Inverter sizing and selection
The inverter is sized and selected almost completely based upon the power demands of the user outlined in the AC load evaluation. The size of the PV source does not affect the size of the inverter and the energy storage system is only a concern if its capacity is significantly undersized relative to the size of the inverter as all types of batteries have a maximum amount of discharge current that they can safely supply.
The basic considerations for sizing and selecting an inverter are the following:
- The voltage must match the system voltage.
- The inverter should be able to meet the continuous power demand for all loads that will operate at the same time.
- The inverter should be able to meet the continuous power demand for all loads and the surge power demand for all loads that will operate at the same time.
This process is intended to serve as a guide, but the best option for a particular project will largely depend upon the available inverter sizes, the cost and the particular project. There are a few common cases in which one may choose an different inverter size than is suggested here:
- It make sense to buy an inverter that is smaller than is suggested by this process to keep costs low as the next size of inverter is significantly more expensive or would require the addition of an additional inverter that is stacked or added in parallel. In this case the users that they will have to manage their loads to avoid overloading the inverter. This makes the most sense with large infrequent loads that the users turn on and off. This approach is not recommended for projects that have large loads that may operate automatically - like a well pump or a refrigerator - as they loads are difficult to manage.
- It may make sense to buy an inverter that is larger than is suggested by this process to enable system expansion in the future or new additional loads.
Step 1: Determine the inverter continuous output rating
All inverters have a continuous output (duty) rating that can be found in Volt-Amperes (VA) or Watts (W) on their specifications sheet, which is the capacity that the inverter has to continuously supply loads. VA = Volts × Amperes, just like Power (P) in Watts, but is used to account for power factor. Regardless of the value used by the manufacturer (VA or W), the continuous duty rating of the inverter should exceed the total power requirement for all loads including power factor from the AC load evaluation.
| Minimum inverter continuous duty rating | = Total VA |
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Step 2: Determine minimum inverter surge rating
Inverters also have additional capacity, called a surge rating or peak power rating, that can be used for short periods of time to supply surge loads. This additional capacity may be found in Volt-Amperes (VA) or Watts (W) and will often have a time interval (1 second, 5 seconds, 5 minutes, 30 minutes). Regardless of the value used by the manufacturer (VA or W), the 1 second surge rating or peak power rating of the inverter should exceed the total power requirement and surge requirement for all loads from the AC load evaluation. The 1 second rating is the most relevant as the majority of the surges created by loads used with an off-grid system will have surges that are short in duration. The 30 minute value can be of interest if the size of the inverter is very close to the Total VA rating from Step 1.
| Minimum inverter surge rating | = Total VA with surge Watts |
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Step 3: Verify inverter size relative to energy storage system size
For standard system designed using the process outlined by OSSP, the energy storage should be adequate to supply the amount of current that the user may require. If a system were to be designed for use primarily with large loads that are used for short periods of time, it should be confirmed that the system is adequate to the task. An energy storage system can only safely supply a certain amount of its capacity for a period of time before being damaged. The size of the energy storage system will have to be increased if it is not adequate. Various inverter sizes can be checked using the calculation and recommended current values below:
| Inverter maximum current draw | = [[Total VA with surge Watts ÷ low voltage disconnect parameter ÷ Total VA with surge Watts |
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Recommended maximum discharge current values:
| Load | Recommendation |
|---|---|
| Average discharge load (5%) | = .05 × Final Ah capacity |
| Heavy discharge load (10%) | = .10 × Final Ah capacity |
| Maximum continuous load (13%) | = .13 × Final Ah capacity |
| Maximum surge load - several minutes (20%) | = .20 × Final Ah capacity |
| Maximum surge load - less than a minute (40%) | = .40 × Final Ah capacity |
