Section 1
The basics
At their core, solar batteries store electricity so it can be used later. This electricity can come from solar panels during the day or directly from the grid when power is available. The stored energy is then used at night or when the grid goes down.
Solar batteries cannot operate on their own. They rely on additional equipment to:
Section 2
Solar terminology explained
Before sizing a battery bank, it’s important to understand a few key terms. If you’re already familiar with solar jargon, feel free to skip ahead.
The term deep-cycle refers to how much of a battery’s capacity can be used regularly. Solar batteries are designed to discharge deeply and recharge repeatedly without damage.
How deeply a battery can discharge depends on its chemistry.
Depth of Discharge indicates how much of a battery’s total capacity can safely be used.
This difference is critical when calculating how much storage you actually need.
Section 3
The basics
There are two main categories of solar batteries:
Sealed Lead-Acid
AGM (Absorbent Glass Mat) batteries are an improved version of traditional flooded lead-acid batteries. They are sealed, maintenance-free, and safer to use indoors.
While the upfront cost is lower, AGM batteries have a much shorter lifespan compared to lithium batteries. This makes them more expensive over time due to frequent replacements.
AGM batteries use fiberglass mats to absorb electrolyte, allowing them to:
New AGM batteries should never be connected in series with older batteries, as this can damage the newer units.
The modern standard for solar
Lithium-ion batteries are the most efficient and advanced option available today. They offer deeper discharge cycles, far higher efficiency, and significantly longer lifespans.
They are lighter, more compact, generate less heat, and are ideal where space is limited.
Lithium batteries are modular and easy to expand. Additional units can be added later without replacing the entire battery bank.
They are available in:
Section 4
Three simple steps
To choose the correct battery size, you must understand what you want to power and for how long.
Before starting, check the power ratings of all appliances you plan to run on battery power.
Which appliances need battery power?
How long must each appliance run?
Will the battery cover load shedding only, or evenings and mornings too?
Any period without solar production will be referred to as downtime.
Add Appliance Power Ratings
Add the power ratings of all the appliances you’d like to run off your solar battery bank.
As I have said, it is a good idea to check each of your appliances for their ratings to get an exact idea of your usage.
Calculate Usage Time
Next, determine how long each appliance is normally used.
Not all appliances run for the same length of time during downtime. For example, a television may be used for several hours, while a microwave is typically only switched on for a few minutes.
This step depends entirely on your personal habits and daily routine, so accuracy here will give you the most reliable battery size estimate.
Define Usage Periods
Depending on your area, outages can last between 2.5 and 4 hours or more.
Essential appliances like fridges, alarms, and routers often run overnight.
Most evening usage occurs between 18:00 and 22:00.
Morning usage is typically high due to kettles, toasters, etc. Solar production is often insufficient early in the day, making battery capacity especially important.
Section 5
A real-world example
Using real household data, total energy usage across mornings, evenings, and nights equals 8.61 kWh.
When selecting a battery, Depth of Discharge must be considered. For example:
An 80% DOD battery requires extra capacity to cover losses
Example battery options:
3 × PylonTech US3000C → 9.975 kWh @ 95% DOD
3 × Dyness 3.6 kWh → 8.645 kWh @ 80% DOD