The battery-based inverter buyer’s guide has a comprehensive list of inverters, and discusses how each specification affects system design. Here we detail the basic inverter sizing steps to select an inverter that has the capacity required to power to your AC loads.
The primary inverter specifications to consider for sizing are the inverter’s continuous output rating and its surge capability. Inverter sizing is also affected by whether the application is off-grid or grid-tied with battery backup.
For off-grid sizing, we simply sum the wattage of all AC appliances that might run simultaneously (the base load). Then we consider any appliances that might surge and add the cumulative surge watts to our base load to estimate the surge capacity requirement. Here is a simple example scenario. (You can either measure these individual appliances’ power with a watt-meter, use their nameplate ratings, or contact the manufacturers.)
In this case, a 2,500 W inverter capable of surging to 50 A or greater at 120 VAC is needed (6,095 W ÷ 120 V = 50.8 A). It is unlikely that all loads will be on and surging at the same time, so some folks may be less conservative in their surge estimates, especially if they can easily provide some load management—for example, not running the washing machine while using the microwave. With a household of more than one, this gets harder to control.
We need to consider the inverter’s two operating modes: when the grid is connected, and when there is a utility outage. When the grid is available, these inverters will be limited to processing the PV array’s power, so the inverter needs to be large enough to pass the available array wattage.
Let’s say we have a 4.5 kW PV array. This array rating is at standard test conditions (STC), and actual output will likely be lower due to higher module temperatures and system inefficiencies. If that full power is available (say during cool, clear weather), it is desirable to have an inverter large enough to process all of it. Otherwise, a portion of the available power will be lost. We’d need an inverter with a minimum continuous output rating of 4,500 W.
When the grid is down, these battery-based grid-tied inverters will switch into off-grid mode to power only a critical load subpanel—which does not normally include all of a home’s loads. Let’s assume that the critical load subpanel has the same load profile as our off-grid system. An inverter with a 4,500 W continuous output rating will meet our critical base load requirement of 2,495 W. As long as the 4,500-watt inverter has a surge rating of about 50 A or greater, we are covered.