Even in 2022, PV storage will still be the hottest topic, and residential battery backup is the fastest growing segment of solar, creating new markets and solar retrofit expansion opportunities for homes and businesses large and small around the world. Residential battery backup is critical for any solar home, especially in the event of a storm or other emergency. Instead of exporting excess solar energy to the grid, how about storing it in batteries for emergencies? But how can stored solar energy be profitable? We’ll inform you about the cost and profitability of a home battery storage system and outline the key points you should keep in mind when purchasing the right storage system.

An residential battery storage or photovoltaic storage system is a useful addition to the photovoltaic system to take advantage of the benefits of a solar system and will play an increasingly important role in accelerating the replacement of fossil fuels with renewable energy. The solar home battery stores the electricity generated from solar energy and releases it to the operator at the required time. Battery backup power is an environmentally friendly and cost-effective alternative to gas generators. Those who use a photovoltaic system to produce electricity themselves will quickly reach its limits. At midday, the system supplies plenty of solar power, only then there is no one at home to use it. In the evening, on the other hand, plenty of electricity is needed – but then the sun is no longer shining. To compensate for this supply gap, the significantly more expensive electricity is purchased from the grid operator.

In this situation, a residential battery backup is almost inevitable. This means that the unused electricity from the day is available in the evening and at night. Self-generated electricity is thus available around the clock and regardless of the weather. In this way, the use of self-produced solar power is increased to up to 80 %. The degree of self-sufficiency, i.e. the proportion of electricity consumption that is covered by the solar system, increases to up to 60 %. A residential battery backup is much smaller than a refrigerator and can be mounted on a wall in the utility room. Modern storage systems contain a great deal of intelligence that can use weather forecasts and self-learning algorithms to trim the household to maximum self-consumption. Achieving energy independence has never been easier – even if the home remains connected to the grid.

Lead-acid batteries are the oldest rechargeable batteries and lowest cost battery available for energy storage on the market. They appeared at the beginning of the last century, in the 1900s, and to this day remain the preferred batteries in many applications due to their robustness and low cost.

Their main disadvantages are their low energy density (they are heavy and bulky) and their short life span, not accepting many loading and unloading cycles, lead-acid batteries require regular maintenance to balance the chemistry in the battery, so its characteristics make it unsuitable for medium to high-frequency discharge or applications that last 10 years or more.

They also have the disadvantage of low depth of discharge, which is typically limited to 80% in extreme cases or 20% in regular operation, for longer life. Over-discharge degrades the battery’s electrodes, which reduces its ability to store energy and limits its life.

Lead-acid batteries require constant maintenance of their state of charge and should always be stored at their maximum state of charge through the floatation technique (maintenance of charge with a small electric current, sufficient to cancel the self-discharge effect).

These batteries can be found in several versions. The most common are vented batteries, which use liquid electrolyte, valve regulated gel batteries (VRLA) and batteries with electrolyte embedded in fiberglass mat (known as AGM – absorbent glass mat), which have intermediate performance and reduced cost compared to gel batteries.

Valve-regulated batteries are practically sealed, which prevents leakage and drying of the electrolyte. The valve acts in the release of gases in overcharged situations.

Some lead acid batteries are developed for stationary industrial applications and can accept deeper discharge cycles. There is also a more modern version, which is the lead-carbon battery. Carbon-based materials added to the electrodes provide higher charge and discharge currents, higher energy density, and longer life.
One advantage of lead-acid batteries (in any of its variations) is that they do not need a sophisticated charge management system (as is the case with lithium batteries, which we will see next). Lead batteries are much less likely to catch fire and explode when overcharged because their electrolyte is not flammable like that of lithium batteries.

Also, slight overcharging is not dangerous in these types of batteries. Even some charge controllers have an equalization function that slightly overcharges the battery or battery bank, causing all batteries to reach the fully charged state.

During the equalization process, the batteries that eventually become fully charged before the others will have their voltage slightly increased, without risk, while the current flows normally through the serial association of elements. In this way, we can say that lead batteries have the ability to equalize naturally and small imbalances between the batteries of a battery or between the batteries of a bank offer no risk.

• Performance: The efficiency of lead-acid batteries is much lower than that of lithium batteries. While the efficiency depends on the charge rate, a round-trip efficiency of 85% is usually assumed.
• Storage capacity: Lead-acid batteries come in a range of voltages and sizes, but weigh 2-3 times more per kWh than lithium iron phosphate, depending on the quality of the battery.
• Battery cost: Lead-acid batteries are 75% less expensive than lithium iron phosphate batteries, but don't be fooled by the low price. These batteries cannot be charged or discharged quickly, have a much shorter life, do not have a protective battery management system, and may also require weekly maintenance. This results in an overall higher cost per cycle than is reasonable to reduce power costs or support heavy-duty appliances.

Currently, the most commercially successful batteries are lithium-ion batteries. After lithium-ion technology is applied to portable electronic devices, it has entered the fields of industrial applications, power systems, Photovoltaic energy storage and electric vehicles.

Lithium-ion batteries outperform many other types of rechargeable batteries in many aspects, including energy storage capacity, number of duty cycles, charging speed, and cost-effectiveness. Currently, the only issue is safety, flammable electrolytes can catch fire at high temperatures, which requires the use of electronic control and monitoring systems.

Lithium is the lightest of all metals, has the highest electrochemical potential, and offers higher volumetric and mass energy densities than other known battery technologies.

Lithium-ion technology has made it possible to drive the use of energy storage systems, mainly associated with intermittent renewable energy sources (solar and wind) and has also driven the adoption of electric vehicles.

Lithium-ion batteries used in power systems and electric vehicles are of the liquid type. These batteries use the traditional structure of an electrochemical battery, with two electrodes immersed in a liquid electrolyte solution.

Separators (porous insulating materials) are used to mechanically separate the electrodes while allowing the free movement of ions through the liquid electrolyte. The main feature of an electrolyte is to allow the conduction of ionic current (formed by ions, which are atoms with excess or lack of electrons), while not allowing electrons to pass through (as happens in conductive materials). The exchange of ions between positive and negative electrodes is the basis for the functioning of electrochemical batteries.

Research on lithium batteries can be traced back to the 1970s, and the technology matured and began commercial use around the 1990s.

• Lithium and Cobalt Oxides (LCO): High specific energy (Wh/kg), good storage capacity and satisfactory lifetime (number of cycles), suitable for electronic devices, disadvantage is specific power (W/kg) Small, reducing the loading and unloading speed.
• Lithium and Manganese Oxides (LMO): allow high charge and discharge currents with low specific energy (Wh/kg), which reduces storage capacity.
• Lithium, Nickel, Manganese and Cobalt (NMC): Combines the properties of LCO and LMO batteries. In addition, the presence of nickel in the composition helps to increase the specific energy, providing greater storage capacity. Nickel, manganese and cobalt can be used in varying proportions (to support one or the other) depending on the type of application. Overall, the result of this combination is a battery with good performance, good storage capacity, long life, and low cost.
• Lithium, nickel, manganese and cobalt (NMC): Combines features of LCO and LMO batteries. In addition, the presence of nickel in the composition helps to raise the specific energy, providing greater storage capacity. Nickel, manganese and cobalt can be used in different proportions, according to the type of application (to favor one characteristic or another). In general, the result of this combination is a battery with good performance, good storage capacity, good life, and moderate cost. This type of battery has been widely used in electric vehicles and is also suitable for stationary energy storage systems.
• Lithium Iron Phosphate (LFP): The LFP combination provides batteries with good dynamic performance (charge and discharge speed), extended lifetime and increased safety due to its good thermal stability. The absence of nickel and cobalt in their composition reduces the cost and increases the availability of these batteries for mass manufacturing. Although its storage capacity is not the highest, it has been adopted by manufacturers of electric vehicles and energy storage systems due to its many advantageous characteristics, especially its low cost and good robustness;
• Lithium and Titanium (LTO): The name refers to batteries that have titanium and lithium in one of the electrodes, replacing the carbon, while the second electrode is the same used in one of the other types (such as NMC – lithium, manganese and cobalt). Despite the low specific energy (which translates into reduced storage capacity), this combination has good dynamic performance, good safety, and greatly increased service life. Batteries of this type can accept more than 10,000 operating cycles at 100% depth of discharge, while other types of lithium batteries accept around 2,000 cycles.
• LiFePO4 batteries outperform lead-acid batteries with extremely high cycle stability, maximum energy density and minimal weight. If the battery is regularly discharged from 50% DOD and then fully charged, the LiFePO4 battery can perform up to 6,500 charge cycles. So, the extra investment pays off in the long run, and the price/performance ratio remains unbeatable. They are the preferred choice for continuous use as solar batteries.
• Performance: Charging and releasing the battery has a 98% total cycle effectiveness while being quickly charged and released in time frameworks of less than 2 hrs– and even faster for a decreased life.
• Storage capacity: a lithium iron phosphate battery packs can be over 18 kWh, which uses less space and weighs less than a lead-acid battery of the same capacity.
• Battery cost: Lithium iron phosphate tends to cost greater than lead-acid batteries, yet usually has a lower cycle cost as a result of greater longevity

The power storage market is growing fast – so it's hard for homeowners to keep track of it. While lead storage systems used to dominate due to their affordability, it's the durable and efficient lithium-ion storage systems that are now driving growth.

Since solar batteries are the most expensive part of residential solar batteries and new storage products are deviating from traditional off-grid designs, it is important to understand the various design options to meet customer expectations within the project budget. When researching options for your solar home battery storage system, there are many choices to consider, and the following are a few of the most common selection criteria：

• If you already have solar panels, choose a residential battery backup system and inverter of comparable power to it.
• If you intend to power even more of your home at once, look for a solar battery with a high-power ranking.
• Furthermore, some systems permit you to stack more than one unit to get the degree of back-up you need.
• If you want to power a much more energy-intensive device (like cooling or sump pumps that need more power to start up than as soon as they're running), seek a battery with a high instantaneous power rating.
• If you want to run your home with your solar battery for a much more extended quantity of time, try to find a battery with a greater useful ability.
• If you really feel that the price of lithium-ion solar batteries and inverters is out of your budget plan, you can look for all-in-one residence power storage space remedies, as well as usually they are more economical than batteries and inverters of the exact same capacity.
• If you intend to get one of the most out of every kilowatt-hour of electrical power you put into your battery, try to find batteries with a high roundtrip performance.
• If you are space-constrained and wish to get one of the most storage out of the least amount of area, look for lithium-ion LiFePo4 solar batteries.
• If you want a battery with one of the most extensive lives to cycle the most amount of times, seek lithium iron phosphate batteries.
• If you desire a battery with the absolute highest possible safety ranking possible, look for lithium iron phosphate batteries.
• If you want a home battery backup system with ensured high quality, seek a reliable house battery system provider or manufacturer.
• Since lithium-ion batteries should not be fully discharged, there are always two indications of their capacity. One is the nominal capacity, i.e., the usable storage capacity, and the actual storage capacity. The nominal capacity is decisive for the purchase.