- Amateur Boat Builders Community

Many variables constitute Inverter selection and there is more than one way to skin a cat. Here, we will describe selection in layman terms – not strictly correct in the purist’s eyes and needs to be swallowed in small bite-size-pieces. In some states, legislation may prevent DIY’ers from working on 12vDC (Very Low Voltage Systems) as well as unlicensed work on 240/110vAC systems. Please check with your state authorities for their specific requirements, keeping in mind that we are only connecting an inverter to the 12vDC (or similar) system however, State requirements will be needed in sizing of the inverter cables (discussed later).


One will soon see that the power draw on normal ships batteries can be extremely high, even with an of AC appliance 800W being used for 10 minutes. We will provide data that may make you reconsider these larger appliances when using and inverter. Finding this line can be difficult unless careful true analyses of ships requirements are taken into account and it may be that power-hungry systems, require a generator in your case.

What is an Inverter?

Xantrex Inverter

An Inverter, in essence, converts DC current to AC current. Given that most sailboats are 12vDC, and 24vDC (some at 36vDC) it is important to have a starting point and work backwards. One needs a clear understanding of the basic differences between DC and AC current before grasping the inverter theory. In this article, our example will use 12vDC and we will connect an AC appliance with an internal electric motor (800w vacuum cleaner).  The proposed inverter will be rated at 1200W.

Inverter Efficiency

The inverter unit uses a small amount of the incoming  (12vDC) current to actually operate or ‘convert’ the power and this in turn reduces the efficiency of the actual inverter unit’s output. Most common, good inverters will state this on their units by saying ‘inverter efficiency’. The key is using the inverter within 10% of its ‘rated inverter output’, in this case:
1200W – (10%) = 1080W.

Most efficiency values are based on the inverter being used at its peak and using the inverter at 50% of its ‘rated inverter output’ will be less inefficient than the value the manufacturer has stated. Charging a laptop or mobile phone for example from the 1200W inverter is a good example and is highly inefficient. This form of ‘Trickle charging’ would be better suited to a car-type adaptor through the ships 12vDC socket. It is key then that the inverter chosen be closest to the power needs for best efficiency.

The thought that, while an appliance is not plugged into the inverter, power is not being used, is ill informed. Leaving an inverter switched on while not being used will drain valuable ship’s power. This is a key reason why inverters should always have their own switch and be switched off at the source when not in use, unless it has a ‘sleep mode’ (not discussed here). Even in ‘sleep mode’ an inverter still drains power.

As a guide, at least 10% of the incoming power to the inverter will be consumed in AC power generation (when using the inverter at its peak output). Given a constant DC power input (say 12vDC at 10A), the 1200Watt inverter (90% efficiency), the output will be:
• (1200Watt / 100) x 90 = 1080W (‘consumed energy’ removed),
• (1080Watt/240vAC) = 4.5Amps

The other option if you want to maintain the 1200W output of the inverter is to increase the input vDC to say 12.6vDC (this being a lot more difficult in most cases and not discussed here). It needs to be said that there is more to efficiency than stated above, however, as a simplified guide; this should provide a starting point and direction in the purchase.

Inverter Cable (Battery to Inverter and return)

The next and very important consideration is to calculate the cable carrying capacity (or required battery cable) from the batteries to the inverter. Here the Surge (or Peak) value of the inverter needs to be used in calculations. On our 1200Watt inverter example, this is 2400W (normally stated on the compliance sticker). This value is a figure that we can run the Inverter for an instant without damaging the unit. It makes sense then that the higher the Inverter Wattage, the higher the battery drain and the cables need to be able to take this increased value without any heating of the cable. As our battery will only provide a voltage of 12vDC, it’s the Amperage that changes.

State legislation in most countries also requires a safety factor on top of this value and this ranges from 15% - 25%, depending on the country. Whether or not a State requirement, it is very wise to err on the safe side when dealing with high amperage 12vDC power. If you use 25% (State Safety Value), all countries are covered. The Inverter Amperage input therefore needs to be:
• 2400Watt (Peak)/12vDC = 200Amps, plus
• 25% (State Safety value) = 250Amps.

These are HUGE numbers and Amperage of this magnitude is be very dangerous, especially with any form of arcing, which is the second reason for a separate Inverter switch where the unit can be deselected when not in use. Additionally, a circuit-breaker cutout system should be installed to prevent any over-amperage in the system for whatever reason. A Cut-out Circuit Breaker (millisecond trip), which we use as the isolation switch, is placed just after the battery on the positive side of the 12vDC circuit. It comes 1.5m directly from the batteries via its own cables (independent to all other ship’s circuitry).


When buying the battery cable, one needs three values:
1. The total distance (to and from) the battery and inverter (2m in our example),
2. The Amperage (280Amp in our example), and
3. A voltage drop value (we will use 3% - voltage drop not discussed here).

This allows the electrician to source the correct cable thickness, or you can search a calculator (such as this). Insert the values and the calculator will supply a ‘Recommended cable size (in mm2)’, in our case 70mm2.  This too is extremely thick cable and has to be to carry the amperage mentioned above. This value mentioned is the square area of the cable, not the width. It can be seen that it is very thick cable indeed and the price reflects that. It is permissible to use 2 x 35mm2 cables in parallel, but they must be EXACTLY the same length. It can be seen now why mounting the Inverter as close as possible to the batteries, will pay handsomely.

Modified Sine Wave (MSW) or Pure Sine Wave (PSV)

The PSV units are more expensive for two strong reasons:
1. Its wave form is closer to that of the AC grid upon which all fragile electrical units are based, and
2. Efficiency of the inverter when not using peak output.

An internet search will help describe the two waveforms. With regard efficiency, the PSW units are far more efficient when not using the full power available from the unit. Additionally, quality inverters come with other features such as ‘sleep mode’ and normally have a ‘conformal coating’ of internal components to prevent damage from harsh environments.  It pays dividends to buy a small PSV over a large MSV, if money is a challenge. You will be far more power efficient in the long run and will look after your more fragile electrical units.

The AC Appliance

Back to our AC motor appliances; there is a spool-up period (where the motor starts and then gets up to speed), where most calculations fall apart. This is because the power draw during the ‘spool-up power requirement’ is higher than its ‘normal running power requirement’. With small appliances, an exact value is hard to quantify. However, a common ‘guesstimate’ is an increase of around 3 times the appliances ‘maximum rated requirement’. It needs to be said that there is technology around called ‘soft-starting’, that can reduce this problem, but this comes at a cost. While its heard that appliances can be started with lesser ‘rated requirements’, its when the system (appliance and/or inverter) fails that owners and worse still, insurance companies start to search for a reason to ‘not pay’ due to non-conformance of ‘best practice’.

In our example with the vacuum cleaner at 800W, to start the vacuum cleaner we will need an inverter capable of a ‘Peak Voltage’ of:
• 800W x 3 = 2400W

Selection Summary

Together with the list below, this should provide a good starting point for inverter selection. There are other considerations that will arise during the search, which can only improve ones choice and this should be used as a guide only.

Our example above uses a motor, which is the worst case scenario in sizing an Inverter, without a motor, the calculation values can be reduced, inturn reducing amperage and therefore the cable size.

1. Always choose a Pure Sine Inverter

a.Choose a unit as close as possible to the largest appliance size, and
b. Seriously consider the ‘start-up (spool-up) power requirement’, or make sure the appliance has a soft-start capability.Install a separate

2. Inverter Switch.
3. Install a separate Inverter circuit-breaker (Nigel Caulders – Boatowners Mechanical & Electrical Manual explains this well. In fact, talks about the whole inverter/battery charging very well).
4. Choose correctly sized cable between battery and inverter.
5. Check your local State Legislation.
6. Buy a quality brand.
7. Consider value at resale (of the vessel). Buyers (and Surveyors) are now more informed and savvy to the legal requirements, just ask any person trying to sell their vessel.
8. Quick Inverter Calculator:

a. Appliance Continuous Load:  (800W) rounded up = 1000W
b. Appliance Start Load:   (800W x 3) = 2400W
c. Inverter Efficiency Compensation: (2400W x 1.1 (10%) x 1.25(State safety of 25%))
= 3300W, rounded up = 3500W
d. Result: 1000W Inverter (with Peak/Surge load of 3500W)
e. NOTE: The ‘Efficiency Compensation’ value is required for cable sizing below

9. Quick Cable Calculator:

a. Battery to Inverter distance:  (1m x 2, there and back) = 2m
b. Current carrying capacity: (3500W/12) = (275A), rounded up = 280A
c. Voltage Drop:   3%
d. Result: 70mm2 cable required


While I try and not take sides, I am a big fan of the new Xantrex and their Charger/Inverter range. It is the ‘bees-knees’ for those who want quality plus all the little perks of conformal coatings, various additional protections etc, all in one box, this then saves on weight (one unit instead of two). I was given the opportunity to test an XC Series Charger and was so impressed that I bought it. It has been flawless in its operation and the initial outlay was worth every cent.

I have also tested the Jaycar MI-5050 Inverter and for those on a very tight budget, this may be right-up-your-alley. They also happen to stock the isolation switch/circuit breaker for this size unit.  You only get what you pay for.

Written by J Coomer
31 March 2011

Preferred Information Guides:

1. Nigel Caulders – Boatowners Mechanical & Electrical Handbook
2. Australian Regulatory Electrical Requirements
3. Xantrex information
4. Jaycar Inverter Information Sheet