Hybrid Inverters should be sized slightly higher than the load or power demand of the appliances they will be powering. Temperature de-rating of the inverter should be at least 1.2 times larger than the highest continuous load demand. Depending on the application this is often the most important specification to be considered when selecting a hybrid inverter. This is especially critical when the hybrid inverter is being used as a back-up power source for dedicated or essential loads.
Tech Note: some inverters will specify the overload capacity and time period that the inverter can allocate extra current to the loads. Always double check this information on the datasheet of the inverter or ask the manufacturer to find out.
Loads that power electrical motors are inductive loads. These are found in a variety of household items and devices with moving parts, including TV’s, stereos, washing machines, air conditioners, refrigerators, water pumps, kitchen ventilators, loads with motors, transformers, relays, compressors and so on. In contrast to resistive loads, in a inductive load, current follows a sinusoidal pattern that peaks after the voltage sine wave peaks, so the maximum, minimum and zero points are out of phase.
The surge or peak power output is a very important consideration for off-grid systems but not always so critical for hybrid systems. If you plan on powering high surge appliances such as water pumps, compressors, washing machines and power tools the inverter must be able to handle these high inductive surge loads. Oversizing the inverter for the inductive loads is an important design aspect for this reason.
Inrush current, or starting current, is the electrical current flowing through the parts of a motor after the motor’s power turns on. During this time, current rapidly changes before any of the parts of the motor actually begin to move. Once a dynamic electrical equilibrium is achieved, at a steady-state current is achieved. It is important to check and calculate the starting currents of these different loads prior to purchasing any hybrid or off grid inverter. Many times if a hybrid system is connected to the grid, the grid will pick up this added inrush current if the inverter can not, however in off-grid mode, the inverter and battery system will need to take the full force of these currents thus indicating the importance of a properly sized inverter.
The repeated short spikes of current with amperage values many times that of the steady state represent a potential disruption to the system, as they can result in the unnecessary tripping of fault devices in a «false alarm» manner.
Loads consisting of any heating element are classified as resistive loads. These include incandescent electric cookers, light bulbs, electric furnaces, electric soldering iron, toaster ovens, space heaters and coffee makers. A load that draws current in a sinusoidal waxing-and-waning pattern in concert with a sinusoidal variation in voltage – that is, the maximum, minimum and zero points of the voltage and current values over time line up – is a purely resistive one and includes no other elements.
In a capacitive load, current and voltage are out of phase as with an inductive load. The difference is that in the case of a capacitive load, the current reaches its maximum value before the voltage does. The current waveform leads the voltage waveform, but in an inductive load, the current waveform lags it.
In engineering, capacitive loads do not exist in a stand-alone format. No devices are classified as capacitive in the way lightbulbs are categorized as resistive, and air conditioners are labeled inductive. Capacitors in large circuits are useful, however, in controlling power use. They are often included at electrical substations to improve the overall «power factor» of the system. Inductive loads increase the cost of a given power system and reduce the amount of power that is converted to another form of energy. Capacitors are installed to offset this drain.
Hybrid and off grid inverter ‘sizing’ is one of the key design aspects for any successful solar plus battery system. There are a number of different tools available to check the size of the inverter you might need as well as the size of the battery. Remember, inverters are sized in kW’s and batteries are often sized in kWh’s.
Symtech Solar offers a basic off grid load calculator which helps in making sure your inverter is sized properly for your loads.
Make a detailed list of each of the loads you plan to have connected to the system and categorize them based on the load types as specified above, ‘resistive’ or ‘inductive’. After doing so you will want to check the devices label for more information on the constant power draw and spike power draw.