There are 8 tips to consider when selecting the proper off grid solar inverter, let us see as following.
1. The off grid solar inverter output voltage and frequency. This will base on your load requirements and usually its same as your country standard supply voltage/frequency.
2. Installing off grid solar inverter or hybrid inverter. Most of off grid solar inverters and hybrid solar inverters have AC backup supply connection, so the inverter bypass to use it for feeding your loads in case of sun absence and batteries discharged. Moreover, you can use this AC backup supply to charge the batteries through internal solar inverter batteries charger.
Suppose you have surplus DC energy generated form the solar array and you have already charged batteries to full and the load is working smoothly based on solar array output power, then the surplus AC energy will feedback to the AC backup supply in case of hybrid solar inverter (2 ways/bidirectional), but in case of off grid solar inverter, the surplus energy cannot return back to the AC backup supply (one directional). So, we can say the hybrid solar inverter can operate as off grid solar inverter but Not vice versa. In this step, you need to take a decision whether you will use solar off grid inverter or hybrid inverter.
3. The off grid solar inverter output power. Just add power consumption for all your loads together, and take care of the inductive loads like motors which have high inrush/surge current at starting period, make good oversizing to avoid under estimation and at the end, the off grid solar inverter power capacity should be 110-120% of your loads total power capacity. Like if your loads are 5kW, then off grid solar inverter power capacity should be 6000 watt and so on.
4. The off grid solar inverter input DC voltage. Once we have selected solar inverter power capacity and brand/manufacturer, there will be corresponding DC input voltage range in inverter specs sheet, which we need to select batteries voltage to match with.
5. The inverter built in solar charge controller will be MPPT or PWM. MPPT is technically better as its able to convert solar panels high voltage to lesser voltage, so it charges batteries with low losses (high efficiency), but it costs higher than PWM type. In the other side, if we can size PWM solar charge controller type properly according to solar panels specs, then we can confirm it will work as well as MPPT charge controller type.
6. Batteries charging requirements. These requirements are maximum batteries charge voltage and cut off voltages, and those 2 values should match solar of grid inverter program range.
7. Solar array max output voltage and MPPT trackers. The PV max DC output voltage must not exceed the inverter max DC input voltage listed in its specs, and MPPT trackers refer to how many inputs the inverter accepts, it will be indicated based on how large is your solar array, and it will ensure better maximum power point tracking performance.
8. Backup power supply (Generator) starter. They are just 2 wire dry contact to start the generator when the inverter was working on DC priority mode and experienced low batteries voltage, so it sends signal to start the backup supply (generator).