt-hours, energy that must be replaced by solar panels or another charging source. When you are working through battery sizing for a portable or backup system, include the inverter’s no-load draw in your daily energy budget, especially if the inverter will be left switched on for long periods.
VA versus watts and the power factor
Some inverter models carry a VA rating rather than a watt rating, or show both. The Victron Wiring Unlimited book explains the difference. Watt is the real power drawn by the equipment and determines the actual power purchased from the utility or the energy consumed from a battery. VA is the apparent power, the product of voltage times current, and is used for sizing wiring, circuit breakers, inverters or generators.
In a purely resistive AC circuit, such as a heater or an incandescent light bulb, voltage and current waves are in phase and the power factor is 1. In that case, watts and VA are equal. When an AC circuit contains inductive loads like motors or transformers, or capacitive loads, a phase shift occurs between current and voltage. The true power in watts is then less than the apparent power in VA. Wiring Unlimited notes that on average a residential AC circuit has an average power factor of 0.8, and for general calculations it is acceptable to use that figure.
When an inverter is labelled with a VA rating, you can roughly estimate the continuous wattage it can supply to a mixed set of loads by multiplying the VA number by the power factor. For example, a 750 VA inverter at a power factor of 0.8 would deliver about 600 W of real power to inductive loads. The Phoenix 750 manual lists the model as 750 VA but also gives a continuous output power of 750 W, illustrating that the relationship depends on the specific design and the loads connected.
Battery cable length and fuse placement
The physical installation of an inverter affects its performance and safety. The Phoenix 750 manual instructs installers to avoid excessive cable lengths and to use the recommended wire lengths and sizes. It also warns that reverse polarity connection will blow the internal fuse and may damage the inverter permanently. The manual states that a fuse is present on the positive DC input and that loose connections can overheat and result in a potential hazard.
Wiring Unlimited explains that correctly calculating fuses and wiring size requires knowing the current in the circuit, and that the VA rating is used for sizing wiring, circuit breakers and inverters. For any permanent installation, the specific wire gauge, fuse rating and torque values must come from the inverter manual and the applicable electrical code. Connecting an inverter to a home wiring system, a main panel or the grid requires a licensed electrician, permits and approval from the utility. This guide does not provide step-by-step instructions for those connections.
When you plan the physical layout of your DIY solar system components, place the inverter as close to the battery bank as practical while keeping it outside the battery compartment. The Phoenix 750 manual cautions that batteries produce corrosive gases and that prolonged exposure will damage the inverter.
Inrush current of AC electric motors
Many common loads contain electric motors, and motors behave differently at startup than they do while running. The book Energy Unlimited, a 2013 publication written mainly for boats and vehicles with lead-acid batteries, discusses how to deal with the inrush current of AC electric motors. When a motor starts, it can draw a current several times higher than its normal running current for a brief moment. This inrush can cause an inverter to trip its overload protection even if the motor’s nameplate wattage is well within the inverter’s continuous rating.
The surge rating on an inverter data sheet is designed to cover this momentary demand. The Phoenix 750 manual’s 1400 W surge figure, for instance, provides headroom above the 750 W continuous rating to start motor-driven tools and appliances. When you select an inverter, list the startup current requirements of any motor loads you plan to run, and check that the inverter’s surge rating and its duration meet or exceed those requirements. The inverters category page can help you explore different types and their specifications.
Rating comparison at a glance
| Rating | What it means | What to compare |
|---|---|---|
| Continuous output power | Power the inverter can supply indefinitely | Match to the total running watts of loads that may be on at the same time |
| Maximum output power | Power available for a few minutes | Useful for short peaks above the continuous level |
| Surge rating | Highest power for a fraction of a second | Must exceed the startup inrush of motors and compressors |
| Output waveform and THD | Purity of the AC sine wave | Lower THD is better for sensitive electronics; pure sine wave suits most loads |
| Efficiency | Percentage of DC input that becomes usable AC output | Higher efficiency reduces energy loss; check the curve across the load range |
| No-load current draw | Current consumed with no load connected | Lower draw preserves battery capacity in small systems |
| VA vs. watts | Apparent power versus real power | Use VA for wiring and breaker sizing; use watts for energy consumption |
Frequently asked questions
Why does my inverter shut down when I try to run a refrigerator?
A refrigerator compressor draws a high inrush current at startup that can exceed the inverter’s surge rating, even though the running power is low. Check the inverter’s surge specification and compare it with the compressor’s locked-rotor or startup current. Energy Unlimited, a 2013 book focused on boats and vehicles, discusses how inrush current of AC electric motors can challenge an inverter and suggests sizing the inverter to handle these short peaks.
Is a pure sine wave inverter always necessary?
Not for every load, but it is the safer choice for a mixed system. The Hong Kong Electrical and Mechanical Services Department advises considering the harmonic contents and voltage stability of the AC output when choosing an inverter. A pure sine wave inverter with low THD, such as the sub-3% figure quoted in the Victron Phoenix 750 manual for that model, closely matches utility power and is compatible with the widest range of devices.
What does the no-load current draw mean for my battery?
No-load current draw is the power the inverter consumes just by being switched on, even with nothing plugged in. The Victron Phoenix 750 manual lists a 12 V model draw of 1.25 A, which works out to roughly 15 W continuously. Over a full day that can consume a noticeable portion of a small battery bank’s usable capacity. If you are building a portable or backup system, either switch the inverter off when it is not needed or choose a model with a low-power saving mode that reduces idle consumption.
For more on how inverters fit into a complete system, see the inverters section and the broader solar system components overview.
