Solar and Storage

thumbnailThe growing frequency of extreme weather events and the threat of a significant earthquake in Utah drives the need for resilient backup power. Solar panels paired with battery storage not only provide robust backup power in the event of an emergency but also help manage day-to-day energy usage and costs. Solar and storage systems are versatile, scalable, and can be paired with a traditional backup generator to provide backup power for critical facilities that require uninterrupted power supply such as hospitals, communication centers, and community emergency shelters.

If you are considering solar panels, it is important to understand the key differences between grid-tied and off-grid systems. Grid-tied systems allow you to produce solar energy to offset electricity purchased from the grid, but do not allow you to store energy for later use. Owners of a grid-tied system complete an interconnection agreement with their utility which allows them to send excess energy to the grid and receive credits which can be used to offset future purchases from the utility. The specific details of interconnection policies and agreements vary by utility. Living with a grid-tied solar PV system is no different than living with utility electricity, except that a portion of the electricity you use comes from the sun. A grid-tied system is the most cost-effective way to generate solar electricity at home, however grid-tied systems do not provide protection from power outages. When the electrical grid fails, the inverters on grid-tied systems are configured to shut down. This allows utility employees to fix the power lines safely without worrying that solar energy systems are still feeding electricity into the power lines. Learn more about grid-tied solar installations.

Although grid-tied systems are the most popular kind of solar installation, the cost of battery storage is falling rapidly which makes solar combined with storage an increasingly viable solution for both reducing energy bills and producing emergency backup power. Learn more about solar and storage using the tabs below or download the full PDF report.


The cost to install solar has fallen about 75% since 2006[1], and solar installations are an increasingly popular way to save money on utility bills. Battery storage costs have fallen by nearly 80% since 2010, making solar with storage an increasingly economic solution for energy management in addition to emergency power.[2]

As you consider solar for your facility, this guide will help you understand how you can incorporate storage into your project or make your project ‘storage-ready’ such that storage can be incorporated cost-effectively in the future.Utah’s solar market continues to grow, and planning for storage by building storage-ready projects opens the door for future cost savings. Understanding best practices for solar and storage systems will prepare facilities to incorporate solar and storage into new construction, scheduled renovations, or even retrofits as storage costs continue to fall.

 Solar-Industry-Prices-2014    Battery Price Projections Chart
The cost of solar energy has fallen more than 75% since 2006. [3]
   The cost of lithium ion batteries is expected to decline rapidly. [4]


[1] GTM Research & Solar Energy Industries Association, U.S. Solar Market Insight 2015 Year-in-Review, March 2016. <>.
[2] McKinsey & Company, “Electrifying insights: How automakers can drive electrified vehicle sales and profitability,” January 2017 <>. 
[3] Solar Energy Industries Association Q2 2015 Solar Market Insight Fact Sheet, <>.

[4] Rocky Mountain Institute, The Economics of Grid Defection, <>, Page 24.


The following considerations will help you install solar and be ready to add storage to your project.

1. Gather data about your energy usage:

In order to optimize your solar and storage system, you will need to understand your building’s energy usage in detail – ideally on an hourly basis, for commercial customers, or a 15-minute basis, for some residential solar customers. [5] Rocky Mountain Power does not provide detailed load data for customers, so if your building’s electricity usage is not already monitored on an hourly basis, identify energy service companies or monitoring equipment that can gather hourly consumption data for 12 months. If hourly load data is not available, gather hourly load data from time periods where energy use deviates from the norm (for example, summer months when peak load is high) and extrapolate for other times of the year, or look for proxy data from a facility with similar operating characteristics.

2. Evaluate and isolate critical loads:

In order to evaluate the emergency benefits of solar and storage, it is important to have data about the energy usage of critical loads. Consult with your facility manager to identify critical functions (including energy and demand needs for each function). Ideally, critical loads should also be isolated on the same circuit. Isolating critical loads during construction or renovation will prepare your facility to add solar and storage at a later date.

3. Determine your backup power goals:

Solar and storage systems can provide backup power for key critical loads, to provide power to an entire facility, or to provide supplementary power to extend the life of a backup generator. Once you have gathered data about your energy usage and critical loads, you can estimate sizing for a solar and storage system to power your facility using tools like the SolarResilient calculator [6].

4. Identify a location for the batteries which is of sufficient size and well ventilated

Batteries must be located onsite and must be directly connected to the solar installation.  The size of the batteries will depend on the battery technology and the anticipated power needs of the building.  Electrical code requirements for batteries address safety concerns and require batteries to be kept on appropriate racking in a well-ventilated location. [7] Anticipate the location of battery storage and make accommodations during construction or renovations.

5. When installing solar, choose a battery-ready solar inverter

Existing solar installations can be retrofitted with battery storage more easily if they include inverters that have the additional functionalities required to integrate battery storage. For more information, refer to the Technical Options section below.

6. Refer to Clean Energy Group’s “Solar+ Storage Project Checklist.” 

This checklist is designed to help building owners and developers solar and storage battery systems. [8]

[5] Commercial customers are typically subject to demand charges or peak or time-of-use energy charges, in which case it is important to have hourly load data. Residential customer rates do not vary hourly, however residential customers who install solar through the Transition Program (after November 14, 2017) will see their generation and consumption netted on a 15 minute basis. For this reason, 15-minute load data is preferable to evaluate the economics of battery storage.

[6] SolarResilient <>.

[7] National Fire Protection Association National Electric Code 70, Article 480 Storage Batteries <>.

[8] Clean Energy Group, “Solar + Storage Project Checklist,” <>.


Solar Panels

Solar panels provide power for a solar and storage system. Solar panels generate direct current (DC) power which must be converted to alternating current (AC) power to provide usable power for a building. Solar panels can be located on rooftops, carports, other structures, or even stand alone in open areas. 


There are several factors to consider when selecting a battery for a solar and storage system, including cost, energy density, expected lifespan, and safety. All batteries store DC power.


  • Lead acid batteries are the oldest rechargeable battery technology and are commonly found in automobile engines. Whereas car batteries are designed to remain near full charge, lead acid batteries designed for storage are designed to withstand repeated charging and discharging, but they are larger in size and have a shorter lifespan compared to lithium ion batteries.
  • Lithium ion batteries are commonly used in laptops and electric vehicles and are lighter and smaller than lead acid batteries. There are several types of lithium ion battery compounds currently on the market. Lithium ion batteries have a longer lifespan than lead acid batteries and they can be charged and discharged more frequently. Proper installation, maintenance, and use of lithium ion batteries is important to avoid overheating, which can create a fire hazard.
  • Flow Batteries are a new type of rechargeable battery. Flow batteries consist of two liquid electrolyte compounds which are pumped across a membrane in one direction to produce electricity and in the opposite direction to charge the battery. Flow batteries are very safe because the electrolytes are stored in separate tanks. They can be cycled 10,000 or more times, however they are currently more expensive than lithium ion and lead acid alternatives.


Recycling batteries

Some of the batteries used for storage contain toxic metals, and proper recycling is important to prevent pollution and avoid environmental impacts.

    • Lead acid batteries are recycled more than any other consumer product in the country. Disposal of lead acid batteries into landfills is illegal in most states.[9] During the recycling process, lead can be easily extracted and reused  multiple times. Recycling centers must first remove combustible material using a gas-fired thermal oxidizer and must mitigate pollution created by the process of burning using scrubbers. [10]

    • Lithium ion batteries do not pose as significant an environmental concern but there are benefits to recycling them. Lithium ion batteries are composed of metals that have little or no recycling value such a cobalt, nickel, and manganese, so the economics of recycling these batteries isn’t favorable. However, as increasing numbers of lithium ion batteries enter the market, recycling of lithium ion batteries is expected to be one of the main sources of future lithium supply.

Charge Controllers

A battery charge controller regulates the DC power produced by the solar array to prevent overcharging the batteries. If the power input to the battery is not controlled it can result in damage to the batteries and poses a safety hazard.


Solar inverters are used to convert DC power produced by solar panels (or the DC power that is stored in batteries) to AC power. A grid-connected solar and storage system must have a specific kind of inverter if it is to provide backup power in the event of a grid failure. A standard solar inverter is designed only for converting DC power to AC power, and it will shut off in the event of a grid failure to protect lineman working on the power lines.

In order for a solar and storage project to function both on and off the grid, the inverter must be able to provide several functions. It must be able to monitor and communicate grid status, convert DC electricity produced by solar panels to AC electricity, provide DC electricity to charge the battery, convert DC electricity stored in the battery to AC electricity for onsite use, and curtail power production from the solar panels as needed to prevent damaging the battery.

      • Dual inverters are used in a DC-coupled solar and storage system and can accomplish all these functions with a single inverter. A DC-coupled battery stores the DC power produced by solar panels without conversion and can also convert the power to AC for use in a building. Some dual inverters, known as Grid Forming Inverters, can also regulate voltage and frequency when the solar and storage system is isolated from the grid. When installing a solar project, choosing a Dual Inverter or Grid Forming inverter for the solar installation will allow for the future addition of storage at a lower cost. See Figure 3, below.

      • Grid-tied inverters are used for grid-tied solar systems, and cannot provide islanding or backup functionality. Grid-tied inverters can be used to convert DC battery power to AC power for use in homes or buildings as long as they remain grid connected.

      • Stand-alone inverters are used for off-grid applications. These convert the DC power from the solar panels and battery to AC power for use in homes or buildings that are not connected to the grid.

An existing solar installation that does not have a Dual Inverter must be retrofitted to accommodate storage by either replacing the existing inverter with a Dual Inverter or adding AC-coupled batteries. AC-coupled batteries store power after it has been converted to AC power by a standard solar inverter. A second battery inverter is required to convert the AC power back to DC in order to charge the battery, and to reverse the conversion when the battery power is needed to charge the building.

While this configuration is necessary to retrofit a grid-tied inverter with storage, an AC-coupled system is less efficient than a DC-coupled system. For this reason, it is recommended that all inverter options are evaluated when installing solar. If battery storage capability is desired in the future then a storage-ready Dual Inverter is likely more cost effective in the long term.

DC Coupled Solar and Storage    AC Couples Solar and Storage
 DC-Coupled Solar and Storage System: 
A single battery inverter converts energy to charge batteries and power the building.[11]
AC-Coupled Solar and Storage System: 
A grid-tied inverter converts DC energy to AC energy.  A second battery inverter converts AC power to DC to charge the battery.[11]


[9] Waste Management World, “The Lithium Battery Recycling Challenge,”

[10] Battery University, “How to Recycle Batteries,” <>

[11] Clean Energy Group, Solar + Storage 101: An Introductory Guide to Resilient Solar Power Systems”



   A microgrid is scalable to serve a single customer or a larger section of the distribution system.[11] 

In order to project a facility from grid outages, a solar and storage system must be able to isolate from the grid and operate autonomously.  A microgrid is an energy system of interconnected loads that consists of one or more form of distributed generation and may also include energy storage that can function while connected to the grid and can also function during grid outages by providing resiliency benefits or emergency power.[12] Microgrids can be utilized to power critical loads on a single circuit, in a single building, or across an entire campus. A microgrid can act as a single controllable entity and can operate in either grid-connected or islanded mode.[13]

Solar and storage can be integrated with generators to extend the life of existing backup power sources. In this case, to maintain generator reliability during a grid outage and to control system voltage and frequency, at least one generator must run at all times, at a minimum of 30% of its rated capacity.[14] Additional generators can be ramped up or down in accordance with changes in load and solar energy output.

Additional information about resilient solar hardware components and systems can be found in the NY Solar Smart DG Hub Hardware Factsheet.[14]

[13] U.S. Department of Energy Office of Electricity Delivery & Energy Reliability

[14] CUNY, NY Solar Smart DG Hub, “Hardware Fact Sheet.”

[15] Ibid.



Solar energy systems are an increasingly popular choice for electricity customers who want to reduce their monthly utility bill and generate clean energy on site.  When paired with battery storage, the benefits of solar are multiplied.  Solar and storage systems can provide a variety of services, from resiliency benefits like emergency power to economic benefits like utility bill savings.  The design of a solar and storage system will depend on the intended function (or functions) of the system.  Solar and storage systems can be broadly grouped into those designed to provide off-grid power and those designed to provide grid-connected power.  Grid-connected solar and storage installations can access a wide variety of resiliency and economic benefits.

implementation models














The City of Houston purchased 17 solar powered shipping containers that can be dispatched as needed in the event of an emergency, such as a hurricane, that disrupts the power grid. The containers function as mobile microgrids that can be used to provide emergency power for charging critical devices or keeping medications cool.  During non-emergency times, the containers will be used to provide mobile power for the Houston Parks Department or for special events.

System size: 3.5 kilowatt solar array



Florida’s SunSmart Emergency Shelter program equipped more than 100 public schools with solar + storage microgrid systems that can power lighting and electrical outlets at the schools if the grid is disrupted by a storm.  Each school can provide emergency shelter for 100 – 500 people.  During normal operations, the schools are able to use the solar panels to offset daily electricity usage and save $1,500 - $1,600 annually.

System size: 10 kilowatt solar array



The Hartley Nature Center in Duluth, Minnesota retrofitted an existing solar array with battery storage to improve resilience and economic security. During the summer of 2017, a power outage forced the center to cancel a week of youth camps, resulting in $14,000 of lost revenue.  If the grid goes down, the battery storage can power basic business operations, allow the building to serve as a community shelter, or even serve as a base of operations for the city’s emergency response efforts.                                               

System size: 13 kilowatt solar array

 hartley nature center


Green Mountain Power built the Stafford Hill Solar Farm to improve resilience following hurricane Irene in 2011. Solar panels, battery storage, a microgrid can provide backup power to a public emergency shelter at the Rutland City High School. In its first year of operation, the project shaved enough peak demand to save its customers approximately $200,000 during a single hour.

System size: 2,500 kilowtt solar array + 4,000 kilowatt battery

 stafford hill solar farm


Green Mountain Power plans to deploy 2,000 Tesla batteries at their customer’s homes for a fee of $15 a month or a one-time fee of $1,500. Customers who choose to participate will benefit from backup power when the grid is down, and the program is estimated to reduce the utility’s peak load by up to 10 megawatts (the equivalent of taking an average of 7,500 homes off the grid.)

System size: 13.5 kilowatt-hour battery




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