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Key Parameters to Look At for Fleet EV Energy Storage

(Rick Sander, CEO, Tuesday, May 26, 2020)

Now let’s assume that your organization is considering integrating battery-based energy storage into your high-power charging infrastructure for your electric vehicles (EVs). The next task is to size your energy storage needs. The parameters you need to consider when sizing your solution are:

  1. What is the power feed that you have into each of your vehicle yards?
  2. What are the operating hours for your EVs, and how long are the hours when they can charge?
  3. How many EVs do you have (specifically, how many in each vehicle yard), both now and over the next 3-5 years, and what is the total battery capacity of the vehicles in each vehicle yard?
  4. What percentage of your EVs’ battery capacity is used in each shift (on average)?
  5. How many solar photovoltaic (PV) power panels can you put into (or above) your vehicle yard?

Here are the steps to go through to determine your energy storage feeds:

  • Determine the total amount of energy available from your utility. The maximum available is the power feed for your yard (a) times the charging hours available (b2). If your yard has a 2MW feed and 12 hours to charge, this amount would be 24MWh (2MW x 12 hours).
  • Determine the total amount of power required to charge your vehicles, which is the total battery capacity in each yard (c3) times the percentage of power used per shift by your EVs (d). If you have 50 EVs per yard with 650kWh each, and they use 90% of their charge, this would be 29MWh.

If #2 is greater than #1 (as in our example), then you either need a larger power feed or energy storage. The amount of energy storage required is #2 minus #1, divided by the efficiency of your storage batteries (usually 90%) – in this case, it would be 4.725MWh: (29.25MWh – 24MH2)/90%.This is roughly 1-1/2 transport containers worth of batteries.

The final question is whether you should add solar PV to the equation. This may be necessary because the maximum amount of energy you can store in your energy storage resources from your electricity utility is the power feed from your utility (a) times the duty hours of your vehicles (24 hours minus “b” above); and your storage can only deliver 90% (the storage battery efficiency) of this (in our example, that would be 2MW times 12 hours times 90%, or = 21.6MWh). If you need more power than that, you will have to utilize solar PV to augment your utility power feed. Our next blog will look into how vehicle to grid (V2G) options changes these calculations.

As a leader in the development and manufacturing of bi-directional, high-power energy systems for both vehicle charging and photovoltaic solar applications, Rhombus is an expert in high-power charging systems for EVs, and especially in the area of V2G charging. We have built hundreds of V2G-capable high-power, high-reliability chargers for a variety of different sizes and classes of EVs. Find out how we can help you by contacting us at, or by reading our Energy Storage Reference Architecture white paper.

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