Battery sizing: Don’t forget the color of the house!

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Greg Smith - SMA America
Greg Smith - SMA America
Greg Smith has been a technical training specialist for the SMA Solar Academy since May 2008. He has 20 years of technical training experience specializing in curriculum development. Greg travels extensively in the United States and Canada teaching customers about SMA products, electrical code compliance and installation best practices. Greg, who holds a master’s degree in human resources administration from Central Michigan University and a bachelor’s degree in corporate training, spent 20 years in the U.S. Navy as a submarine sonar technician and master training specialist prior to his career at SMA.

I wish I had a dollar for every time someone asked me, “I have a *insert number* square foot house. How big of a battery do I need?” My reply is always the same:

What color is your house?

The answer to that question is about as important as the size of the house. Oh, and there is no such thing as a “standard house” when it comes to determining consumption. Sure, you can find data that will tell you that the ‘average US household’ consumes about 900 kWh a year, but that won’t help much when the customer adds a jacuzzi, surround sound system and a lighted disco ball after you sized their system for ‘average’ consumption.

It is all about the loads. The cool kids who have been doing battery backup and stand alone systems will tell you that this is the single most important piece of information that is required for these applications. Everything else is just extra stuff that can be figured out later. A properly filled out load calculation sheet will be worth its weight in gold- ok, probably a lot more than that since an excel spreadsheet doesn’t have any real weight, but you get what I mean.

Battery systems are evolving far beyond the source of power when the grid goes down, however. These systems are now being used in commercial and residential applications to hide those transient pesky spikes in consumption that the utilities love to charge you a whole day for. Peak shaving can save many times over what a battery system costs.

If you haven’t heard the term “self consumption” (when referring to battery/renewable energy systems) then you need to get out more often. Self consumption isn’t a new dieting fad or new age introspective mediation program- it is a term used to label an RE/battery system that allows the user to consume all of the self generated RE (i.e. solar, wind, hydro, etc.) so that very little of those grid electrons ever make it into the house. The idea is to use the RE to power the loads at certain times when the sun is shining the brightest or the wind blowing the hardest. Some systems are so intelligent that they can turn on large consumers at solar noon and then turn them off when the utility rates are higher.

Time of use is a programmable function in modern battery systems that will limit the amount of energy pulled from the grid when the utility rates are at their highest- i.e. when most people are getting home from work or when they are waking up. Discharging the battery at these strategic times of day will save the customer a LOT of money in the long run.

Things to think about as the customer/installer:

  1. List all the loads that will be in operation and for how many hours of the day they will be running. This will determine the rough size of the battery bank.
  2. What is the largest load in the system including its surge? This will determine the size of the battery inverter. If you have a well pump that draws 50A on startup, but your battery inverter can only deliver 20A then people are going to get thirsty when the grid goes down.
  3. From the calculations: Watts = battery inverter size; watt hours = battery capacity.
  4. Are there other sources of energy? Solar, wind, hydro, generator, etc. all provide power to the loads and thus reduce the amount of energy taken from the grid and/or the battery bank.
  5. How big are the other sources? The energy form the RE sources contribute to offsetting the consumption from either the grid or the battery. This means a smaller battery bank can be used thereby saving some up front costs.

These 5 considerations should be enough to really start nailing down a battery bank size for either self consumption, peak shaving, time of use and even a little battery backup thrown in.

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