Best off-grid solar system
Jason Svarc
Choosing the best off-grid solar system is not easy as off-grid systems are many times more complicated than grid connected systems. In this article we will highlight some of the many considerations which must be taken into account and discuss the various off-grid system types available. We also explain why a good quality ‘inverter/charger’ is vital to building a reliable off-grid system. Finally we will dive into the different brands available and determine which systems are best suited to different applications.
Many people believe an off-grid system can be easily put together (which may be true in the case of a small caravan or cabin) but in reality these systems need to be carefully designed by an experienced solar installer or SAPS system designer.
Solar and battery storage systems must be installed by an experienced licensed electrical professional. Modern off-grid solar systems are typically high voltage and can generate and store huge amounts of energy which can result in damage, fire or serious injury if the installation does not meet all relevant regulations, standards & guidelines.
The 5 main parts of an off-grid system
Battery Inverter/charger
Solar inverter (AC) or Solar charge controllers (DC)
Battery bank
Solar panels
Backup Generator
Basic layout of a AC coupled off-grid system - Well suited for modern off-grid homes and businesses
Off-grid configuration types - AC or DC coupled
In addition to AC generation sources, DC coupled solar can also be used to charge battery systems using solar charge controllers. The system configuration type such as AC or DC coupled is based on the inverter/charger and it’s compatibility with different solar inverters (AC) or solar charge controllers (DC). Many modern multi-mode inverters can be both AC and DC coupled which creates a very secure flexible system.
Key considerations for off-grid solar systems
Daily average energy consumption (kWh) - Summer and winter
Peak load (kW) - Maximum power drawn from appliances
Average continuous load (kW)
Solar exposure - Location, climate, orientation, shading
Backup power options - During poor weather or shutdown
With the above considerations in mind the key component of an off-grid power system is the main battery inverter/charger often referred to as a multi-mode inverter as they can usually operate as either an off-grid or grid-tie system.
The inverter/charger is both the heart and brains of a stand-alone power system and manages not only the battery, but energy flow from all other sources such as solar, wind, micro-hydro or a backup generator.
The battery inverter/charger’s job is to supply a steady ‘pure sine wave’ or clean AC power and must be able to meet the load requirements of the appliances under all conditions. In addition it must also be able to charge the batteries from sources such as solar, wind or a generator. Many appliances such as water pumps, fridge compressors and air-conditioning units require very high surge (peak) power during startup and the inverter must be able to to cope with this surge. This is where many cheaper inverters can fail. If multiple appliances are running at the same time then the inverter must also be able to supply continuous power under all conditions including higher temperatures, so must be sized correctly to account for temperature derating. Again this is where cheaper (transformerless) inverters can often trip, especially during high summer temperatures.
Technical Guide
Recommended for solar Professionals - Technical guide to selecting and sizing off-grid and energy storage systems.
A solar professional should be able to put together what is known as a load table to help determine which type and size inverter is best suited to your individual needs. The load table is also used to size the solar array, battery and backup generator.
Best Off-grid Battery options
Lead-acid
Until recently traditional lead-acid deep cycle battery systems were the most common and reliable option for off-grid systems. Lead-acid batteries are a proven technology and can last up to 15 or more years if they are not held at elevated temperatures (above 40 degC) and are not regularly discharged too low. Lead-acid batteries require precise battery charging following a specific charge cycle and temperature sensors to adjust voltage settings so most well known off-grid inverters offer programmable charge voltage settings along with sensors to precisely charge under all conditions. Lead-acid deep cycle batteries are still widely used and offer several advantages over lithium including:
Lead-acid deep cycle battery options - 2V, 6V or 12V options and various types - Gel, AGM, Flooded
Advantages
Compatibility with most inverters.
Proven and trusted technology.
Safe, very low risk (sealed Gel).
Long life (if not over discharged)
Easily recycled.
Disadvantages
Lower efficiency - around 80%
Low energy density - Very heavy
Usable capacity limited - Max 50% on regular basis
Not modular - Fixed size once installed
Cannot sustain partial state of charge
High temperatures can drastically reduce battery life
Lithium-Ion
More recently lithium battery systems have become extremely popular due to the high efficiency (95 to 98%), compact size, lightweight and scalability. In contrast lead-acid battery banks have a fixed size or capacity whereas lithium modular systems do not suffer this limitation. This flexible sizing allows for additional capacity to be added at a later stage, which is a real bonus for both installers and customers alike. Lithium batteries have a much higher energy density compared to lead-acid and thus are lighter and more compact. A huge advantage of lithium is the ability to sustain a low state of charge (partial state of charge) for a prolonged amount of time without any negative effects such as sulfation which is a common problem with lead-acid batteries.
Advantages
Very high efficiency - Approx 97%
Very high energy density - Light weight and compact
High charge and discharge rates allowed
No degradation with partial state of charge
Modular and scalable systems
Safe and low risk (if charged correctly)
10 year warranty
Disadvantages
Can shutdown under high temperatures (45+ degC)
Can trip off under continuous high loads.
Can be difficult to recycle at end of life.
May not function without a compatible inverter.
Lithium Battery Types
Managed lithium - Many lithium battery systems contain a BMS (battery management system) which requires a dedicated communication link to the inverter in order to operate. If the inverter isn’t compatible with the battery then they cannot function together and will not work. A issue with this type of system is that if the communications is disconnect or suffers interference then often the inverter or battery will shutdown to ensure the battery is not damaged.
The LG chem RESU 48V batteries are compatible with almost all the leading off-grid inverter brands. The list now includes Selectronic, Schneider Electric and Victron Energy. The BYD B-Box Pro is also compatible with a growing range of hybrid and off-grid inverters including Victron, SMA and Selectronic.
Modular lithium LFP battery system from PowerPlus Energy.
Non-managed Lithium - There are also ‘non-managed’ lithium battery options available which do not require a fixed communication in order to operate such as Simpliphi PHI and LiFe from Powerplus Energy. These batteries are self-managed with an internal BMS to monitor state of charge, temperature and cell voltages. The big benefit with this type of battery is that they can be used to replace existing lead-acid batteries and do not require any special communications or connections.
What’s best for off-grid applications?
In my experience non-managed lithium battery options are a more reliable choice for off-grid systems as communications used by managed batteries can be problematic, trip off or become unstable especially at low state of charge. They are also generally more modular and capable of creating much larger capacity battery systems.
See out detailed solar battery review including lead-acid batteries Vs Lithium from LG and Simpliphi
The heart of an off-grid system - The Inverter/charger
Modern off-grid solar systems use multi-mode inverters/chargers to manage batteries, solar, and back-up power sources such as a generator. The inverter/charger is the main energy management unit and be can be either AC coupled with solar inverter/s or DC coupled with solar charge controller/s, and in some cases both. Read more about AC and DC coupled systems here.
Early off-grid systems used basic battery inverters and were very simple in comparison as the battery charger was a separate unit. These simple inverters generally had low power rating below 2kW and thus were only capable of powering low energy appliances like lights, fans and small pumps.
The modern multi-mode inverter/chargers are much more powerful (up to 20kW) and can have enough capacity to run high consumption appliances like air-conditioners, large pumps, electric ovens and hot water systems which require high continuous or surge current. Naturally these powerful inverter/chargers are also much more expensive than common grid-connected solar or small capacity hybrid energy storage systems and can cost anywhere from $1500 to $6000* depending on the rated power output. (* 48V inverter/chargers up to 8kW)
Inverter output - Continuous and peak power
There are many inverters available with different output ratings from as low as 2.4kW up to very large 20kW continuous output. Most of these inverters can supply double or more of the continuous rating for a short amount of time to handle surge power spikes from motors, compressors, pumps etc. This surge rating is critical for off-grid systems to operate under all the various load conditions expected without tripping out or shutting down unexpectedly. This output power rating can also be reduced under higher ambient temperatures.
For more detailed information about selecting off-grid and hybrid inverters see our ‘Technical guide to sizing a hybrid inverters and off-grid power systems’.
best Off-grid Inverter/chargers
There are many off-grid inverter/chargers available for basic small-scale systems, however, to power a modern home or building requires an advanced multi-mode inverter/charger of which there are five well known and trusted options:
Selectronic - SP PRO series (Australia)
Victron Energy - Multiplus or Quattro (Netherlands)
SMA - Sunny Island series (Germany)
Schneider Electric - Conext SW or XW+ (Italy)
Outback Power - Radian or FXR series (USA)
These inverters are all available in a range of different power output ratings (kW) to suit different applications depending on the continuous and peak power requirements. In addition these inverter/chargers can also be both AC or DC coupled using compatible solar inverters or solar charge controllers.
Selectronic SP PRO
Best off-grid system 5kW+ power output
When comparing the leading multi-mode inverters specifications and power ratings under all conditions the SP PRO inverters from Selectronic outperform all others with both the highest 30 minute rating and highest peak/surge rating. However it is the advanced energy management software, 63A (15kW) pass through power capability and high charge ratings that put the SP PRO into a league of its own. Premium quality comes at a premium price and the SP PRO inverters are one of the most expensive options available, however considering the generally high cost of off-grid systems, paying a little more a is well worth it for the power and features needed for a reliable off-grid system. The impressive features together with a leading 7 year warranty place the SP PRO at the top of our list.
See the full Selectronic SP PRO review here
SP PRO basic specifications
Type: Multi-mode Inverter/charger (AC or DC coupled)
Use: Solar storage, back-up (UPS), off-grid
Sizes available (power output): 3.0kW, 4.5kW, 5.0kW, 7.5kW, 20kW
Pass through power: 15kW built-in (28kW external contactor)
Compatible Battery types: Lead-acid, lithium-ion, flow, sodium-ion
Battery System Voltages: 24V, 48V, 120V
Highlights
Very high pass through power capability
Very high surge power output
Battery temperature monitoring for longer battery life (lead-acid)
Battery sense monitoring and mid string sensor for very precise battery charging (lead-acid Gel and AGM)
Very powerful and user friendly software package with remote access monitoring.
Unique pre-charge feature for safer startup
Compatible with many popular lithium battery systems
Generator auto start, control and monitoring
AC / DC coupling capability
3-phase capability
Note:
Separate solar inverter or solar DC regulator required
Full off-grid capability when used with certified Fronius or ABB solar inverters
Additional generator contactor required if used with dual AC sources.
Price bracket – From AUD$4500 (4.5kW)
See detailed specification datasheet - Selectronic SP PRO series
48V off-grid system using Selectronic SP PRO inverter/charger coupled with a Fronius Primo solar inverter and a 26kWh LiFe lithium LFP battery system from PowerPlus Energy.
2. Victron MultiPlus
Best off-grid system 2 to 5kW power output
Victron Energy based in the Netherlands has grown to become an international high quality manufacture with a wide product range including battery chargers, sinewave inverters, inverters-chargers, DC/DC converters, transfer switches, battery monitors, charge controllers and more. Victron offer the most cost effective range of multi-mode inverters with a huge range of sizes including the new MultiPlus II range of inverters/chargers. For many applications the Victron is a great quality lower cost option especially for DC coupled systems.
See the full Victron Energy Review here
For smaller off-grid systems the most cost effective combination is Victron inverter/charger and DC coupled solar charge controllers with the BYD B-Box Pro lithium LFP battery system.
Basic specifications: (5 sizes available) plus other variations
Type: Inverter/charger (DC or AC coupled)
Use: Solar storage, back-up (UPS), off-grid
Solar Array Size (Solar input): N/A
Sizes available (power output): 0.7kW, 1.3kW, 1.6kW, 2.4kW, 4.0kW,
Pass through power: 3.6kW - 11.5kW
Compatible Battery types: Lead-acid or lithium-ion
Battery System Voltage: 12V, 24V, 48V
Features:
Economical Inverter/charger option
Med-High surge power output
Pass through power capability
Battery temperature monitoring for longer battery life
Generator auto start and monitoring available
Dual AC outputs for non-essential loads (high-power units only)
Programmable software with remote access and Wifi App
3-phase capability + parallel for higher output
Note:
Separate solar inverter or solar DC regulator required
Victron size the inverters in kVA not kW (ref data sheet)
Full capability only when used with Victron regulators and color control panel (CCGX) and Venus GX
Larger capacity Quattro inverters also available (up to 15kVA)
Price bracket – from AUD$1600 (2.4kW)
See detailed specification datasheet - Victron Multiplus inverter specs
24V DC coupled off-grid solar system with 2 x Victron Bluesolar charge controllers, 2.4kW solar array and Victron Phoenix 2.4kW battery inverter.
3. SMA Sunny Island
Best off-grid system for extreme environments
SMA Solar Technology is a leading German solar energy equipment supplier founded in 1981 and is one of the world's largest manufacturer of solar inverters. The well regarded Suny island inverters are a proven performer in harsh outdoor environments in particular sub zero temperatures.
Basic specifications: (3 sizes available)
Type: Inverter/charger (AC coupled)
Use: Solar storage, back-up (UPS), off-grid
Solar Array Size (Solar input): n/a
Sizes available (power output): 3.3kW, 4.6kW, 6.0kW
Pass through power: 11.5kW
Compatible Battery types: Lead-acid, lithium-ion (refer to manufacturer)
Battery System Voltage: 48V
Features:
High pass through power capability
High surge power output
Battery temperature monitoring for longer battery life
Powerful software package with remote monitoring
Generator auto start and monitoring (optional)
Dual AC inputs (grid and generator)
Very good IP54 weather rating
AC / DC coupling capability
3-phase capability + parallel for higher output
Note:
Separate solar inverter or solar DC regulator required
Full capability when used with SMA or compatible solar inverters
External automatic transfer switch required to enable battery backup
When configured as a hybrid inverter, grid isolation (backup mode) is not instantaneous - 2-3 sec delay.
Price bracket – From $3800
See detailed specification datasheet - SMA Sunny Island specs
About the Author - Jason Svarc is an accredited off-grid system design specialist who has been designing and building off-grid power systems for over 9 years during which he also taught the off-grid design course at Swinburne University. Having designed, installed and monitored many off-grid systems for many different households over the years he has gained vast experience and knowledge of what is required to build a quality, reliable and high performing off-grid solar power system.