Inverter Load Calculator for Backup Circuits

An inverter will allow your appliances to continue to recieve power in the event that the electrical grid fails. In order to ensure that the inverter can provide power to your appliances, however, it is necessary to size the inverter correctly. Many individuals attempt to calculate the wattage that an inverter will need to draw from the appliances by simply adding the numbers that are found on the appliances manufacturer stickers.

In most cases, however, the result of such an addition will not reflect the correct size that the inverter should be. Instead, you must calculate the power requirement of the appliances in order to determine the power that each of the appliances will draw. For instance, appliances may draw different amounts of power when they are performing one task compared to another; a refrigerator, for instance, will draw more power when its compressor starts up compared to when it is performing its typical running tasks.

How to Size Your Inverter

In order to correctly size the inverter for an electrical system, it is necessary for individuals to understand the concept of diversified running load. Diversified running load is the total amount of power that will be drawn by the appliances at any given moment. In most instances, an inverter does not have to provide power to each appliance at maximum power simultaneously; the power required by each appliance will vary over time.

As a result, you can calculate the diversified running load by determining the total wattage of each appliance and then multiplying the number by a factor between 0.7 and 0.85; the factor represents the fact that only 70 to 85% of the total power of the appliances will be drawn at any one time. Additionally, it is also often recommended to add 20% to this calculation to account for the potential addition of additional appliance to the system. In addition to calculating the diversified running load of the appliances, it is also necessary to ensure that the inverter is sized to handle the surge power that motors and compressors typically draw in many appliances.

Surge power is the amount of power that is required by these appliances at the time that they start up. As a result, each of these motors will draw between two and three times there running wattage at the time that they start up; if the inverter is not able to supply that much power to the appliance, the inverter will fail or shut down. As a result of the recommended use of a plug-in watt meter to measure the starting wattage of appliances like refrigerators or sump pumps; the surge power that an appliance draws will often be higher than the wattage rating that is displayed on the appliances manufacturer stickers.

Providing for the surge power of the appliances will prevent the inverter from shutting down. Beyond calculating the power that will be drawn by the appliances, another of the factors that must be considered is the battery bus voltage. Battery bus voltage is a mathematical value that can be used to determine the amount of current that will flow through the system.

For instance, if an individual is using a 12-volt battery system to supply power to appliances that require 3,000 watts of power, the 12 volt battery system will need to supply 250 amps of DC current. The higher amount of DC current that a 12 volt system requires, however, requires the use of very thick cables and fuses; high DC current can also result in voltage sag within the system. In contrast, however, if an individual used a 48 volt battery system instead, the 48 volt battery system will only need to supply 60 amps of DC current.

48 volt systems, therefore, allow for the use of thinner wires, which is less likely to overheat during the operation of the system. In determining the time that the inverter will run before the batteries become empty of the stored electricity, the runtime of the system, another factor must be considered. Runtime is related to the capacity of the batteries that will be used in the inverter and the number of appliances that are to be supplied with power.

One type of battery that can be utilized in an inverter system are LiFePO4 batteries. LiFePO4 batteries can be discharged to 90% of their power capacity without damaging the battery. AGM batteries, in contrast, should only be discharged to 50% of their capacity otherwise their life will be significantly reduced.

Additionally, the idle draw of the inverter must be accounted for; inverting systems will typically draw between 30 and 40 watts of power even when the inverter isnt supplying power to any appliance. Beyond each of these factors that relate to the sizing of the inverter, additional factors must also be considered. For instance, the environment in which the inverter will be used can impact the performance of the inverter.

For instance, if the inverter will be located within a garage or similar location that is often hot, the temperatures may reduce the performance of the inverter by 5 to 10% of its rated output. Additionally, cold temperatures can impact the performance of the batteries that are used in the inverter. Additionally, the inverter will need to be properly ventilated to allow for the operation of its cooling fans; battery cables that are too thin or too long may lead to a loss of voltage.

By taking each of these factors into consideration and ensuring that each is properly accounted for in the power calculations that are made prior to installation of the inverter system, youll be ensured of the ability of the appliances to function properly during the power outages.