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Hybrid/off-grid battery storage selection guide

Jason Svarc

This is a technical guide to selecting a hybrid or solar battery system.

Also see the basic guide to sizing the various home solar/battery systems here.

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:

Battery Inverter Power - continuous output

Most battery inverters (hybrid or Interactive) are available in a number of different sizes determined by the continuous output power rating measured in kW. 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 derating factor should be taken into account especially in warmer climates.

Battery Inverter Power - Surge 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 30 seconds. Generally the high-end multi-mode or interactive inverters have the highest surge ratings.

Solar Array Size (Solar input)

The maximum solar array size in kW of hybid or interactive systems is usually determined by the size of the inverter used. For off-grid systems the battery size (kWh) should also be considered when sizing the solar array.

Most hybrid systems have an in-built solar inverter or MPPT regulator. If the hybrid system contains a solar regulator (such as a hybrid system with built-in battery storage or all-in-one hybrid inverter) this will determine the maximum size solar array which can be used with the system (usually around 6kW). The high-end interactive or multi-mode inverters can work with multiple solar inverters or DC regulators and therefore can accommodate much larger solar arrays, which can also be expanded at a later stage if required.

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 interactive inverters can pass through additional power from the grid or be connected to a back-up generator.

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 (much like a LAN) to operate safely and efficiently.

Battery Voltage

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. The Tesla Powerwall was one of the first battery systems to operate at a high voltage (HV) and is connected in-line with the solar array which generally operates at a similar voltage (300-500V). Note: Unlike the traditional solar DC Regulators, hybrid inverters cannot work with multiple battery voltages.

Battery capacity

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 90-100%. Therefore the battery type and capacity needs to be carefully selected to cater to the energy requirements. In the case of a typical home battery system this is the peak (evening) energy use from 4-5pm until midnight (average 5-12kWh). However for off-grid systems the battery system will need to be able to store enough energy for 3-5 days of bad weather. With an average (inefficient) 3 bedroom home using from 15-20kWh a day this would require a battery system of 80-120kWh.

Hybrid Example: If peak energy use (from 6-12pm) is 6kWh then you would require a 12-14kWh lead-acid or 7-8kWh lithium system to cover peak use.

Software and Energy management

To enable a hybrid system to correctly 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 requires advanced energy management and monitoring capabilities. Interactive inverters typically have the most advanced software packages. These systems also incorporate specialised battery monitoring and temperature sensors to prolong battery life (in reference to lead-acid batteries). Complete systems with built-in battery storage also utilise advanced energy management systems and sensors. However some of the all-in-one hybrid inverters have limited capabilities which can result in reduced battery life and 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: