For a less technical introduction see the guide to sizing a basic home grid connect and off-grid solar battery system.
There are many specifications to consider when selecting an off-grid or solar hybrid inverter or complete hybrid system with built in battery storage, so to make it easier we have explained the key features below:
1. Battery/Hybrid Inverter Power Output
There are two main types of modern hybrid or multi-mode inverters available.
- Dedicated off-grid battery inverter/chargers with heavy duty transformers provide very high surge and peak power output, explained in detail below.
- Hybrid all-in-one systems which use lightweight transformer-less inverters using 'switching transistors' are cheaper and easier to manufacture, this makes them much more cost effective for standard grid-tie systems.
Continuous Power Output
Most battery inverters (hybrid or Inverter/charger) are available in a wide range of sizes determined by the continuous output power rating measured in kW. Note: some inverters power ratings are provided in kVA which can be misleading. The general conversion ratio used for kVA to kW is 0.8*kVA = kW. For example a 5kVA inverter equates to 4kW inverter power rating.
The inverter should be matched (sized) to the loads or power demand of the appliances it will be powering. Depending on the application this is often the most important specification to be considered when selecting a hybrid inverter especially when using a hybrid inverter as a back-up power source for dedicated or essential loads.
For off-grid installations the inverter sizing is critical and must be sized to meet the full load (demand) under all conditions. It is important to note that the inverter output is derated (reduced) at higher ambient temperatures, for example a 5kW inverter which is rated and 20degC may only output a continuous power of 4kW at 40degC. This de-rating factor should be taken into account especially in warmer climates.
Surge or Peak Power Output
The surge or peak power output is very important for off-grid systems but not always critical for a hybrid system. However if you plan on powering high surge appliances such as water pumps, compressors, washing machines and power tools the inverter should be able to handle the high surge loads.
The amount of time the inverter can maintain the surge power output is also very important, but can be misleading depending on how it is described by the manufacturer. For example some inverters may specify the surge output of say 8kW while others may specify 8kW for 60 seconds. Generally the high-end multi-mode or interactive inverters have the highest surge ratings for the longest amount of time. The Selectronic SP PRO is known to have the highest surge rating of any battery inverter/charger on the market.
Backup Power Output - Continuous
As highlighted in the chart above many all-in-one hybrid inverters have reduced or limited backup power when operating in backup or emergency supply mode. This can also be further limited by the battery capacity output rating depending on the battery size used. However there are several all-in-one hybrid inverters (Solax, Redback & SolarEdge) do not have reduced power output in backup mode. The dedicated battery inverter/chargers (interactive inverters) such as the Selectronic SP PRO and Victron Multiplus do not have any such limitations.
2. Solar Array Size - Solar PV Input
After sizing the PV array based on the energy consumption profile, location, losses, etc (as calculated by a solar professional) the next step is to determine the maximum solar array size in kW based on the specific hybrid or off-grid system used, which is usually limited by the inverter size. For off-grid systems the battery capacity (kWh) must also be considered when sizing the solar array.
Most hybrid systems have an intergrated solar inverter or MPPT. If the hybrid system contains a solar inverter, such as an all-in-one hybrid inverter, this will determine the maximum size solar array which can be used with the system (usually around 6-7kW). In comparison the high-end interactive battery inverter/chargers such as the SP PRO and Victron Multiplus can work with multiple solar inverters or DC regulators in both AC or DC coupled configurations. These systems can accommodate much larger solar arrays, which can also be expanded at a later stage if required.
3. Pass Through Power
This enables the inverter to supply additional power from the grid under high loads, when the batteries are low and when solar energy is not available. The ability to pass through additional power from the grid (or generator in an off-grid system) can greatly simplify the installation by not requiring separation of essential and non-essential loads. Note: Generally only high-end hybrid/off-grid interactive inverter/chargers can pass through additional power from the grid or be connected to a back-up generator.
4. Compatible Battery Type
Before the recent release of affordable lithium battery systems most inverters where designed to operate with the widely available lead-acid batteries (Gel, AGM & flooded). Lead-acid batteries are far more common but are larger, heavier and can emit gases which require ventilation, whereas lithium-ion batteries are lighter, more compact and are considered safe to store inside a garage. Most lithium battery systems have an integrated battery management system (BMS) which requires an inverter with compatible communications (network protocol) to operate safely and efficiently.
All hybrid/off-grid inverters are designed to be used with a specific nominal DC battery voltage, the most common being 48V. Since most lithium battery systems are 48V this is not a problem, however many small capacity inverters use 12V or 24V so may only be compatible with lead-acid battery banks of the same voltage. Selectronic, SMA and Schneider have a range of high-end 48V hybrid/off-grid inverters while Victron Energy and Outback Power supply both dedicated 12V, 24V & 48V off-grid inverters.
The first Tesla Powerwall was one of the first battery systems to operate at a high voltage (400V) and is connected in-line with the solar array which generally operates at a similar voltage (300-500V). The SolarEdge StorEdge and Fronius Symo hybrid (3-phase) inverters both work with the high voltage battery systems.
Note: Unlike the traditional DC coupled solar controllers or regulators, all modern hybrid inverters cannot work with multiple battery voltages.
Battery capacity measured in kWh is the total amount of energy a battery system can store, however depending on the battery type and specifications not all of the capacity is usable. Common Lead-acid deep-cycle batteries (AGM & Gel) can only be used or discharged to 40-50% of total capacity, whereas Lithium-ion and new generation battery technologies can be discharged to 80-100%. Therefore the battery type and capacity needs to be carefully selected to cater to the energy requirements.
Hybrid Vs Off-grid - A typical grid-connected home with peak (evening) energy use from 5pm until midnight might average 8 kWh, roughly a 12 kWh lithium battery would be sufficient. However for off-grid systems the battery system will need to be able to store enough energy for 3-4 days of bad weather. With an average (inefficient) 3 bedroom home using 15-20kWh over a whole day this would require a much larger, very expensive battery system of 80-100kWh.
Hybrid Example: If peak energy use (from 6-12pm) was 6kWh then you would require roughly a 14kWh lead-acid battery or 7-8kWh lithium system to cover the peak consumption.
5. configuration - AC or DC coupled
As solar battery systems became larger and more advanced AC coupled systems evolved as one of the best configurations due to the use of low cost, easy to install string solar inverters. Most modern off-grid AC coupled systems use advanced bi-directional multi-mode inverters coupled with one or more compatible solar inverters. AC coupled systems are generally more efficient during the day when there is high AC power demand such as air-conditioning systems, modern kitchen appliances and pool pumps.
However the new generation high voltage DC coupled battery systems (400V) are becoming more and more popular with the growing range of advanced HV hybrid inverters now on the market.
See the complete AC vs DC coupled system review article.
6. Software and Energy Management
To enable hybrid or off-grid power systems to optimise energy use and prolong battery life a high level of power management and battery monitoring is required. The software used to run hybrid systems thus require advanced energy management and monitoring capabilities and this is where the high-end Interactive inverters really shine as they have the most advanced software packages and built-in control systems such relay and digital inputs and outputs. These systems also incorporate specialised battery monitoring and temperature sensors to prolong battery life when used with lead-acid battery banks.
Several advanced hybrid inverters such as those from SolarEdge and Redback Technologies also include smart control features. For additional monitoring and control add on energy monitoring systems like Reposit Power can provide more advanced remote monitoring and smart control features.
Complete hybrid systems with built-in battery storage also utilise advanced energy management systems and sensors however some of the cheaper all-in-one hybrid inverters have limited capabilities which can result in less efficient use of stored energy.
See our Hybrid/off-grid inverter and energy storage summary for direct comparison of all available hybrid and energy storage systems: