Power inverters
Computers, hi-fi and microwave ovens use considerable power, so if you're considering using them on board you'll probably need power inverters.
An inverter is probably one of the first extras most owners consider buying for their boat, whatever size it is. The ability to use 240V equipment when you are on passage or moored away from a shore supply is prized by most people. At the same time, an inverter is one of the easiest items to install, providing you understand the basic steps involved. Inverters range in size from 50W to 4kW, and can power most items that you now plug into the mains, but you need to understand their limitations, and for that you need to understand what makes them tick.
How they work An inverter takes low-voltage direct current (DC) from batteries, usually 12V or 24V, and converts it to high-voltage alternating current (AC), either 110V, 230V or 240V. The process is the same as your battery charger, except in reverse, and some combi models double as chargers, using the same basic electronics inside. Early inverters used transformers to step the volts up, and were heavy, but most modern models use solid-state electronics, both to step up the voltage and to convert the current from DC to AC, and are correspondingly lighter.
Domestic mains supply voltage follows a sine-wave form with a rounded top as the current alternates. But, while this wave form is easily produced by a rotating generator in a power station, it's more complex and costly to produce electronically and wastes more power. Instead many inverters use a modified sine-wave, or quasi sine-wave, which has a flat topped or square curve. The modified sine-wave powers most AC equipment, but can struggle with some items, particularly electronics with internal power supplies and digital timers like computers, TVs and microwave ovens. Recently the internal power supplies for most electronic equipment have changed to switch-mode systems, which better handle poorer quality incoming AC. At the same time, the standard of the best modified sine-waves has improved markedly, close to that of a pure sine-wave.
Yacht batteries and marine batteries
An inverter can provide high outputs but all this power has to come from your batteries. Short burst high power appliances like a microwave oven, kettle, toaster or hair-dryer are fine, but to run heaters or machines for long, you need a very large battery bank or engine power.
As we have said, virtually anything that you now plug into the mains can be run from an inverter, but the size of your unit governs what it will power. Up to 500W you can run computers, TV, hi-fi, battery chargers for your mobile phone and cameras, and even a 240V domestic fridge, though not always all at the same time.
Up to 1,000W, and you can add a small travel hair-dryer. For most people however the big plus is being able to run a microwave oven and for this you will need at least 1,500W, preferably 1,800W. Don't be confused by the 60OW or 80OW rating on most microwaves. This is the useful cooking power they generate, not the amount of power going in, which will be double this figure.
And at this size, your inverter should also power a standard hair-dryer, plus possibly a kettle, toaster and coffee-maker, though these may require 2kW.
Installing an inverter is within the capabilities of a competent DIY electrician, but if you've any doubts, leave it to a professional. A 2kW, 12V inverter will be drawing up to 200A from your batteries, more than many engine starter motors, so you need large diameter cables, short runs, and good connections. For the higher output models, use 50mm cables (35mm for lower outputs), a maximum of 1.5m long with properly crimped lugs, not screw connectors as they work loose. Anything less and you could lose too much power down the line, which means reduced performance and possibly tripping the inverter. You need proper cable from a battery dealer or automotive electrical supplier. Measure the exact length you want before you buy. They will probably crimp the terminals on for you, but check the diameter of the studs. Most batteries will be 8mm, but the inverter may be l0mm.
If you have to mount the inverter further away, use 70mm cable, or two 35mm cables in parallel for both positive and negative.
Unless a main input fuse is already fitted, you'll have to fit a 250A fuse in the supply line. It's also a good idea to have a separate battery master switch in line, so you can disconnect the unit completely. This must be capable of taking 250A continuous load. Check the size of its terminal studs - they will usually be l0mm.
The DC input terminals on some units were very close together, risking short circuit. If there are no plastic terminal covers, fit your own.
Your battery bank is a major consideration when fitting any but the smallest inverter ...If you take 200A out of a fully-charged 200Ah bank, the voltage at the battery terminals will drop from 12.6V to 11.0V at the inverter. If the bank is only half-charged, the voltage could be down to 10.5V, close to the low voltage tripping point of 10.0-10.5V.
If you're fitting a 21kW inverter, you should have a minimum of 400Ah batteries, preferably 600Ah. For a 1 kw unit, you will need 200Ah, preferably 300Ah.
All these figures assume the engine is not running, which is the usual situation when moored. If you start it up, you will get an input from the alternator, and the battery volts will rise, which will improve the situation, but you should not rely on this.
The condition of your batteries is also important. High continuous current drains will hammer the bank, and quickly find out any weak cells. Gel or AGM batteries will be better able to handle continuous heavy loads.
The 230V output from the inverter will be either one or more sockets on the front, or you may have to hard-wire a cable internally Again, only do this if you are sure of your proficiency. UK three-pin outlets are best fitted sideways or upside down so that large plugs or power-supplies dont foul the base.
Because the inverters have to be close to the batteries, they will usually be mounted in the engine space, or at least away from the galley area. But because most of them have a continuous residual current drain in standby-mode, you don't want to leave them permanently switched on. A remote control panel allows you to turn off the inverter when it's not needed.
Ideally you should fit a change-over switch in the output circuit to switch the incoming AC power between shore supply, generator, and inverter. It's important that you don't have two different power sources feeding into your ring main at the same time. Make sure you get the polarity right when connecting the DC or you could damage the unit
Any piece of electronic equipment will give a high short-term output, but will quickly cut out as it heats up. Best practice is to use the continuous rating to describe the unit. Having said that, the intermittent rating is important, as some pieces of AC equipment, particularly those with motors in them, have a start-up surge that needs a short burst of higher power.
Most power inverters drain a continuous current when switched on, even when you're not using them. This standby current will be at least 2A, sometimes more, which, over 24 hours, it could drain 50Ah or more from the batteries.
This is why a remote control is important. Some models have a powersave mode. This sends the unit to sleep while nothing is connected and wakes it up when it's needed. The drawback to this is that tiny currents, like the timer light on a microwave, will not trigger the unit, so the microwave won't start. Your mobile phone charger will also probably not activate the inverter, so you still need to turn it on manually.
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