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Solar Toolbox - Inverters

An inverter is simply an electronic device used to convert DC electricity into AC, usually with an increase in voltage. The inverters used in renewable energy systems fall into two distinct categories:

  1. Battery based: Inverters that convert the DC electricity from batteries to operate AC equipment such as those that are plugged in to most house hold electrical outlets, or
  2. Direct Grid Tie: Inverters that convert the DC electricity from solar modules or wind turbines to feed directly into the utility AC electrical grid.

Inverter selection is first based on your application as it relates to the above two categories, then breaks down into sub categories, particularly with battery based inverters. Click each of the two categories above, or scroll down the page to continue.

The following chart can be used to help with inverter selection:

Inverter Selection

Battery Based Inverters:

Battery based inverters convert the DC electricity from a battery bank to operate AC equipment. Because of this, inverter selection is based initially on ensuring it will properly power all the connected AC loads that you intend to operate, but there are many other criteria that also need to be addressed.

  1. Define the size of the AC load that the inverter will operate, then
  2. Decide which waveform is appropriate for you intended loads. Modified sine wave inverters are primarily used in applications where power quality is not a concern. So called "pure" sine wave inverters are used everywhere utility grade power is required, and have become the norm for most continuous usage applications.
  3. Determine your DC system voltage. Battery based inverters operate at anywhere from 12, 24 or 48 VDC. Voltage selection is usually defined by the size of the inverter (most larger inverters operate at 24 or 48 VDC), or the size and/or type of DC input (Many wind turbines, solar modules and solar regulators operate at 12, 24 or 48 VDC and need to be matched with the same voltage inverter).

Secondary considerations then focus on issues such as:

  • Will you need a transfer switch? Some mobile class inverters have the ability to transfer loads to shore power when available. These inverters do not charge batteries.
  • Will you need an inverter/charger? The chargers built in some inverters are used to recharge the battery bank from an AC power source, not to regulate solar or wind input. All inverter/chargers have built-in transfer switches. If so, how "smart" do you need this inverter to be? Inverter/chargers can be used for:
    • Battery Back-up: The simplest form of inverter/charger can be used to transfer power to critical loads during a utility power outage. These inverters will transfer all load responsibilities to the utility or generator when voltage is present at the AC input of the inverter.
    • Utility or Grid Interactive: These inverters are capable of selecting when to use inverter or battery power based on available battery voltage and user defined presets such as time of day usage or low battery cut out.
    • Utility or Grid Intertie: These are the most sopisticated inverters as they are capable of pushing energy out beyond the dedicated load panel and into the main grid power stream. This energy is then used by non-dedicated circuits in the home, or pushed right out beyond the home or business. These inverter/chargers are used in net metering arrangements with local distribution companies.


Defining Battery Based Inverter Size:

As previously stated, battery based inverters are sized on the load they are intended to carry. Some experts suggest that the only real way to do this is to calculate the running load in watts of every connected load that the inverter will carry. We feel that this often results in a severly oversized inverter that will run very inefficiently. Most inverters run efficiently at or above 70% capacity. While we don't want to see you run out of inverter capacity, there should still be a happy medium. We use the following formula to find that medium.

Make a list of all the appliances you intend to run from the power inverter, and write down the wattage of each appliance.  Highlight all the appliances that you may need to have running at the same time, and add up their wattages to determine the maximum combined wattage that your inverter will need to support. Add 20% to get the continuous rating required. 
Continuous rating = maximum combined wattage + 20%.

If your inverter will be used to run large appliances with motors, you will need to calculate a surge rating or a startup load. This allows for surges that occur during appliance start ups. Usually motor starts are 2x the wattage of the appliance and 3x if it is a large induction motor.
Start up load = (wattage of largest appliance x at least 2) and (wattage of largest induction motor x 3).

Compare the surge rating of your appliances with the surge capacity of the inverter, and make sure your inverter is oversized for surge capacity by at least 50% of this capacity.

Visit our "calculators" tech help page for worksheets.

Direct Grid Tie Inverters

Direct Grid Tie Inverters convert the DC electricity from solar modules or wind turbines to feed directly into the utility AC electrical grid. Inverter selection is based on the size of the solar or wind electrical input that is feeding them. They do not maintain any loads themselves, and these systems can not generate electricity without utility voltage present. This is possible in areas that allow:

    • Net metering: Whereby the solar system stores excess solar generated electricity with the utility in the form of energy credits when it is producing more power than you are using.
    • Feed in Tarrifs: Whereby the solar system electrical power generation is separately metered from your electrical load and you are paid for the solar energy recorded by the meter. In Ontario, this program is called the "Standard Offer Program, or SOP".

Design and Sizing Issues:

  1. Net Metering: Sizing battery-less grid tie systems against electrical load demand is done using your average monthly kWh usage on your utility bill and your average sun hours per day (yearly). Knowing these two values allows you to define your solar system size in a net metered environment to net zero your utility bill over a one year period. If we estimated an average home energy usage at 1000 kWh per month, then the calculation for this would be:
  2. (1000 kWh monthly usage / 30 days) / 4.2 hrs* = 7.9

    The above solution identifies that a 7.9 kW nameplate rated solar array** would be suitable for completely offsetting 1000 kWh per month average electrical usage under a standard net metering agreement.

  3. Feed in Tarrifs: Sizing battery-less grid tie systems against electrical load demand is done similarly to a net metering application, with one important distinction: The feed in tarrif must be weighed into the calculation of solar system size. The easiest way to do this is to divide your average monthly electrical bill in dollars by the feed in tarrif price. For instance, if we use the Ontario Standard Offer Contract solar price of 42 cents per kWh we discover that:

($ 150.00 per month bill) / ($ 0.42) = 357

The above solution identifies that a solar system producing 357 kWh per month (on average) would provide an income of $ 150.00 per month (on average). We can now take this number and the average sun hours per day (yearly, as discovered in the solar resources section here) to define your solar system size. For example:

(357 kWh per month / 30 days) / 4.2 hrs*= 2.83

The above solution identifies that a 2.83 kW nameplate rated solar array** will provide an average of 357 kWh per month in electricity. This energy would then be sold to the utility to offset your electrical bill of $150 per month.

* 4.2 hrs based on values obtained from Photovoltaic (PV) potential mean daily global insolation of a south-facing, tilt = latitude fixed array in London Ontario Canada

**The nameplate rating does not take into consideration any environmental or wiring losses.

Inverter Sizing/Selection:

Once you have defined your nameplate rated solar array (the amount of solar needed in watts), then you can begin inverter selection. Grid tied inverters are sized based on the size of the solar array, but module selection and string sizing is critical to the proper functioning of the inverter.

Key Issues:

  1. DC voltage: This is the most critical number in the equation. Inverter manufacturers will list the DC input voltage window which must be adhered to in order for the inverter to run its MPPT tracking, and to prevent equipment failure. Module manufacturers will list their module voltage at maximum power, and at open circuit but these values are at standard test conditions. Module manufacturers will then typically list the voltage co-efficient which describes the module characteristics under non-standard conditions. In Canada, this is a particularly tricky number to assign. You need to pick a system voltage which will allow the inverter to properly run in the heat of the summer when module voltage is at its lowest. You also need to pick a system voltage which will not allow the voltage to rise above the high limit of the inverter in the cold winter when module voltage is at it's peak.
  2. DC Amperage: Once you have the DC voltage (above) that your array will operate at, the listed maximum input DC amperage of the inverter can be used to identify the number of solar strings that can be operated by the inverter. Running a direct grid tie inverter at less than the maximum allowable DC amperage is not a problem, but you can never allow more than the maximum.

Direct grid tie inverter manufacturers have taken great steps to help you define your inverter and solar configuration based on the two above issues. Most manufacturers have on-line calculators which can be used to build your system.


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