Inverter Should Meet The Application

To choose an inverter, you should first define your needs. Where is the inverter to be used? Inverters are available for use in buildings (including homes), for recreational vehicles, boats, and portable applications. Will it be connected to the utility grid in some way? Electrical conventions and safety standards differ for various applications, so don’t improvise.

Selection Of Battery System Voltage

All conductors used in electrical devices resist the flow of electric current by varying degrees depending upon their electrical property called “Resistance”. Flow of current through a resistance produces voltage drop and heat. The heat energy is wasted and contributes to the loss of efficiency. For a specified value of resistance, the heat loss is proportional to the square of the current e.g. 2 times increase of current produces 4 times heat dissipation. Hence, it is desirable to use conductors with the minimum possible resistance and also to reduce the value of current to reduce the loss of power due to heat dissipation. By increasing their size (area of cross-section) to practically permissible limits, the resistance of the conductors can be reduced. Electrical power (e.g. DC power) is a product of the Voltage and the Current i.e. Power = Voltage X Current. This equation shows that a particular value of power can be obtained by either increasing the current and reducing the voltage or by increasing the voltage and reducing the current. From the consideration of power loss due to the heating effect of current as explained above, it is desirable to consider higher Voltage and lower Current option. In view of the above, a higher battery system voltage improves efficiency due to consumption of lower current resulting in lesser power dissipation inside the inverter and in the DC side wiring. Also, lower current at higher voltage will require smaller size of wiring which makes the system wiring cheaper and easier. The DC input voltage of the inverter must conform to that of the DC electrical system / the battery bank. 12 Volts is recommended for small, simple systems. 24 and 48 Volts are the common standards for higher capacities.

Electrical Standards

The inverter’s AC output must conform to the conventional power in the region in order to run locally available appliances. The standard for residential / light commercial AC utility service in North America is Single Phase 120 VAC and Split 120 / 240 VAC at a frequency of 60 Hertz (cycles per second). In Europe, South America, and most other places, it is 230 volts at 50 Hertz.

The rated output power (in Watts) of the inverters is normally specified for resistive type of loads that have unity (1) Power Factor. In a reactive type of load, the phase angle Ø of the sine wave-form of the current drawn by the load may lead or lag the sine wave-form of the AC voltage source. In this case, the power factor of reactive loads is lower than unity (1) – generally between 0.8 and 0.6. A reactive load reduces the effective power that can be delivered by an AC power source.

Specifying The Power Rating Of An Inverter

The Power Factor of an AC power source (like an inverter) that is supplying an AC load is controlled by the AC load and NOT by the AC power source. For example, a resistive load will have a Power Factor of 1, a motor load will have a Power Factor of 0.8, a non Power Factor Corrected Switched Mode Power Supply may have a Power Factor of 0.5 to 0.6 etc. Hence, the power rating of an AC power source should normally be specified as Apparent Power in Volt Amperes (VA) as this rating is not influenced by Power Factor as against specifying it in Active Power in Watts which is dependent on Power Factor. Sometimes, the power rating of the inverter is designated as Active Power in Watts (W) at Power Factor = Unity (1). This rating will be equivalent to the Apparent Power Rating in VA (Because at Power Factor = unity (1), the Apparent Power = The Active Power). If the power rating of the inverter has been designated as Active Power in Watts (W) and the Power Factor has not been indicated, it should be assumed as unity (1). Please note that in cases where the Power Factor is less than unity (1), the Apparent Power (VA) is higher than the Active Power (Watts) because the Apparent Power (VA) is the vectorial sum of the Active Power (Watts) and the Reactive Power (VAR). Instantaneously, the AC power source has to provide the total Apparent Power (VA) of the load and, therefore, should be sized based on the Apparent Power (VA) of the load and not on the Active Power (Watts) of the load. If the power consumed by the load is designated as Active Power (Watts), it should be converted to the Apparent Power (VA) by dividing the Active Power (Watts) of the load by the Power Factor of the load. For example, a load rated at Active Power consumption of 600 W at Power Factor = 0.8 will require an Apparent Power of 750 W (Active Power of 600 W divided by Power Factor of 0.8 = Apparent Power of 750 Watts)

Power Rating – “Continuous” and “Surge”

How much load can an inverter handle? Its power output is normally rated in Watts. Read details under Section 2 titled“Characteristics of Sine Wave AC Power”.There are two levels of power rating - Continuous Rating and Surge Rating. Continuous means the amount of power the inverter can handle for an indefinite period of time. When an inverter is rated at a certain number of Watts, that number generally refers to its continuous rating. The “Surge Power Rating” indicates the power to handle instantaneous overload of a short duration to provide the higher power required to start certain type of devices and appliances. If the duration of the“Surge Power Rating” is not specified, it may be assumed to be < 1 sec.

Loads that require initial “Surge Power” to Start

Most loads have inrush currents that may be up to 3 to 10 times their steady-state currents. Therefore, the size of the inverter relative to its load is one of the most important considerations when sizing the power handling capacity of an inverter. If an inverter cannot feed the surge power, it will get overloaded and may simply shut down instead of starting the device. If the inverter’s surge capacity is marginal, its output voltage will dip during the surge. This can cause a dimming of the lights in the house, and will sometimes crash a computer. There is also a possibility of premature failure of the inverter and the load.

Power Rating Of Microwaves

The power rating of the microwave generally refers to the cooking power. The electrical power consumed by the microwave will be approximately 2 times the cooking power. The power rating of the inverter should, therefore, be more than 2 times the cooking power of the microwave. For example, a microwave rated at a cooking power of 800 Watts should be powered from an inverter rated >1600 Watts.

Powering A Water Supply Pump

A water well or pressure pump often places the greatest demand on the inverter. It warrants special consideration. Most pumps draw a very high surge of current during start up. The inverter must have sufficient surge capacity to handle it while running any other loads that may be on. It is important to size an inverter sufficiently, especially to handle the starting surge (If the exact starting rating is not available, the starting surge can be taken as 3 times the normal running rating of the pump). Oversize it still further if you want it to start the pump without causing lights to dim or blink. In North America, most pumps (especially submersibles) run on 240 VAC, while smaller appliances and lights use 120 VAC. To obtain 240 VAC from a 120 VAC inverter, use a 120 VAC to 240 VAC transformer. If you do not already have a pump installed, you can get a 120 Volt pump if you don’t need more than 1 / 2 HP.

Sizing Chart For Typical Loads That Require High Starting Surge

The manufacturers’ specification for power rating of the appliances and devices indicates only the running power required. The surge power required by some specific types of devices as explained above has to be determined by actual testing or by checking with the manufacturer. This may not be possible in all cases and hence, can be guessed at best based on some thumb rules The Surge Power Rating of inverters is normally specified as 2 times their Continuous Power Rating. In a majority of the cases, the time period for sustaining the Surge Power is not specified and should be assumed to be less than 100 milliseconds. As the duration of the Surge Power Rating is very short and is normally not specified, this rating should not be considered for sizing purposes. Only the Continuous Power Rating should be considered for sizing of the inverter Table 1, page 6 lists some common loads that require high surge power on start up. A “Sizing Factor” has been recommended against each which is a multiplication factor to be applied to the rated running Watt rating of the load to arrive at the Continuous Power Rating of the inverter (Multiply the running Watts of the device / appliance by the Sizing Factor to arrive at the size of the inverter).

Idle Power / Self Consumption / Power Save

Idle power / self-consumption 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. Some inverters have a Power Save option where the inverter outputs momentary full power of a few cycles every 3 to 5 sec (generally 1 cycle of around 16 ms {for 60 Hz} output).

Phantom and Idling Loads

Most of the modern gadgets draw some power whenever they are plugged in. Some of them use power to do nothing at all. An example is a TV with a remote control. Its electric eye system is on day and night, watching for your signal to turn the screen on. Every appliance with an external wall-plug transformer uses power even when the appliance is turned off. These little loads are called “phantom loads” because their power draw is unexpected, unseen, and easily forgotten. A similar concern is “idling loads.” These are devices that must be on all the time in order to function when needed. These include smoke detectors, alarm systems, motion detector lights, fax machines, and answering machines. Central heating systems have a transformer in their thermostat circuit that stays on all the time. Cordless (rechargeable) appliances draw power even after their batteries reach a full charge. If in doubt, feel the device. If it’s warm, that indicates wasted energy.