An inverter is a black box device which converts energy stored in a 12V or 24V battery into 240V household AC. The implication is obvious: you can now run appliances such as TV, DVD, power tools and other mains powered devices from the boat battery, with some limitations.
You might question whether a generator would be more appropriate for this task, considering that a portable generator can provide 700 Watts. The answer is that the inverter is better because it is compact, silent, does not require fuel or maintenance and is easily installed. But beware! The inverter has an Achilles heel.
Let’s do an equation. An appliance rated at 1250W such as a microwave oven consumes 5.2Amperes at 240VAC.
But because the power is being derived from a 12Volt battery, the current becomes a massive 115ADC, meaning the average 100Ah battery is good for about 25 minutes before requiring recharging. That is fine if those 100Ah can be replenished before defrosting the next frozen lasagna by running the engine for a couple of hours or solar power for two days or more. However, in some cases practicality dictates that frequently used, high current loads are better served by a generator.
Waveforms
Ideally inverters should produce a waveform which is as close as possible to 240V mains electricity. When purchasing an inverter, there will be a choice of sinewave or modified sinewave (or square-wave) output.
Modified sinewave inverters produce a stepped waveform which will not operate some electronic equipment such as TVs, microwaves with digital timers, bread makers, some phone and laptop battery chargers, light dimmers and variable speed tools.
Modified sinewave inverters will power pure resistive and inductive loads such as incandescent lighting, most power tools, and some computers and recharging. Some video equipment is okay but you may get lines across the screen. Fluorescent lighting can be tricky due to a power factor correction capacitor in the fluoro input. Some equipment, even if it does operate on modified sine wave power, will be damaged over time.
Pure sinewave inverters produce current replicating mains power. They’re more efficient and are capable of running all AC loads. There is usually a significant on-cost which is more than justified by their superior features. Pure sinewave inverters are essential for sensitive electronic equipment and are strongly recommended as the best choice for all applications.
It is generally not practical to use inverters continuously for high loads such as heaters and cookers. Another important no-go are products using a compressor – fridges, air conditioners and air compressors require a lot of start up current which will place considerable strain on the inverter.
Sizing
Ranging from 150W to 3,000W continuous, inverters are capable of absorbing surges of up to three times the continuous rating for limited periods. Sizing is determined by the maximum amount of power required at one time. If you are only intending to recharge the mobile phone, there is no point in exceeding 150W. But be aware that some loads have a high inrush current which can cause drama with an underrated inverter, e.g. some TVs have a capacitive power input resulting in a start up current over the surge rating, which will cause the inverter to shut down.
Battery connection
It is essential to make sure that the connection between the battery and the inverter is as substantial as possible and includes a fuse. Cigarette lighter sockets are okay for loads up to 100W or 8A. Anything more than that requires a hardwired installation or a high current rated plug/socket with heavy duty cabling. I’ve seen installations using speaker wire on the DC side. This causes huge voltage drops and a potential fire hazard.
Desirable features:
- High efficiency. A good inverter will operate at about 95% efficiency.
- Protection against output short circuits, input reverse polarity and overload. Low battery alarm and disconnect is desirable.
- Good quality input and output cabling and connections.
- Electrical isolation is essential so that there is no path for the 240VAC to appear at the battery terminals, thus preventing possible electrocution.
WARNING: AC current produced by inverters is just as dangerous as household power. It can kill! 240 volt wiring must be installed by a licensed electrician.