How to choose an inverter and smart battery charger

How to choose an inverter or smart battery charger

Advice on choosing an inverter or smart battery charger

On this page:


  • Don’t Go Cheap and Nasty
  • A Clean Output: Sine Wave vs. Modified Sine Wave
  • Sizing & Power Capacity
  • Effect of Operating Temperature
  • Efficiency
  • Internal Protection
  • Idle Power
  • Automatic On Off
  • When Not to Use an Inverter
  • Common Appliance and Load Questions
  • Why We Recommend Xantrex
  • Smart Battery Chargers

    Don’t Go Cheap and Nasty

    WARNING: Many inverter chargers now available on the market are of poor quality and potentially damage appliances & equipment you’re powering. There are growing number of products entering the market that are made for the lowest possible price, with no regard to over all reliability or actual performance.

    When shopping around for a power inverter ask yourself if you want a cheap inverter that, over the long run may end up costing you more and possibly damage your valuable equipment; or a good quality unit that may cost a little more but will give you many years of faithful fault free service?

    This article is designed to help you understand the ins and outs of inverters, what to look at for and how to go about selecting the right inverter for you.

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    A Clean Output: Sine Wave vs. Modified Sine Wave

    Some inverters produce ‘cleaner’ power than others. A ‘sine wave’ inverter produces clean power; anything else is dirty. A sine wave is the ideal form of AC power and is what is delivered by the national grid to your house.

    A ‘modified sine wave’ inverter is less expensive, but it produces a distorted square waveform. In truth, it isn’t a sine wave at all. The misleading term ‘modified sine wave’ was invented by advertising people. Engineers prefer to call it ‘modified square wave’.

    Sine wave vs. Modified sine wave

    Modified sine wave inverters can have detrimental effects on many electrical loads:

    • Reduction in energy efficiency of motors and transformers by 10 to 20 percent.
    • Wasted energy causes abnormal heat which reduces the reliability and longevity of motors and transformers and other devices, including some appliances and computers.
    • The choppy waveform confuses some digital timing devices.
    • About 5 percent of household appliances simply won’t work on modified sine wave power at all.
    • A buzz will be heard from the speakers of nearly every audio device.
    • An annoying buzz will also be emitted by some fluorescent lights, ceiling fans, and transformers.
    • Some microwave ovens buzz or produce less heat.
    • TVs and computers often show rolling lines on the screen.
    • Surge protectors may overheat and should not be used.

    Modified sine wave inverters were tolerated in the 1980s, but since then, true sine wave inverters have become more efficient and more affordable. Some people compromise by using a modified wave inverter to run their larger power tools or other occasional heavy loads, and a small sine wave inverter to run their smaller, more frequent, and more sensitive loads. Modified wave inverters in renewable energy systems have started fading into history.

    Advantages of Pure Sine Wave inverters over modified sine wave inverters:

    • Output voltage wave form is pure sine wave with very low harmonic distortion and clean power like utility-supplied electricity.
    • Inductive loads like microwave ovens and motors run faster, quieter and cooler.
    • Reduces audible and electrical noise in fans, fluorescent lights, audio amplifiers, TV, Game consoles, Fax, and answering machines.
    • Prevents crashes in computers, weird print out, and glitches and noise in monitors.

    If you are powering the following equipment then we recommend using a pure sine wave inverter:

    • sensitive electrical or electronic items such as laptop computers
    • stereos
    • some fluorescent lights with electronic ballasts
    • laser printers and photo copiers
    • certain specialized applications such as medical equipment
    • digital clocks
    • bread makers with multi-stage timers
    • variable speed or rechargeable tools
    • Power tools employing "solid state" power or variable speed control
    • Some battery chargers for cordless tools
    • Some new furnaces and pellet stoves with microprocessor control

    Sizing & Power Capacity

    An inverters power output is rated in watts (watts = amps x volts).

    There are three levels of power rating:

    1. a continuous rating (output power). This means the amount of power the inverter can handle for an indefinite period of hours. When an inverter is rated at a certain number of watts, that number generally refers to its continuous rating.
    2. a surge rating. This is a higher number of watts that it can handle for a defined period of time, typically 10 or 20 minutes.
    3. The third level of power rating, peak output surge capacity. This is critical to its ability to start motors and pumps (if you are powering these devices you will check these specifications).

    For help in sizing your system, have a look at our usage tables below:

    Appliance Usage Chart

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    Effect of Operating Temperature

    The power output of an inverter is dramatically decreased as its internal temperature rises (this is sometimes called its 5, 10 & 30 minute rating; but in reality if the inverter cannot remove the heat quick enough, then the power will rapidly drop off). Many of our models are rated at a staggering 40°C, such as Prosine, with a classic comparison between a Prosine 1000 and a low cost 1500watt modified as follows. The following chart provides a comparison between the Prosine 1000i rated at 40°C and a common 1500watt inverter rated at 25°C.

    Inverter Output at Operating Temperature

    Note: the new Xantrex Prosine designs allow for rapid removal of warm air from the inside workings.

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    It is not possible to convert power without losing some of it (it’s like friction). Power is lost in the form of heat. Efficiency is the ratio of power out to power in, expressed as a percentage. If the efficiency is 90 percent, 10 percent of the power is lost in the inverter. The efficiency of an inverter varies with the load. Typically, it will be highest at about two thirds of the inverter’s capacity. This is called its "peak efficiency." The inverter requires some power just to run itself, so the efficiency of a large inverter will be low when running very small loads.

    In a typical home, there are many hours of the day when the electrical load is very low. Under these conditions, an inverter’s efficiency may be around 50 percent or less. The full story is told by a graph of efficiency vs. load, as published by the inverter manufacturer. This is called the "efficiency curve." Read these curves carefully. Some manufacturers cheat by starting the curve at 100 watts or so, not at zero!

    Because the efficiency varies with load, don’t assume that an inverter with 93 percent peak efficiency is better than one with 85 percent peak efficiency. If the 85 percent efficient unit is more efficient at low power levels, it may waste less energy through the course of a typical day.

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    Internal Protection

    An inverter’s sensitive components must be well protected against surges from nearby lightning and static, and from surges that bounce back from motors under overload conditions. It must also be protected from overloads. Overloads can be caused by a faulty appliance, a wiring fault, or simply too much load running at one time.

    An inverter must include several sensing circuits to shut itself off if it cannot properly serve the load. It also needs to shut off if the DC supply voltage is too low, due to a low battery state-of-charge or other weakness in the supply circuit. This protects the batteries from over-discharge damage, as well as protecting the inverter and the loads. These protective measures are all standard on inverters that are certified for use in buildings.

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    Idle Power

    Idle power is the consumption of the inverter when it is on, but no loads are running. It is "wasted" power, so if you expect the inverter to be on for many hours during which there is very little load (as in most residential situations), you want this to be as low as possible. Typical idle power ranges from 15 watts to 50 watts for a home-size inverter. An inverter’s spec sheet may describe the inverter’s "idle current" in amps. To get watts, just multiply the amps times the DC voltage of the system.

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    Automatic On Off

    Inverter idling can be a substantial load on a small power system. Most inverters made for home power systems have automatic load-sensing. The inverter puts out a brief pulse of power about every second (more or less). When you switch on an AC load, it senses the current draw and turns itself on. Manufacturers have various names for this feature, including "load demand," "sleep mode," "power saver," "autostart," and "standby."

    Automatic on/off can make life awkward because a tiny load may not trigger the inverter to turn on or stay on. For example, a washing machine may pause between cycles, with only the timer running. The timer draws less than 10 watts. The inverter’s turn-on "threshold" may be 10 or 15 watts. The inverter shuts off and doesn’t come back on until it sees an additional load from some other appliance. You may have to leave a light on while running the washer.

    Some people can’t adapt to such situations. Therefore, inverters with automatic on/off also have an always-on setting. With it, you can run your low-power night lights, your clocks, fax, answering machine and other tiny loads, without losing continuity. In that case, a good system designer will add the inverter’s idle power into the load calculation (24 hours a day). The cost of the power system will be higher, but it will meet the expectations of modern living.

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    When Not to Use an Inverter

    Air conditioners are highly uneconomical to work off an inverter. Most large inverters these days can power AC’s with no problem, but the battery bank will nearly always let the system down. If you must run an air conditioner, look for an inverter that load shares with a generator, as the generator will be the primary power source, with the Inverter assisting in start up loads. Battery chargers will be more important here, so ensure the right sized charger is used. YOU CANNOT run a battery charger off a Power Inverter connected to the same battery bank. This generates a loop sequence that only discharges your batteries quicker that the charger can recover them. This is due to the losses and inefficiencies within the inverter and battery charger.

    Large heating elements such as frypans, toasters and fast boil kettles are very uneconomical to run from a power inverter. They require a large inverter system, quick battery charging capabilities and sufficient storage within the battery system. Generally to do this you would require around 400amps storage @ 12 volts, along with an inverter over 2000watts. The charging would have to come from a large battery charger running from an AC generator (forget solar in this case). The easiest solution – just run your generator to begin with.

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    Common Appliance and Load Questions

    Do I need a pure sine wave inverter to run a television?

    In short, No. By using a modified sine wave inverter you will find that lines will appear across the screen (see above). On the subject of televisions, most small systems need up to 10 times their power rating just to star t (e.g. a 35cm TV that needs 55 watts to run, may need up to 550watts just to start). Our XPower units all have auto reset on overload to help star t televisions, but they may need 2 or so attempts to get a TV up and running (using the XPower 300).

    What do I need to run my C-pap (respirator) machine?

    For C-pap machines to function correctly they require a pure sine wave power output. These machines tend to draw around 200watts, which equates to 20amps per hour. For a normal night of sleep (7-8 hours), you would need a battery bank of around 200amp hours (and a suitable recharging system). The Alessi 300watt units are perfect for basic C-pap machines, contact us for details if your machine has a heater inbuilt.

    What size inverter do I need to run a microwave?

    Microwaves are commonly run from inverters in many applications with a standard microwave drawing around 1000 – 1200 watts of power. It is becoming more common that microwaves refuse to work from low cost modified sine wave inverters, and the best units are generally the 1000 watt pure sine wave units (such as Prosine). Microwaves will probably be the largest load your batteries will ever power, but they are the most efficient due to the shor t run time required from a microwave (we even recommend a microwave be used over a kettle to boil your water).

    How can I run my fridge from an inverter?

    First of all, if your fridge is 12 volts supply, then you can forget using an inverter; you don’t need it (you will draw more power from your batteries than required). If the fridge is 240 volts powered, then it is important to find out the duty cycle, or total time run per day to work out the fridge power usage per day. Fridges are very hard to size up an inverter system, so always look for a larger powered inverter than you think, and be prepared for some testing and trialing to find the best suited inverter for your needs.

    Can I run my computer / laptop from an inverter?

    The answer to this is very much the same as the Televisions above. Modified sine wave inverters can be used on either a computer or laptop, however if the laptop is to only ever be powered from the inver ter then a pure sine wave inverter (such as the Alessi 200) should be used, as the modified sine wave inver ters will actually destroy the laptop battery pack. If your laptop / inverter usage is very intermittent, then a modified sine wave unit will be suitable. Modified Sine wave inver ters do not affect the workings of the computer in any way – it is just a matter of interference on the screen (not present in LCD screens).

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    Why We Recommend Xantrex

    Xantrex is a quality US manufacturer and a world leader in advanced power electronics, building products since 1983. Xantrex provides a wide range of AC and DC systems and solutions that provide AC power anywhere. With models ranging from 150 watts up to 4500 watts, Xantrex also provides battery chargers, charge controllers and battery monitors. Xantrex products are smaller, smarter and more efficient than traditional power conversion equipment and are widely used in applications such as renewable and distributed power, mobile power and commercial applications.

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    Check out some amazing everyday deals on Inverters

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    Why use a Smart Battery Charger

    1) Microprocessor controlled smart chargers are smaller and lighter than most other chargers with similar capabilities. This allows for easier installation and less required space for mounting.

    2) Precise “Set and Forget” charging technology which is highly recommended when charging deep cycle batteries. This involves a multi-step process of cycling through a charge method of Bulk (or Boost), Absorption and Float stages (See details below). This process ensures a fast, safe and complete charge.

    3) A quiet operation cycle with no hum or buzz as commonly found in chargers. Normally the only noise you hear is the in-built cooling fan on some models.

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    How Do Smart Chargers Differ?

    Most chargers on the market today aren’t true three step chargers. Here is a quick overview of what can happen to your expensive batteries by not using a multi-step smart charger on a regular basis:

    A) Taper chargers – Simple Ferro resonant chargers deliver current at a reducing rate as the battery voltage rises during charging, until the current tapers to zero at a certain voltage. Unfortunately, there is no single voltage setting that allows the charger to remain connected to ensure a full charge is reached. If the voltage is set above the safe float voltage, the battery will be overcharged and may suffer damage from boiled-off electrolyte over time. If the voltage is set at or below the float voltage, then the battery may never be charge properly. These chargers are NOT “Set and Forget” chargers.

    B) Two Step chargers – These chargers typically charge at a constant current until the battery reaches a preset voltage, and then the charger voltage is reduced to a safe float voltage. There are two problems with this, if the preset voltage is at or below the battery absorption voltage, the loss of the absorption stage of charging will result in an undercharged battery. If the preset voltage is above the absorption voltage, then the battery will be overcharged. Repeated overcharging will cause loss of electrolyte and eventual battery damage.

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    How our Smart Chargers Work

    Our range of smart chargers utilise high frequency power switching circuits to convert the normal mains 240 volts AC (via either shore or generator power), to the required low voltage DC required to charge the batteries. As dictated by battery manufacturer’s recommendations, deep cycle batteries require a three stage charge sequence for perfect, fast, accurate charging. This sequence is as follows:

    Step 1 – Bulk or Boost charge; The battery is charged at full rated output current of the charger until the battery reaches its final charging voltage, known as its absorption voltage. In this step, around 80% of the batter y is recovered as fast as possible .

    Step 2 – Absorption Charge; With the charger voltage held steady, the remaining 20% is replaced with the charger allowing the current to drop as the battery approaches its full charge.

    Step 3 – Float; Finally, in the float stage the charger voltage is lowered and held at a constant and safe predetermined level. This prevents the battery from being overcharged, yet allows the charger to supply enough current to make up for the self discharge losses of the battery, while supporting any additional loads connected to the battery (such as DC lighting and refrigerators). This stage allows for the charger to be used as a DC power supply.

    Some smart battery chargers such as Truecharge also have a 4th step, which is a timed recharge. The Truecharge for example switches the charger from float to boost after 21 days.

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    Maximising Battery Life Using a Smart Charger

    The life of a battery can be greatly extended if proper charging regimes and practices are followed. One of the most important rules is to reduce cycling your batteries below 50% of their rated capacity (this is called Depth of Discharge). Your charger should also be sized accordingly to the battery capacity of around 10-20% of the house battery bank only (do not include starting batteries in this equation). For example, a 12 volt, 400amphour bank of batteries would be best suited with a 40amp battery charger for accurate charging.

    A further way to extend your batteries life is to regularly equalise your battery. Equalisation is a controlled ‘overcharge’ that helps to remove any wear built up from normal charge / discharge cycles. For frequently used batteries it should be done around once every 4-8 weeks, while infrequently used batteries should be done around 3-6 months (Warning: check your battery manufacturers specifications to see if they can be equalised). Some batteries such as Gel Cell type batteries cannot be equalised.

    The Truecharge range, Xantrex Battery Charger, Freedom Combi, PS and SW inverter / chargers all have an excellent equalisation mode that can be adjusted to your battery type.

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    Charger Set Up Q & A

    My smart battery charger has more than one charge outlet. I have two batteries so how do I connect the charger?

    If the batteries are connected in parallel (positive joined to positive, and negative joined to negative), then you only have one battery bank. If this is the case, simply use one of the outlets from the charger to connect to your batteries (don’t forget to cross connected your battery connections). If the batteries are totally separate (e.g. one start battery and one house battery), then simply connect one positive outlet from the charger to each battery bank, and join both battery negatives in common (this is then connected to the charger – not to the chassis).

    Can I use a small portable generator to run my smart battery charger?

    The answer to this is yes, but with some caution. Just follow these simple rules:

    1) Ensure your generator never ever runs out of fuel while running the battery charger. In the final splutters the AC voltage delivered from the generator can damage your expensive battery charger.
    2) Be sure to purchase a good generator that has a clean AC output. If in doubt simply connect a surge diverter (like you would use for your computer) to the generator.
    3) The Truecharge 40 battery charger will run very comfortably from many of the common market generators around the 700 to 1000 watts range.

    Does my smart battery charger drain the battery when there is no AC power available?

    In short NO. Smart chargers such as the Truecharge and CPS range use isolation diodes for the multiple banks, which results in drawing less than 1 micro amp drain from the battery - insignificant. This is far less than the self discharge rate of the battery. The Truecharge 10i model does not use isolation diodes and draws 12 milliamps when no AC is present. This is comparable to the self discharge rate of the battery. If the charger/battery is disconnected from any other charge source or load the charger will typically drain the battery by 2 Ahr in 1 week, therefore we recommend disconnecting the charger after a week if there is no AC power available for extended periods.

    Will my smart charger recharge a battery that has been discharged to less than 10 volts?

    In short NO, if the batteries are too discharged, no smart charger can really start into boost mode. The Truecharge and CPS chargers will pulse the voltage up slowly until the battery reaches a suitable terminal voltage to kick into boost mode. If your battery is too discharged sometimes it is best to use a non-regulated battery charger for a very short time to throw some charge into the battery.

    How far away can the battery charger be mounted from the batteries?

    The answer is as close as possible to the batteries, however if the correct size cable is used to reduce voltage drop then no problems should occur. Remember your battery bank needs to get over 14 volts DC (12 volt system) to be adequately recharged, and most chargers charge at 14.4 volts in boost. If you have .6 of a volt drop in your battery cables, then your batteries will take much longer to recharge.

    What happens if a charger that is too small for the batteries is used?

    If the battery bank is too large for the charger, they will eventually reach full charge, however it will take much longer (perhaps days in an extreme case) for the battery to reach a 90% charged state. At that point the charge current would drop below the chargers output and the charger would go into absorption phase to complete the charge to 100%. The drawbacks of using too small a charger for a given battery are:

    • The battery may not reach full charge
    • Excessively long time to reach full charge
    • Battery held at higher voltage for longer than necessary, accelerating in electrolyte loss.

    The best rule of thumb is to ensure that your charger is within 10-30% of the total house battery bank capacity (e.g. the best size for a 100 amp hour battery is 10amps, however a 20amp charger could be used if desired.

    What happens if your battery charger is too large for the bank of batteries to be used?

    Although not recommended, a larger battery charger can be used on a small battery bank providing their recharge cycle is monitored. Again the rule of thumb applies to work to the 10-30% of the total house battery bank. If the charger has temperature compensation it is wise to use it. Remember this point, the batteries will be recharged quicker, but the charger will taper off very quickly into the absorption mode so the full current may not be delivered to the batteries.

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