What’s the Difference Between Nominal and Useable Energy Capacity?

Referring to energy capacity (kWh) can be done so in both nominal and useable energy capacity specifications that will be listed on almost every battery spec sheet.

The main difference between nominal and useable energy capacity is the amount each refers to. Nominal refers to the absolute total, and useable is the energy owners can consume.

Why do batteries have nominal & useable capacity? Solar batteries are designed with an useable capacity to ensure the battery lasts longer. Elements like regular 100% discharges degrade the battery faster, so operating within safer operating ranges can prolong the battery’s lifespan.

What is Battery Power Output?

The power output, power rating, or power capacity refers to how much energy a battery can deliver at any given moment, usually measured in kilowatts (kW) – the same as solar systems.

Like energy capacity, Power output can be measured in one of two ways, continuous and peak power output. Peak power output is how much electricity a battery can deliver in a short moment, whilst continuous power output refers to the consistent power output a battery can provide.

Continuing our example, Tesla Powerwall 3 offers up to 11kW of power output. Higher output means that a battery can support higher loads like heavy appliances or multiple devices simultaneously.

Power output is generally limited by the solar inverter which is responsible for converting the battery's DC power into AC power for your home. Output power can also vary depending on whether your home is single or three-phase.

What is State-of-Charge (SoC)?

State-of-charge or ‘SoC’ refers to the current energy level of a battery as a percentage of the nominal total energy capacity. Like we see on our phones, SoC is simply how much battery life you have at any given moment, 100% being full and 0% empty.

In the world of solar energy, state-of-charge can help us understand how we can better use our energy based on timing, energy generation, and battery depletion.

What is Depth-of-Discharge (DoD)?

As opposed to SoC, DoD or Depth-of-Discharge measures the used side of the battery, also relative to the battery’s nominal capacity. In a real-use case, a battery that has 60% remaining would have a 40% DoD as it looks at the other half of the stored energy.

DoD is used mostly for measuring discharge cycles which can shorten battery lifespan, compared to reduce discharge cycles which can prolong it.

What are Battery Charge & Discharge Cycles?

One battery cycle is when a battery charges from empty (0%) to full (100%) and empties again.

Whilst most solar batteries don’t operate between 0-100%, batteries can still complete full cycles. For example, a solar battery that charges from 30-80% and then discharges back down to 30%, it has achieved half a cycle by charging and discharging 50%. This over 2 days reached a full battery charge cycle.

Why do we use charge & discharge cycles? Using charge and discharge cycles enables better measurement of the battery’s lifespan and wear.

How many cycles are batteries warranted for? A popular solar battery manufacturer we install warrants their batteries for 70% energy retention over 10 years OR 4,000 cycles – whichever comes first.

What does that mean in the real-world? Let’s say you use your battery everyday and on average the battery does half a charge and discharge cycle. Ignoring the 10-year aspect, with a warranty of 4,000 cycles, it would take you 21.9 years to reach 4,000 cycles.