Solar battery storage

The idea of battery storage in the home is not new. Off-grid solar photovoltaic (PV) and wind turbines generating electricity have been using battery storage for a long time especially in very remote areas in Australia, they are used to store excess power now to be used at a later time. It is quite possible that in 5-10 years the majority of houses with Solar PV panels will have battery storage also.

A battery captures any unused solar power generated during the day, to be used later at night, on days with low sunlight or where extra power is required to power a heavy consumer of electricity in the home. Getting battery storage installed with PV panels is becoming more and more popular. Worldwide people are becoming more attracted to the idea of being as independent as possible from the national grid. There are two main reasons why people go with battery storage, environmental reasons as you are generating and using all of your own clean green energy that you are generating, and also it is a way of protecting yourself from increasing electricity costs and increasing carbon taxes.

If your Solar PV system and battery capacity is large enough you could potentially offset the large majority of your consumption in your home. Using the electricity from your battery will work out cheaper per kilowatt-hour than using electricity from the grid, also if you are on a night rate tariff or with potential new smart tariffs on route you can charge your battery from the grid on the cheaper rate during poorer weather conditions and use that power during the higher tariff times. 

Grid-connected solar pv systems vs off-grid solar pv systems

There are three main ways your home can be set up for electricity supply.

Grid-connected solar with no battery

The is the most common set-up for homes with solar pv panels. The solar panels supply power during the day, and the home generally uses this power first, then if there is excess electricity it either goes to the grid through your ESB meter or else you can install an immersion diverter which will send the excess to you hot water cylinder via the immersion until your thermostat reaches max temperature and then it will export to the grid

Grid-connected hybrid solar system with a battery

These have solar panels, a battery, a hybrid inverter, and a connection to the main electricity grid. The solar panels supply power during the day, and the home generally uses the solar power first, using any excess to charge the battery. At times of high power usage, or at night and on low-sunlight days, the home draws power from the battery, and as a last resort from the grid.

Off-grid

This system has no connection to the main electricity grid. All the home’s power comes from solar panels, and possibly some other types of power generation either generator back up or ideally wind. The battery is the main power source at night and on low-sunlight days. The final back-up is usually a diesel-powered generator, which may also kick in when there is a sudden high demand for power or on a dull day with  no wind. Which is not very common in Ireland.

Off-grid systems are usually much more complex and expensive than grid-connected systems. They need more solar and battery capacity than a typical grid-connected system and may also need inverters capable of higher loads to cope with peak demands. Homes that run off-grid need to be particularly energy-efficient and the load demand needs to be well-managed throughout the day.

Off-grid solar pv systems usually only make sense for remote properties where a grid connection isn’t available or would be extremely expensive to install.

What happens in a blackout?

For most grid-connected systems, having a battery doesn’t necessarily protect you in the event of a blackout. You may still lose all power to your home, despite having solar panels producing power and a charged battery ready and waiting. This is because grid-connected systems have what’s known as “rapid shut down device installed”. During a blackout, the grid and any engineers working on the lines must be protected from excess electricity generation (such as your solar panels) pumping power unexpectedly into the lines. For most solar PV systems, the simplest way to provide anti-islanding protection is to shut down entirely. So, when it senses a grid blackout, your solar PV system shuts down and you have no household power at all.

Typically a switch over is required. Most commonly you would choose to allow only critical household circuits to operate in that situation, such as the fridge and lighting. This would require extra wiring work than a standard PV Panel installation

These are the key technical specifications for a home battery.

Capacity

How much energy the battery can store, usually measured in kilowatt-hours (kWh). The nominal capacity is the total amount of energy the battery can hold; the usable capacity is how much of that can actually be used, after the depth of discharge is factored in.

Depth of discharge (DoD)

Expressed as a percentage, this is the amount of energy that can be safely used without accelerating battery degradation. Most battery types need to hold some charge at all times to avoid damage. Lithium batteries can be safely discharged to about 80–90% of their nominal capacity. Lead-acid batteries can typically by discharged to about 50–60%.

Power

How much power (in kilowatts) the battery can deliver. The maximum/peak power is the most that the battery can deliver at any given moment, but this burst of power can usually only be sustained for short periods. Continuous power is the amount of power delivered while the battery has enough charge.

Efficiency

For every kWh of charge put in, how much the battery will actually store and put out again. There’s always some loss, but a lithium battery should usually be more than 90% efficient.

Total number of charge/discharge cycles

Also called the cycle life, this is how many cycles of charge and discharge the battery can perform before it’s considered to reach the end of its life. Different manufacturers might rate this in different ways. Lithium batteries can typically run for several thousand cycles.

Lifespan (years or cycles)

The expected life of the battery (and its warranty) can be rated in cycles (see above) or years (which is generally an estimate based on the expected typical usage of the battery). The lifespan should also state the expected level of capacity at the end of life; for lithium batteries this will usually be about 60–80% of the original capacity.

Ambient temperature range

Batteries are sensitive to temperature and need to operate within a certain range. They can degrade or shut down in very hot or very cold environments.

Types of battery

Lithium-ion

The most common type of battery being installed in homes today, these batteries use similar technology to their smaller counterparts in smartphones and laptop computers. There are several types of lithium-ion chemistry. A common type used in home batteries is lithium nickel-manganese-cobalt (NMC), used by Tesla and LG Chem, Huawei(100% discharge on Huawei Battery).

Another common chemistry is lithium iron phosphate LiFePO, or LFP which is said to be safer than NMC due to lower risk of thermal runaway (battery damage and potential fire caused by overheating or overcharging) but has lower energy density. LFP is used in home batteries made by BPE and Sonnen, among others.

Pros

• They have a long lifespan as they can charge and discharge thousands of cycles.
• They can be discharged heavily typically to 80–90% of their overall capacity.
• They should last for 10+ years in normal use.

Cons

• They need to be recycled to recover valuable metals and prevent toxic landfill, but large-scale programs are still in their infancy. As home and automotive lithium batteries become more common, it is expected that recycling processes will improve.

Lead Acid Batteries

The good old lead-acid battery technology that helps start your car is also used for larger-scale storage. It’s a well-understood and effective battery type. Most manufacturers making batteries are moving away from lead acid, without significant developments in performance or reductions in price, it’s hard to see lead-acid competing long-term with lithium-ion or other technologies.

Pros

• They are relatively cheap, with established disposal and recycling processes.

Cons

• They are bulky.
• They are sensitive to high temperatures, which can shorten their lifespan.
• They have a slow charge cycle.

How long do solar batteries last?

In principle, most solar battery types should be able to last 10 years or more under normal usage and if not subjected to extreme temperatures. That is, they should be able to last as long as their warranty period at minimum, which for most models is 7-10 years. A good indicator of lifespan batteries is how many cycles they are warranted for.

Are solar batteries worth it?

If this question were asked 2 to 3 years ago, the answer would definitely be no. But with the prices of batteries reducing and grant aid available for battery storage it is making it a more attractive offering. Currently in Ireland the fact that there is no feed in tariff for your excess electricity this makes having a battery a far more appealing offering. Depending on how much the feed in tariff is and if grant aid is continued for batteries from the SEAI, that will see if batteries continue to be a worthwhile solution for your excess electricity. Currently, a lithium-ion battery and hybrid inverter will typically cost between €8,000 and €15,000 (installed), depending on capacity and brand, with grants available up to €3000. With prices falling, in two or three years, if there is no grant aid it may still be worthwhile to include a storage battery with any solar PV system. As it stands with grant aid available this is making the batteries worthwhile.

Many people are investing in home battery storage now, or at least ensuring their solar PV systems are battery ready. We recommend at least that you would go with a battery ready inverter. This will future proof your system so even if a battery is out of reach right now, you can easily add one again down the line.

Terminology

Watt (W) and kilowatt (kW)

A unit used to quantify the rate of energy transfer. One kilowatt = 1000 watts. With solar panels, the rating in watts specifies the maximum power the panel can deliver at any point in time. With batteries, the power rating specifies how much power the battery can deliver.

Watt-hours (Wh) and kilowatt-hours (kWh)

A measure of energy production or consumption over time. The kilowatt-hour (kWh) is the unit you’ll see on your electricity bill because you’re billed for your electricity usage over time. A solar panel producing 300W for one hour would deliver 300Wh (or 0.3kWh) of energy. For batteries, the capacity in kWh is how much energy the battery can store.