Solar Battery Storage: Basic Terms You Need To Know & Understand

Solar Battery Storage Capacity

These days, solar batteries are generally sold using kWh.

For example, Powerwall 2 has a capacity of 13.5 kWh, while Enphase batteries are smaller at 1.2kWh each.

Technically speaking, battery capacity is best measured in Ampere/Hours (Ah) and the number or size of batteries based on many factors, including peak load, discharge rates, depth of discharge and cycles (see below!).

However, this is all a bit technical for most customers. With the new battery control equipment (e.g., Enphase AC micro-inverters and Solar Edge Inverters) and the fact that the battery systems themselves are generally grid-connected (which means that running out of battery power occasionally is not a big deal), the need for a lot of this knowledge is no longer as important.

Therefore, if you buy a 10 kWh battery, you can expect to use close to 10 kWh of power per day/cycle (but watch out for the Depth of Discharge!!).

Solar Battery Storage Peak Draw

Peak draw is the maximum power you can draw from the batteries at any one time.

In other words, you may, for example, have a 10kWh solar battery, but it may only let you draw 2kW of power at any one point in time – so you would have to use it for at least 5 hours or more to get your 10kWh of energy.

The reason for this is that with most solar batteries, the higher the Peak Draw, the shorter the life of the battery itself.

This is really important when choosing a battery system. If you wanted to run an appliance that used, say, 8kW of power (e.g., an air conditioner) for one hour, you would need to check not only the battery's capacity but also the Peak Draw.

Using the above example, you would need 40kWh of solar battery storage to run your air conditioner for one hour! This would, therefore, only make sense if you wanted to run the air conditioner for five hours every day.

Solar Battery Storage Cycles

A ‘battery cycle’ is one complete discharge and recharge.

For most customers, you would cycle your solar battery once per day (although some may be able to cycle twice a day if using off-peak power).

Importantly, a battery's life is measured by the number of cycles you would be expected to get from it. This is important when considering the battery that is right for you.

A battery may have an expected life of 10 years or 3500 cycles (eg 365 days a year times 10 years-ish). If you use all of those 3500 cycles in the first 5 years, the battery is unlikely to last much beyond that 5-year mark.

Solar Battery Storage Depth of Discharge

Another really important number is the Depth of Discharge (look for DOD on brochures).

This means how deeply you can discharge the solar battery per cycle. For example, if a 10kWh battery has a DOD of 90%, you could use 9kWh per day/cycle. If it has 50%, you can use/draw 5kWh and so on.

Most Lead Acid and Gel batteries have a very low DOD – sometimes as low as 20% (i.e. of 10kWh, you can only draw 2kWh per cycle).

However, the newer Lithium-Ion batteries generally have a DOD of 90% or even 100%, which means the amount of energy you can use is approximately the same as that on the label.

In reality, this is normally done by labelling them with the usable capacity instead of the technically correct capacity, but as long as you know your usable capacity, you will be fine.

Also, most of the Lithium-Ion batteries are computer-controlled, so it is not physically possible to discharge them below their DOD – a vast improvement on the older technology where if you discharged them too low too often, you could prematurely destroy the battery.

Solar Battery Storage Warranty

Always check the actual warranty of a solar battery.

Most batteries will have, say, a 10-year warranty. However, there will usually be a number of cycles (as per the cycles paragraph above) specified in the warranty. The warranty will apply to WHICHEVER COMES FIRST – eg 10 years OR 3500 cycles.

Lithium-Ion Battery

A Lithium-Ion (Li-ion) battery has most commonly been used in consumer electronics such as mobile phones and laptops.

They are one of the most common types of batteries in portable electronics due to their high energy density and slow loss of charge when not in use. The Li-ion battery is a rechargeable battery that can have a long life with many thousands of cycles if properly maintained.

This makes it possible for a hybrid solar system to have an expected life of over ten years.

Lithium Iron Phosphate Battery

The lithium iron phosphate battery (LFP, or even better… LiFePO4!) is a type of lithium-ion battery that has a lower energy density than the more common Lithium Cobalt Oxide Battery but offers a longer lifetime and better power density (the rate that energy can be drawn from them).

Lead Acid Battery

Lead acid batteries have been around for a long time – since 1869, to be exact – and they are the oldest type of rechargeable battery.

Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, its ability to supply high surge currents means that the cells have a relatively large power-to-weight ratio.

A battery bank needs ventilation, drainage and constant maintenance to ensure both the battery’s life and safety, making it impractical for a residential installation in most households.

Solar Inverter

The inverter converts the D.C. power from the batteries to 240V A.C. power used in our homes. Some systems have bidirectional inverters that also convert AC to DC to allow for charging the batteries from the grid.

Hybrid Solar Inverter

A hybrid inverter is for use with both batteries and a solar system.

In general terms, a hybrid inverter would normally measure a household’s usage and intelligently determine when to charge batteries and when to use battery power to save the most money.

It is worth noting that there is no formal definition of “hybrid inverter,” so there are inverters promoted as hybrids that will need to be upgraded before use with batteries.

Solar Regulator

A regulator is designed to protect the batteries from becoming overcharged, which can shorten the life of the battery.

Solar regulators also detect when the batteries are fully charged and prevent the batteries from overcharging. A regulator a quite a small piece of electronics and it often comes with an LCD screen.

With the newer residential battery systems, these are usually part of the inverter.

You now know the fundamentals of solar batteries. But there's even more to discover! Our comprehensive guide to solar battery storage dives into topics like its benefits, system design considerations, and pricing. Empower yourself more with in-depth knowledge to make the right choice for your home.