ESS Energy Losses

What Are ESS Losses?

Due to various efficiency losses and auxiliary loads, energy storage systems will often cause increases to a site’s energy usage and incur energy losses. Total energy losses include but are not limited to the following:

PCS & Battery Charge/Discharge Efficiencies
Resistance Related Losses
HVAC & Cooling Fans
BMSs
Monitoring Equipment

How ESS Losses Are Calculated

Losses are calculated by measuring the total amount of energy that has been inputted to the ESS against the total amount of energy that been outputted by the ESS, adjusted for the change in the ESS’s energy capacity over the same time. Due to where the loads are measured, the input energy accounts for the energy involved in charging the ESS, as well as any auxiliary loads.

Example Diagram of Where ESS Energy Input and ESS Energy Output Are Measured

ESS Losses Formula:

ESS Losses = (Measured Energy Input – Measured Energy Output) + (Starting Capacity – Ending Capacity)

Losses Calculation Example 1:

At 1:00pm the ESS has a capacity of 100kWh.

Between 1:00pm and 5:00pm 200kWh is inputted to the ESS.

Between 1:00pm and 5:00pm 200kWh is outputted by the ESS.

At 5:00pm the ESS has a capacity of 50kWh.

Total Energy Losses from 1:00pm to 5:00pm is 50kWh.

Losses Calculation Example 2:

At 1:00pm the ESS has a capacity of 100kWh.

Between 1:00pm and 5:00pm 120kWh is inputted to the ESS.

Between 1:00pm and 5:00pm 0kWh is outputted by the ESS.

At 5:00pm the ESS has a capacity of 200kWh.

Total Energy Losses from 1:00pm to 5:00pm is 20kWh.

Typical Losses to Expect

There is no blanket answer as to what is considered “typical” losses. Losses will vary greatly depending on the hardware at the site, the equivalent battery cycles, environmental variables, and other site-specific variables. Sites with less active energy storage systems will typically have a higher percentage of losses, but lower kWh of losses when compared to more active energy storage systems. This is because the percentage of losses is determined by comparing the energy losses to the energy delivered to the site, and the energy delivered to the grid. Therefore, if a system is less active, less energy will be delivered to the site and grid, while the auxiliary loads are still drawing power over time. See the examples below for an example on how the same ESS can have vastly different loss percentages and kWh under different conditions.

Example of a Less Active Site:

A site is targeting Demand Charge Management and is not allowed to export to the grid. The ESS at the site is a 30kW/120kWh unit. This site has an auxiliary ESS load that draws 40kWh per month. Besides auxiliary loads, the ESS operates at 95% efficiency. This site’s load profile is such that there is a relatively flat baseload, with one or two, large, two-hour spikes per month. The ESS predicts a large spike for the month is going to happen, so it charges to full capacity. When the demand spike occurs, the ESS discharges at its max power discharge rate of 30kW for 2 hours to shave the peak as effectively as possible, resulting in 57kWh being delivered to the site (30kW * 2 hours * .95). The rest of the month there are no other peaks to shave and the ESS is idle. This means that the breakdown of the energy that left the ESS would be the following:

Energy from the ESS to the Site: 57kWh (57%)

Energy from the ESS to the Grid: 0kWh (0%)

Energy from the ESS to Losses: 43kWh (43%)

Example of a More Active Site:

A site is targeting TOU Arbitrage and is not allowed to export to the grid. This site has an ESS auxiliary load that draws 40kWh per month. Besides auxiliary loads, the ESS operates at 95% efficiency. The ESS at the site is a 30kW/120kWh unit. The ESS is operating on a schedule where each day it charges to full capacity during the off-peak TOU window, then discharges its full capacity during the four-hour on-peak window. It does this for all 30 days of the month. The total energy being delivered from the ESS to the site for a 30-day month is 3,420kWh (30kW * 4 hours * 30 days * .95). The total energy being lost for a 30-day month would be 220kWh (30kW * 4 hours * 30 days * .05 + 40kWh). This means that the breakdown of the energy that left the ESS would be the following:

Energy from the ESS to the Site: 3,420kWh (94%)