How to Size a Portable Generator for Home Use

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Portable Generators are a reliable source of power in the absence of utility power. They provide electrical power to supply our critical power needs when the utility company is unable to supply us electrical power due to fault in their transmission system or during maintenance interventions in their power infrastructure or in the worst case of a natural disaster such as earthquake or a hurricane that has destroyed section of the power grid.

What Size of Portable Generator Do I need ?
The size of portable generator you need depends on your power requirement when the need arises to use the generator. Do you require the generator to power all of your electrical appliances at once? Or do you require the generator to power some critical electrical load during power outage? The bigger your power needs, the bigger the size of your generator and the more expensive your portable generator will be!

Running Watts of an Electrical Appliance
The running watts of an electrical appliance is the power it can draw continuously with rated voltage and current. It is usually calculated as:

Running Watts = Rated Voltage x Rated Current.

Note that the above formula will give power in volts-amps or VA but assuming a power factor = 1 which is rarely the case, we get power in watts. This approximation is done to enable easy sizing of a portable generator for home use.

The running watt can easily be calculated by using the rated voltage and current on the name plate of the appliance. Generators are also rated for their running watts. It is the power the generator can deliver continuously at rated voltage, current and frequency. A generator must not be made to continuously carry load beyond its running watts for a very long time otherwise the generator’s life will be shortened and the device becomes damaged in a short time.

Surge Watts or Start up Power of an Electrical Appliance
Certain devices and appliances have an electric motor or compressor in them. They require additional watts to start them. This additional watt may also be referred to as the surge watt of the device. The surge watts required by these devices may sometimes be two or three times the watts required to run the device. Heat producing devices also called resistive loads such as light bulbs, toasters or coffee makers do not require surge watts at start up. A generator must have enough surge watts capacity to handle devices that require surge watts at start up to prevent a nuisance tripping of the main power breaker in the generator.
As shown above, a generator must have sufficient surge capacity to carry loads requiring additional power during start up. Consider a refrigerator that works for one third of the time within a given time cycle. Each time the refrigerator compressor starts, a generator powering the refrigerator must have sufficient surge power for the compressor each time it comes on!

How to Calculate the Size of Portable Generator Required
To properly size a generator, care should be taken to analyse the load the generator is to power so that both running watts and surge watts can be correctly calculated. To calculate the size of generator:

Add up the total running and surge watts for each appliance. Multiply the total sum gotten by a contingency of 15 – 20 % to get the capacity of your generator.

As a guide during the sizing calculation for domestic application;
Surge watts for refrigerators and air conditioners = 2 x running watts 
Surge watts for motors (surface or submersible pumps) = 3 x running watts
Microwave Oven = 1.5 x running watts

Sample Sizing Calculation
Suppose the following loads are to be powered by a portable generator:

Electrical Load
Number
Running Watts
(W)
Refrigerator 1 800
Submersible pump 1 600
Lighting loads lot 150
Air Conditioner (1hp) 1 800
Deep Freezer 1 500
Microwave 1 600
Computer 1 300
TV 1 400



Determine power rating for generator  as shown below:
Electrical Load
Number
Running Watts
(W)
Surge Watts
(W)
Refrigerator 1 800  2 x 800 = 1,600
Submersible pump 1 600 3 x 800  = 2,400
Lighting loads lot 150 0
Air Conditioner (1hp) 1 800 2 x 800 = 1,600
Deep Freezer 1 500 2 x 500 = 1,000
Microwave 1 800 1.5 x 800 = 1200
Computer 1 300 0
TV 1 400  0
Total
4,350 7,800
Total Power Required = 4,350 + 7,800 = 12,150W

Add 15% contingency = 12,150 x 1.15 =13,972.5W

Size of Generator needed 

= 15,000W or 15KVA   standard Size







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How to Calculate Inverter Power Rating and Inverter Battery Backup Time

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Inverter systems are a common feature in our homes and workplace where they play a prominent role in the ensuring uninterruptible power to sensitive loads and devices. For home applications, there is the need to adequately size your inverter to be able to meet the expected load demand. 

Inverters convert DC voltage to AC voltage. They have a battery system which provide adequate backup time to provide continuous power in the home. The inverter system then converts the battery voltage to AC voltage through electronic circuitry. The inverter system also has some charging system that charges the battery during utility power. During utility power, the battery of the inverter is charged and at the same time power is supplied to the loads in the house. When utility power fails, the battery system begins to supply power via the inverter to the loads in the home as shown below:

How to Size and Calculate the Inverter Power Requirement
Inverter power is rated in VA or KVA.
Power in VA = AC Voltage x AC Current in Amps
Power in KVA = AC Voltage x AC Current in Amps/1000
Power in Watts = AC Voltage x AC Current in Amps x PF
Where PF = power factor 
Power in KW = AC Voltage x AC Current in Amps x PF/1000
Also  Power in W = Power in VA x PF
         Power in KW = Power in KVA x PF

Suppose we want to size an inverter to carry the following loads:
1. Lighting load, 300W
2. 3 Standing fans of 70W, each
3. 2 LCD TV, 100W
4. 1 Home Theatre Music System, 200W
5. 1 Juice extractor, 150W

Applying Power in KW = Power in KVA x PF
Power in KVA  = Power in KW/PF = Power in KW/0.8    (Nominal PF = 0.8, which is standard for homes)

Total load in Watts = 300 + (3 x 70) + 200 + 200 + 150 = 1060W = 1.06KW
Power in KVA = 1.06/0.8 = 1.325
An inverter of standard rating 1.5KVA is required to carry the loads above.

How to Calculate Inverter Battery Backup Time
The backup time for batteries in an inverter system depends on the number of batteries as well as their capacity in Amp-hours.

Inverter battery backup time is calculated as:
Back up time = Battery Power in Watt hour (Wh)/Connected Load in Watts (W)
Battery Power in Wh = Battery Capacity in AH x Battery Voltage (V) x Number of Batteries

Let us shorten the formula by using the following Symbols:
Let BUT = battery backup time in hours
            C = battery capacity in AH 
    V = battery voltage in volts
    N = Number of batteries in series or parallel as the case may be.
    $P_L$ = connected load in watts (W)

Now

$$BUT = {\frac{C*V*N}{ P_L}}$$

In our example above, suppose we have selected a 24V, 1.5KVA inverter system that is to use two 12V batteries in series connection and suppose further that the capacity of our batteries are 200AH each, then :

C = 200AH
V = 12V
N = 2
$P_L$   = 1,060W

$$BUT = {\frac{200 * 12 * 2}{1060}} = 4.53 hrs$$

How UPS (Uninterruptible Power Supply) Systems Works

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UPS stands for Uninterruptible Power Supply. A UPS system is an autonomous source of alternate power that is used to supply sensitive electronic loads such as computer centers, telephone exchanges and many industrial-process control and monitoring systems. These applications require power that is availability and of good quality.

A UPS solution for sensitive electrical loads is used to provide a power interface between the utility and the sensitive loads, providing voltage that is:
1. Free of all disturbances present in utility power and in compliance with the strict
        tolerances required by loads.
2. Available in the event of a utility outage, within specified tolerances

UPS systems satisfy requirements in 1 & 2 above in terms of power availability and quality by:
1. Supplying loads with voltage complying with strict tolerances, through use of an
        inverter
2. Providing an autonomous alternate source, through use of a battery
3. Stepping in to replace utility power with no transfer time, i.e. without any interruption in the               supply of power to the load, through use of a static switch.

These characteristics make UPS units the ideal power supply for all sensitive applications because they ensure power quality and availability, whatever the state of utility power.

Basic Parts of a UPS System
A UPS comprises the following main components:
1. Rectifier/charger, which produces DC power to charge a battery and supply an inverter
2. Inverter, which produces quality electrical power free of all utility-power disturbances, notably           micro-outages and that is within tolerances compatible with the requirements of sensitive                     electronic devices.
3. Battery, which provides sufficient backup time to ensure the safety of life and property by                   replacing the utility as required
4. Static switch, a semi-conductor based device which transfers the load from the
        inverter to the utility and back, without any interruption in the supply of power

Types of Static UPS Systems
Types of static UPSs are defined by standard IEC 62040. The standard distinguishes three operating modes for UPSs which are:
1. Passive standby (also called off-line)
2. Line interactive
3. Double conversion (also called on-line)

These definitions concern UPS operation with respect to the power source including the distribution system upstream of the UPS. IEC Standard 62040 defines the following terms:
a. Primary power: power normally continuously available which is usually supplied by
        an electrical utility company, but sometimes by the user’s own generation
b. Standby power: power intended to replace the primary power in the event of
        primary-power failure
c. Bypass power: power supplied via the bypass

UPS Operating in Passive Standby Mode

Operating Principle:
The inverter is connected in parallel with the AC input in a standby as shown below:
UPS in Passive Standby Mode. Photo Credit: Schneider Electric

Normal Mode Operation
In normal mode operation, the load is supplied by utility power via a filter which eliminates certain disturbances and provides some degree of voltage regulation (IEC 62040 specifies some form of power conditioning). The inverter operates in passive standby mode.

Battery Backup Mode Operation
In battery backup mode operation, when the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a very short less than 10 ms transfer time. The UPS continues to operate on battery power until the end of battery backup time or the utility power returns to normal, which causes transfer of the load back to the AC input (normal mode).

Application
This configuration is a compromise between an acceptable level of protection against disturbances and cost. It can be used only with low power ratings less than 2 kVA.

Limitations
This UPS operates without a real static switch, so a certain time is required to transfer the load to the inverter. This time is acceptable for certain individual applications, but
incompatible with the performance required by more sophisticated, sensitive systems
(large computer centers, telephone exchanges, etc.). Furthermore, the frequency is not regulated and there is no bypass.

UPS Operating in Line-interactive Mode
The inverter is connected in parallel with the AC input in a standby configuration, but also charges the battery. It thus interacts with the AC input source as shown below:
UPS in Line-interactive Mode. Photo Credit: Schneider Electric

Normal Mode Operation
In normal mode operation, the load is supplied with conditioned power via a parallel connection of the AC input and the inverter. The inverter operates to provide output-voltage conditioning and/or charge the battery. The output frequency depends on the AC-input frequency.

Battery Backup Mode Operation
In this mode of operation, when the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a transfer without interruption using a static switch which also disconnects the AC input to prevent power from the inverter from flowing upstream. The UPS continues to operate on battery power until the end of battery backup time or the utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).

Bypass Mode Operation
This type of UPS may be equipped with a bypass. In the bypass mode, If one of the UPS functions fails, the load can be transferred to the bypass AC input (supplied with utility or standby power, depending on the installation).

Application and Limitation
This UPS configuration is not well suited to regulation of sensitive loads in the medium to high-power range because frequency regulation is not possible. For this reason, it is rarely used other than for low power ratings.

UPS Operating in Double Conversion (On-line) Mode

Operating Principle:
In this type of UPS, the inverter is connected in series between the AC input and the application as shown below:
UPS in Double-Conversion Mode. Photo Credit: Schneider Electric

Normal Mode Operation
During normal operation, all the power supplied to the load passes through the rectifier/charger and inverter which together perform a double conversion (AC to DC to AC), hence the name.

Battery Backup Mode Operation
In battery backup mode, When the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a transfer without interruption using a static switch. The UPS continues to operate on battery power until the end of battery backup time or utility power returns to normal, which causes transfer of the load back to the AC input (normal mode).

Bypass Mode Operation
This type of UPS is generally equipped with a static bypass, sometimes referred to as a static switch. The load can be transferred without interruption to the bypass AC input (supplied with utility or standby power, depending on the installation), in the event of UPS failure, load current transient (inrush or fault currents) or load peaks. The presence of a bypass assumes that the input and output frequencies are identical and if the voltage levels are not the same, a bypass transformer is required.

For certain types of load, the UPS must be synchronized with the bypass power to ensure load-supply continuity. Furthermore, when the UPS is in bypass mode, a disturbance on the AC input source may be transmitted directly to the load because the inverter no longer steps in. Another bypass line, often called the maintenance bypass, is available for maintenance purposes. It is closed by a manual switch.