Grid Backup Solar PV Systems


By Mark Simmons, Senior Energy Consultant

Energy Experts USA, msimmons@energyexpertsusa.net


I started developing Grid Backup systems when working in Colombia, South America, where Net Metering doesn't exist.  I had a good technical designer, which I am not.  

Grid Backup means that the Grid is your backup power supply.  It's a solution to attacks on Net Metering, as it works without NEM.  It became affordable (arguably) about four years ago, and as the price of batteries continues to fall it may become the norm.  It's already the norm in Hawaii, where we can't Net Meter anymore.  It provides sufficient payback when power (plus incentives) equal more than about $.24/kWh.  

If the local utility reduces value of net metered kWh switch to Grid Backup, as then every kWh is still worth retail value.  Since we're getting attacked across the country, learn about this now, to make sure your business can thrive regardless of what the utilities do to net metering.

The objective is to catch all the daytime overproduction of the solar system in the batteries.  When the sun goes down automatically the electrical panel grabs power from the batteries, until they reach their discharge level. Then automatically the panel grabs a few kWh from the regular electric meter, until sunrise.  The batteries are then refilled by solar production and the cycle continues.  

Lead acid batteries are lowest cost, but last only 5 to 8 years with a 30% standard Depth of Discharge (DoD) each night. If you ran them at, say, 50% DoD each night you'd have more storage cycling every day, so a smaller battery pack could be used, but you would get significantly shorter battery life.  Get sealed batteries instead of wet cells, so there's no maintenance. Lithium Ion batteries can go with 80% DoD and last 8 years or so, but they're pricey.  Nickel Iron wet cell batteries run at 80% DoD for 30 years, but cost a lot more. There are other battery solutions, and this is an area of intense development. Lithium Iron Phosphate (LiFePo) run at 50% DoD with 14 years life, and they cost less than Lithium Ion, so these are my go-to batteries presently.  

To determine battery sizing you need to track the daytime and nighttime electricity usage of the building, whether house or business. If you can get an Intervals Report from the utility you can size it properly.  If the utility won't provide one you can put monitoring equipment on the electrical system for some period of time- a day, a week or so, depends on the situation- and get the usage patterns that way.  But to do it perfectly you'd need to run each season, to get the variations.  

I use a simple rule of thumb here in Denver. The typical home will use 1/3 of its power during the day, 2/3 at night.  So we size the batteries to catch 2/3 of the solar production. Grid Backup might waste a small amount of power, as once the batteries are full and the panel has all the power it needs there's nowhere for the excess production to go.  But wasting a few kWh for the greater goal seems acceptable. Just try not to waste much.  

If in doubt and the project budget can afford it, oversize the batteries a bit.

You might be able to get information from the utility as to typical residential usage patterns.  With commercial, unless you get the intervals report from the utility, you must test.

Typically these are 48 volt systems.  

 

Grid Backup's Side Benefit

You have some power when the Grid goes down. This was a key selling point in Colombia, where the Grid goes down a couple hundred times a year, sometimes for hours.  When the Grid goes down you can adjust to use 80% DoD, for more power until the Grid come sback up.  Then rest to standard DoD and you haven't significantly reduced battery life.

Added Costs

Batteries, a battery-style inverter, charge controller(s), more wiring, a battery box, extra labor on both DC and AC side.  Jurisdictions are just starting to learn how to approve and permit a battery system on a grid connected building. This can stretch permit approval and inspections.  The initial testing and design take more time.  Your designer and perhaps electrician might have a steep learning curve.

What Grid Backup is Not

It's not connected to the Grid at all.  It typically goes to the main panel, or a subpanel, so you are not touching any equipment owned by the utility.  If you use a subpanel you transfer critical loads- those the customer wants to be sure operate in case of grid outage. This is a handy side-use of Grid Backup.

It's not peak demand shaving.  That technology exists– a combination of software and carefully tracked peak loads, regularly modifying timing of battery discharge throughout the year, to trim the peaks.  

It's not Off Grid.  It typically requires about half the battery capacity needed for Off Grid, because when Off Grid you want two or more days off battery capacity before turning on your backup generator.

It's not meant to permanently sustain all loads. That's Off Grid.

What Grid Backup is, Ultimately

Freedom for the solar industry from utility mandates.  Right now we have a new commercial building in Colorado that would be 100% served by a 40kW PV system.  But the rural electric co-op utility will only allow 25kW to be grid connected.  A 15 kW Grid Backup system solves it.

Now you have the basics, and know how your business can thrive without Net Metering.

Good Luck!


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