Choosing a battery is often the most significant investment in a solar project. While most users focus on the physical size or brand name, the real value lies in the usable energy and cycle life.
A good battery decision is never just "how many kWh can I afford?" It is a balance between backup runtime, peak power needs, charge/discharge behavior, space, and how frequently the battery will be used. Understanding the basics helps you avoid expensive upgrades later.
kWh vs. kW: The Tank vs. The Pipe
The most common confusion is between Kilowatt-hours (kWh) and Kilowatts (kW). Think of your energy system like water plumbing:
- kWh (Capacity): This is the size of the water tank. It tells you how much total energy is stored. A 5kWh battery stores 5 units of energy.
- kW (Power): This is the size of the pipe. It tells you how much energy can flow out at once. A 5kW inverter can power a 3kW oven, but a 2kW inverter cannot.
This distinction matters because a client can have enough battery capacity (kWh) but still experience trips if the system cannot deliver enough power (kW) at the same time. Battery choice and inverter sizing must therefore be coordinated.
Advertised capacity vs usable energy
When comparing batteries, ask for the usable kWh, not only the nominal rating. Two batteries may both be marketed as "5kWh," but the usable energy depends on the allowed Depth of Discharge (DoD), BMS limits, and system settings.
Practical example: a 5kWh battery at 90% usable DoD provides about 4.5kWh of usable energy. That difference becomes significant when planning overnight backup.
Depth of Discharge (DoD)
Not all stored energy should be used. Depth of Discharge refers to how much of the battery's capacity has been used relative to its total capacity.
Modern Lithium Iron Phosphate (LiFePO4) batteries used by Iselle Africa are typically rated for 80% to 95% DoD. This means if you have a 5kWh battery, you can safely use 4.5kWh. Older Lead Acid batteries could only handle 50% DoD before suffering permanent damage.
DoD should be understood as a lifespan management tool, not just a number on a spec sheet. Running a battery deeper and more frequently may be acceptable for backup-critical sites, but it can shorten service life if the design has little headroom. Good sizing reduces how hard the battery has to work every day.
Why backup duration estimates vary
Clients often ask, "How many hours will this battery last?" The correct answer is: it depends on the load. A battery that runs lights, routers, and a fridge for several hours may drain quickly if you add kettles, irons, or multiple pumps. Runtime estimates should always be tied to a defined essential-load list.
Cycle Life & Longevity
A "cycle" is one full discharge and recharge. High-quality lithium batteries are rated for 6,000+ cycles. If you cycle your battery once a day, that equates to roughly 16 years of service.
To maximize this lifespan, we recommend:
- Temperature Control: Keeping batteries in a cool, ventilated area.
- C-Rating Management: Not pulling too much power (kW) too quickly, which generates heat.
- Firmware Updates: Ensuring the Battery Management System (BMS) is always optimized.
Cycle life ratings are usually tested under specific laboratory conditions. Real-world longevity also depends on installation quality, ventilation, charging behavior, and whether the battery is routinely pushed to its limits during outages.
How battery sizing affects project cost and experience
Battery size influences more than backup duration. It affects user experience, generator dependence (if any), how often loads need to be managed, and how much solar energy can be stored for night use. Under-sizing batteries often creates the feeling that "solar is not working" when the real issue is simply insufficient storage for the client’s expectations.
- Smaller battery strategy: lower upfront cost, but requires stricter essential load management.
- Larger battery strategy: better comfort and outage resilience, but higher capital cost and longer payback.
- Modular strategy: start with a right-sized baseline and plan for expansion if usage grows.
Questions to ask before choosing a battery
- What loads must stay on during outages, and for how long?
- Will the battery cycle daily (load shifting) or mostly during outages only?
- Is the installation area cool, ventilated, and protected?
- Can the system be expanded later without replacing the inverter?
- What warranty terms apply, and what records are needed for claims?
If you are still estimating options, use the pricing calculator for a budgetary range, then compare that with your load priorities using the System Sizing 101 guide.