Power Inverter FAQ

Frequently Asked Questions about Power Inverters

• What does a power inverter do, and what can I use one for?
• Using an inverter for basic emergency home backup power
• What size inverter should I buy? (and Formula to convert Amps to Watts)
• Watts Used by Common Appliances and Tools (Usage Chart)
• Do I need a Modified Sine Wave Inverter, or a Pure Sine Wave Inverter?
• How do I hook up the Inverter? What size cable should I use, and is it included?
• What is an overcurrent protection device? Why do I need one?
• What type of battery should I use (automotive or deep cycle)?
• How long can I run the power inverter on my battery?
• How do I connect two or more batteries together?
• Using a Microwave Oven with a Power Inverter
• Using a Photographic Strobe with a Power Inverter
• Using a Laser Printer with a Power Inverter
• Television and Audio Suggestions
• Appliance Cautions
• Safety Precautions and Installation Tips (Please Read)
• If you don't find your answer here, Read our Power Inverter Blog ... More Info, Questions & Answers about Inverters

What does a power inverter do, and what can I use one for?

For more information, please visit our website.

A power inverter changes DC power from a battery into conventional AC power that you can use to operate all kinds of devices ... electric lights, kitchen appliances, microwaves, power tools, TVs, radios, computers, to name just a few. You just connect the inverter to a battery, and plug your AC devices into the inverter ... and you've got portable power ... whenever and wherever you need it.

The inverter draws its power from a 12 Volt battery (preferably deep-cycle), or several batteries wired in parallel. The battery will need to be recharged as the power is drawn out of it by the inverter. The battery can be recharged by running the automobile motor, or a gas generator, solar panels, or wind. Or you can use a battery charger plugged into an AC outlet to recharge the battery.

Using an Inverter for Emergency Home Backup Power

A very simple way to use an inverter for emergency power (such as during a power outage), is to use a car battery (with the vehicle running), and an extension cord running into the house, where you can then plug in electrical appliances.

Click here to read an in-depth Article on Emergency Home Backup Power

What size inverter should I buy?

We carry many different sizes, and several brands of power inverters. See our Inverters Page for specifications on each of our models.

Short Answer: The size you choose depends on the watts (or amps) of what you want to run (find the power consumption by referring to the specification plate on the appliance or tool). We recommend you buy a larger model than you think you'll need (at least 10% to 20% more than your largest load).

Example: You want to power a computer with a 17" monitor, some lights, and a radio.

Computer: 300 Watts 2 - 60 Watt lights: 120 Watts Radio: 10 Watts Total Needed: 430 Watts

For this application, you would minimally need a 500 W inverter, and should give some thought to a larger one, as there will likely be a time when you wish you'd bought a bigger model ... in this example, you might decide you'd like to run a fan while you compute, or let the kids watch TV.

Longer Answer: Determine Continuous Load and Starting (Peak) Load: You need to determine how much power your tool or appliance (or combination of them that you would use at the same time) requires to start up (starting load), and also the continued running requirements (continuous load).

What is meant by the terms "continuous- watts" and "peak surge- watts" is that some appliances or tools, such as ones with a motor, require an initial surge of power to start up ("starting load" or "peak load"). Once started, the tool or appliance requires less power to continue to operate ("continuous load")

Helpful formulas:

To Convert AMPS to WATTS:

Multiply: AMPS X 120 (AC voltage) = WATTS
This formula yields a close approximation of the continuous load of the appliance

To Calculate approximate Startup Load:

Multiply: WATTS X 2 = Starting Load
This formula yields a close approximation of the starting load of the appliance, though some may require an even greater starting load. NOTE: Induction motors such as air conditioners, refrigerators, freezers and pumps may have a start up surge of 3 to 7 times the continuous rating.

Most often the start up load of the appliance or power tool determines whether an inverter has the capability to power it.

For example, you have a freezer with a continuous load of 4 amps, and a start up load of 12 amps:

4 amps x 120 volts = 480 watts continuous
12 amps x 120 volts = watts starting load

You would need an inverter with peak-surge rating greater than watts.

FORMULA to convert AC Watts to DC Amps:

AC Watts divided by 12 x 1.1 = DC Amps
(this is the size vehicle alternator you would need to keep up with a specific load; for example, to keep up with a continuous draw of watts, you would need a 91 amp alternator)

Click for a Chart of Estimated Watts Used by Common Appliances and Tools

Do I need Modified Sine Wave, or Pure Sine Wave?

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

a) Output voltage wave form is pure sine wave with very low harmonic distortion and clean power like utility-supplied electricity.

b) Inductive loads like microwave ovens and motors run faster, quieter and cooler.

c) Reduces audible and electrical noise in fans, fluorescent lights, audio amplifiers, TV, Game consoles, Fax, and answering machines.

d) Prevents crashes in computers, weird print out, and glitches and noise in monitors.

e) Reliably powers the following devices that will normally not work with modified sine wave inverters:

• Laser printers, photocopiers, magneto-optical hard drives
• Certain laptop computers (you should check with your manufacturer)
• Some fluorescent lights with electronic ballasts
• 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
• Digital clocks with radios
• Sewing machines with speed/microprocessor control
• X-10 home automation system
• Medical equipment such as oxygen concentrators

We carry a full line of Pure Sine Wave and Modified Sine Wave Power Inverters here at DonRowe.com. Modified Sine Wave works well for most uses, and is the most common type of inverter on the market, as well as the most economical. Pure Sine Wave inverters (also called True Sine Wave) are more suited for sensitive electrical or electronic items such as laptop computers, stereos, laser printers, certain specialized applications such as medical equipment, a pellet stove with an internal computer, digital clocks, bread makers with multi-stage timers, and variable speed or rechargeable tools (see " Appliance Cautions" below). If you wish to use those items with an inverter, then choose a Pure Sine Wave inverter. If you mostly want to run lights, TV, microwave oven, tools, etc, a Modified Sine Wave inverter is fine for your needs.

We often are asked if computers will work with Modified Sine Wave. It's been our experience that most (with the exception of some laptops) will work (though some monitors will have interference such as lines or a hum). However, if you have any doubt about any appliance, tool or device, particularly laptop computers and medical equipment such as oxygen concentrators, we recommend that you check with its manufacturer to be sure it is compatible with a Modified Sine Wave inverter. If it is not, choose one of our Pure Sine Inverters instead.

The difference between them is the Pure Sine Wave inverter produces a better and cleaner current. They are also considerably more expensive. You might find it practical to get a small Pure Sine Wave inverter for any "special need" you may have, and also a larger Modified Sine Wave inverter for the rest of your applications.

How do I hook up the Inverter? What size cable should I use, and is it included?

Many small inverters (450 watts and under) come with a cigarette lighter adapter, and may be plugged into your vehicle's lighter socket (although you will not be able to draw more than 150 to 200 watts from the cigarette lighter socket). The small units also come with cables that can be clamped directly to a battery. If you want an inverter that will plug into your cigarette lighter, you must choose one that is 450 watts or less.

Larger inverters (500 watts and over) must be hard-wired directly to a battery. The cable size depends on the distance between battery and inverter, and will be specified in the Owner's Manual.

When connecting the inverter to the battery always use an overcurrent protection device, such as a fuse or circuit breaker, and use the thickest wire available, in the shortest length practical.

See our Cables Page for recommendations for each of the inverters we sell.

General recommendations:

Inverter Size < 3 ft 3ft - 6ft 6ft - 10ft 400 Watts 8 6 4 750 Watts 6 4 2 Watts 4 2 1/0 Watts 2 1 3/0 Watts
1/0 2/0 250 Watts
1/0 3/0 350 Watts
3/0 4/0 500

NOTE: These are general recommendations for inverters that utilize a single cable set (one positive and one negative cable) only and may not be correct for all inverters or applications. Additionally, some inverters require two or more cable sets and therefore may require a different cable size than listed.

Cable size recommendations may vary among inverter brands and models; check the Owner's Manual for the model you purchase before you buy the wire for it.

The maximum length generally recommended is 10', and shorter is better. If you need more length, it is much better to put it on the AC side (as with an extension cord from inverter to appliance) than on the DC side.

Cables with battery terminals (ring terminals or stud terminals) to hook up your inverter are available here.

What is an overcurrent protection device? Why do I need one?

Batteries are capable of supplying large amounts of current, and thousands of amperes could be present if a short circuit were to occur. A short circuit can damage your system, cause a fire and be hazardous to your health. Incorporating an overcurrent device is an effective line of defense against a short circuit occurrence. An overcurrent protection device is typically a fuse or circuit breaker that goes inline on the positive cable, between the inverter and battery, to protect your system. A fast acting fuse or circuit breaker will blow within milliseconds under short circuit conditions, preventing any damage or hazards.

It is important to appropriately size your fuse or circuit breaker for both your inverter and cables. An oversized fuse could result in cables exceeding their ampere capability, resulting in the cables becoming red-hot and dangerous. Consult your owners manual for the recommended sized fuse or circuit breaker and cable gauge for a safe installation.

Fuses and circuit breakers to protect your inverter are available here.

What type of battery should I use (automotive or deep cycle)?

Small Inverters: Most automobile and marine batteries will provide an ample power supply for 30 to 60 minutes even when the engine is off. Actual time may vary depending on the age and condition of the battery, and the power demand being placed on it by the equipment being operated by the inverter. If you use the inverter while the engine is off, you should start the engine every hour and let it run for 10 minutes to recharge the battery.

500 Watt and larger Inverters: We recommend you use deep cycle (marine or RV) batteries which will give you several hundred complete charge/discharge cycles. If you use the normal vehicle starting batteries they will wear out after about a dozen charge/discharge cycles. If you do not have a deep cycle battery, we recommend that you run the engine of your vehicle when operating the power inverter.

When operating the inverter with a deep cycle battery, start the engine every 30 to 60 minutes and let it run for 10 minutes to recharge the battery.

When the inverter will be operating appliances with high continuous load ratings for extended periods, it is not advisable to power the inverter with the same battery used to power your car or truck. If the car or truck battery is utilized for an extended period, it is possible that the battery voltage may be drained to the point where the battery has insufficient reserve power to start the vehicle. In these cases, it's a good idea to have an extra deep cycle battery for the inverter (installed close to the inverter), cabled to the starting battery. It is recommended to install a battery isolator between the batteries.

How long can I run the inverter on my battery?

To estimate how long a battery/appliance combination will operate together, use this handy calculator. (Tip: If the calculator output equals 0 hours, the total Amp/Hrs of the battery bank are insufficient to run the load. Try adding additional Amp/Hrs to the battery bank field in order to run the desired wattage.)

You can also use these formulas to calculate how long your appliance will operate on your battery.

For a 12 Volt System:

Link to Jiwei

(10 x (Battery Capacity in Amp Hours) / (Load Power in Watts)) / 2 = Run Time in Hours

For a 24 Volt System:

(20 x (Battery Capacity in Amp Hours) / (Load Power in Watts)) / 2 = Run Time in Hours

Tip: Deep cycle (marine) batteries generally have the highest reserve ratings. They are also capable of withstanding repeated drains of power and recharging.

Tip: Engine start batteries should not be discharged below 90% charged state, and marine deep cycle batteries should not be discharged below 50% charged state. Doing so will shorten the life of the battery based on most battery manufacturers recommendations.

Note: If you intend to use power tools for commercial use, or any load of 200W for more than 1 hour regularly (between battery recharging) we recommend installing an auxiliary battery to provide power to the inverter. This battery should be a deep cycle type and sized to meet your run time expectations with the engine off. The auxiliary battery should be connected to the alternator through an isolator module to prevent the inverter from discharging the engine start battery when the engine is off.

How do I connect two or more batteries?

It may be advisable to operate the inverter from a bank of 12 Volt batteries of the same type in a "parallel" configuration. Two such batteries will generate twice the amp/hours of a single battery; three batteries will generate three times the amp/hours, and so on. This will lengthen the time before your batteries will need to be recharged, giving you a longer time that you can run your appliances.

You can also connect 6 Volt batteries together in "series" configuration to double the voltage to 12 volts. Note that 6 Volt batteries must be connected in pairs.

Operating a Microwave with a Power Inverter

The power rating used with microwave ovens is the "cooking power" which refers to the power being "delivered" to the food being cooked. The actual operating power requirement rating is higher than the cooking power rating (for example, a microwave with "advertised" rating of 600 watts usually corresponds to almost watts of power consumption). The actual power consumption is usually stated on the back of the microwave. If the operating power requirement cannot be found on the back of the microwave, check the owner's manual or contact the manufacturer.

Operating a Photographic Strobe with a Power Inverter

A photographic strobe or flash generally requires a pure sine wave inverter capable of surging to at least 4 times the Watt Sec rating of the strobe. For instance, a strobe rated at 300 watts requires an inverter capable of surging to watts or more.

For additional information, please read this Samlex Application Note.

Operating a Laser Printer with a Power Inverter

A laser printer generally requires a pure sine wave inverter capable of surging at least 6.5 times the maximum wattage rating of the printer. For instance, a laser printer rated at 500 watts requires an inverter with a surge rating of at least 3,250 watts.

An inkjet printer does not maintain the same requirements as a laser printer. Inkjet printers can be operated normally with a modified sine wave inverter rated to handle the printers wattage requirement.

For additional information, please see our Inverter Blog and this Samlex Application Note.

Television and Audio Suggestions

Although all our inverters are shielded and filtered to minimize signal interference, some interference with your television picture may be unavoidable, especially with weak signals.

Here are some suggestions that may improve reception:

1. First make sure that the television antenna produces a clear signal under normal operating conditions (i.e., at home plugged into a standard 110AC wall outlet). Also insure that the antenna cable is properly shielded and of good quality.

2. Change positions of the inverter, antenna cables and television power cord.

3. Isolate the television, its power cord and antenna cables from the 12 volt power source by running an extension cord from the inverter to the TV set. Insure that any excess AC power cord is a distance away from the TV set.

4. Coil the television power cord and the input cables running from the 12 volt power source to the inverter.

5. Attach a "Ferrite Data Line Filter" to the television power cord. More than one filter may be required. These are available at electronic supply stores including Radio Shack (Radio Shack Part No. 273-105)

NOTE: Some inexpensive audio systems may discharge a slight "buzzing" sound when operated with an inverter. This is caused by deficient filters in the audio system. The only solution to this problem is using a sound system with a higher quality power supply.

Appliance Cautions (for Modified Sine Wave Inverters):

DO NOT plug small appliances into the inverter AC receptacles to directly recharge their nickel-cadmium batteries. Always use the recharger provided with that appliance.

DO NOT plug in battery chargers for cordless power tools if the charger carries a warning that dangerous voltages are present at the battery terminals.

Not all fluorescent lamps operate properly with a modified sine wave inverter. If the bulb appears to be too bright, or fails to light, do not use the lamp with the inverter.

Some fans with synchronous motors may slightly increase in speed (RPM) when powered by a modified sine wave inverter. This is not harmful to the fan or to the inverter.

Certain rechargers for small nickel-cadmium batteries can be damaged if plugged into a modified sine wave inverter. In particular, two types of appliances are susceptible to damage:

• Small, battery-operated appliances such as flashlights, cordless razors and toothbrushes that can be plugged directly into an AC receptacle to recharge.
• Certain battery chargers for battery packs that are used in some cordless hand-tools. Chargers for these tools have a warning label stating that dangerous voltages are present at the battery terminals.

DO NOT use a modified sine wave inverter with the above two types of equipment.

The majority of portable appliances do not have this problem. Most portable appliances use separate transformers or chargers that plug into AC receptacles to supply a low-voltage DC or AC output to the appliance. If the appliance label states that the charger or adapter produces a low-voltage DC or AC output (30 volts or less), there should be no problem powering that charger or adapter.

Safety Warning: 110 Volts of current can be lethal. Improper use of a power inverter will result in property damage, personal injury, or loss of life. Please read and follow carefully the instructions in the Owner's Manual provided with every inverter for important safety considerations and precautions.

General Safety Precautions and Installation Tips:

Frequently Asked Questions about Inverters

How much battery capacity do I need with an inverter?

As a rule of thumb, the minimum required battery capacity for a 12-volt system is around 20 % of the inverter capacity. For 24-volt inverters, it is 10 %. The battery capacity for a 12-volt Mass Sine 12/, for instance, is 240 Ah, while a 24-volt Mass Sine 24/ inverter would require at least 150 Ah. The indicated battery capacity is only for the inverter. The capacity required for other loads should be added to it.

How much power does an inverter consume?

Mastervolt sine wave inverters have an output efficiency of more than 92 %, which is the maximum that can be achieved with modern technology. If you connect an 850 W coffee maker to a Mass sine wave inverter, consumption will be 850 W divided by the onboard voltage of 12 volt, approx. 70 A. Of course, a coffee maker will only be in use for a short period of time, so the consumption measured in Ah will be relatively modest (76 x 5 min. = 6.5 Ah). As a rule of thumb you should divide the connected capacity by 10 for 12 volt and by 20 for 24 volt. This also includes all the power losses in the cables, fuses and the inverter.

Is there a stand-by switch on the inverter?

Definitely! Although the no-load consumption is extremely low, most Mastervolt inverters and Combis are even equipped with two energy saving solutions. Activating the Economy mode reduces battery consumption by an extra 10 %. This function requires an adjustment to the dipswitch/jumper settings, which can possibly be realised via the free downloadable software. The economy mode ensures that the output voltage drops slightly to 208 V, which also reduces the power consumption from the battery. Low-load devices like microwaves and DVD clocks function as normal at this lower voltage.With some inverter models it is even possible to activate a stand-by mode. In this mode the inverter sets a tiny pulse on the 230-volt installation, checking for any connected appliances. As soon as the pulse detects consumption, voltage returns to normal strength to power the connected equipment. The economy mode ensures that the power consumption of the inverter stays at virtually zero.

Can I power a computer with an inverter?

Yes, you can. All Mastervolt sine wave inverters can easily and safely supply a computer without the slightest problem or risk. In fact, the output voltage from an inverter is often better than that from the electricity grid or shore power. This is why Mastervolt inverters, combined with a battery charger and a battery set, are often used as a back-up system in places where the grid connection is unreliable. Laptops can also be powered by a Mastervolt inverter.

Can a microwave be powered with an inverter?

Any microwave model can be connected to a Mastervolt inverter. Bear in mind that an 800-watt microwave consumes about to watt from the 230-volt system, and that the capacity of the inverter and battery must be able to handle this. Apart from that, the total consumption of the microwave-inverter combination is moderate: Using the microwave for five minutes will use around 12 Ah on a 12-volt system and 6 Ah on a 24-volt system.

Are there any appliances that cannot be powered by an inverter?

You can connect almost any appliance to an inverter, with a few practical exceptions. In practice you must be careful with equipment that consumes a lot of power, such as electrical heaters or air conditioning. While the inverter itself has no problems with these loads, the battery capacity is often too limited for long-term usage of these loads.Appliances that are only used for a limited time period, such as washing machines, driers or a small hotplate, should be fine as long as the battery has sufficient capacity. In these cases it is advisable to use a power source such as an alternator, which also powers the battery. Keep in mind that heavy users such as engines, pumps, air conditioning and fridges have a starting current that is between 5 to 12 times higher than the nominal absorbed capacity. The inverter has to be able to supply accordingly. Mastervolt inverters are developed to provide high peak currents.

How much current will an inverter draw from my batteries?

This depends on the equipment connected to the inverter. There is a simple method to calculate how much power your inverter is using: For 12-volt inverters, divide the connected load by 10; for 24-volt inverters, divide by 20.

Example: How much does an inverter consume with a 400 W load connected? For a 12 V inverter such as a Mass Sine 12/, consumption will be 400/10 = approx. 40 amps. For a 24 V inverter, say a Mass 24/, the corresponding figure is 400/20 = approx. 20 amps. It is important to remember that this is only an approximation: The actual consumption will tend to be some 5 to 15 % less, depending on the load type.

How thick should my battery cables be?

Using the method described above, calculate how much power your inverter will be drawing at maximum capacity (120 amps in the case of a Mass Sine 12/, for example). Then count 3 amps per mm². In the above example this is 120 divided by 3 = 40 mm². The standard cable thickness closest to that is 35 mm². This rule applies to cables up to three metres in length. If the inverter is further away from the battery, choose the next size up (50 mm² for instance).

Does an inverter need a lot of ventilation?

An inverter needs very little ventilation - two approx. 60 cm² ventilation openings are usually enough. Larger inverters, from W upwards, need twice that size. Inverters used in high ambient temperatures, and those expected to be operating at full capacity for a long period, require openings that are four times as large.

Can an inverter be used in parallel with the generator or the grid?

No, stand-alone inverters cannot function in parallel with a generator or grid connection. A Mastervolt Mass Combi is the solution if you need more power than is available from the grid connection or the generator. These models can compensate for the lack of grid or generator power using the batteries. A Masterswitch or Systemswitch needs to be installed when grid or generator power is connected to the onboard grid simultaneously with an inverter. The main function of the Masterswitch is the automatic and safe switching between grid, generator and inverter power.

What is no-load?

No-load is the amount of current used by the inverter when no loads are switched on, while the inverter is switched on.

What is efficiency?

An inverter uses a small amount of energy during the conversion process. The difference between the input power and the output power is expressed in percentages. The efficiency of modern inverters is more than 92 %. This means that a maximum of 8 % of the power consumption is used to convert battery voltage to 230V/50Hz. A connected load of 250 watt, for instance, requires less than 270 watt to be supplied by the batteries.

Efficiency versus output power:

What are trapezoidal or square wave inverters?

Square wave and trapezium inverters are often available in DIY shops and supermarkets at low prices. These cheap inverters are not suitable for all loads, however, and therefore not safe to use. Modern inverters generate a sine wave-shaped output current similar to or even better than that of the public grid and perfectly suited to powering sensitive equipment. Trapezoidal inverters, also called modified sine wave, are the ancestors of the modern sine wave inverters. As the name suggests, they generate an output voltage in the shape of a trapezium. This type of voltage is inappropriate for sensitive equipment. The square wave inverter is the predecessor of the trapezoidal inverter and represents the first generation of inverters. It is also very unsuitable for delicate equipment.

Can I power an air conditioning system via the inverter?

It is perfectly possible to power a small air conditioning installation of, for instance, - BTU via an inverter. While it is important to remember not to leave the air conditioning running for too long, cooling down a cabin before going to sleep is fine as long as the battery bank and inverter are correctly sized. Also remember to pay attention to the start-up current, which can be up to eight or twelve times higher than nominal capacity. A BTU air conditioning uses approximately 700 watts. For a 230 V system this means a current consumption of approximately 35 amperes. Cooling down a cabin within two hours requires 70 Ah for a 24 V system and 140 Ah for a 12 V system. It is important to make sure that the inverter is large enough to provide the starting current for the air conditioning, and the battery also needs to be able to supply the required capacity within two hours as well.

Can I cook with electric appliances using an inverter?

Cooking is definitely possible with an inverter as long as the battery set is reasonably large and the inverter has a capacity of at least 2 kW. The preparation of a complete meal will, in general, require the generator to be activated or power to be plugged in. Switching on the generator to heat up a cup of soup or cook a steak or an egg is rather wasteful, since the generator will not even warm up properly before it is turned off again. This is bad for the generator and the environment. Instead, we recommend that you power only one or two of the hotplates via the inverter – a snack can then still be prepared or heated without needing to start the generator. After using the cooker, recharge the battery fully via the battery charger or alternator.

Inverter selection table