The Challenge of Storing Energy


All sources of energy have some level of intermittency, they are not always available when needed. The reliability range is wide, utility supplied electric or gas energy seldom fails while we know that the sun is not available to operate photovoltaic (PV) or thermal systems at night. Our needs for energy also vary from critical full time (example: network computer servers), to critical when needed (example: emergence lighting), to use it while it is available with no problem with loss of operation (example: cathodic protection systems).

Key to reliable power is redundancy, having multiple sources of power. For utilities this usually means multiple generating stations and multiple transmission paths to ensure delivery of power. For applications not connected to a reliable power grid, the alternate power source can be small emergency generators or battery backup.

Most renewable energy sources, solar and wind being the main sources, are intermittent by their very nature. If continuous power is required, they need to be backed up by energy storage or generators. The most common combination is PV power systems with batteries. In these systems the PV system is designed to provide enough energy to meet the energy needs and recharge the battery. As such the size is based the worst case during the year with safety factors added for year to year differences, longest expected periods of low sunshine, and the expected degradation of equipment as it ages. The result is rather high energy costs because at many times of the year the energy produced by the PV array exceeds needs and is not harvested.

More popular these days are PV systems that are utility connected and any excess energy is either banked with the utility (net metering) or sold to the utility (net billing). Since most of these utility connected systems generate energy only when the utility is functioning, there is also the ability to design the system to include energy storage. There are many options, see the AZSC separate article on (title and article needed covering AC vs DC coupled storage, demand reduction, backfeed limiting, etc. based on 2019 products).

There are many aspects to energy storage, size can range from very small (battery or capacitor) to very large (pumped hydro), many technologies, safety aspects, to name a few.  There is no easy way of describing this wide range of technologies, but we will start with some definitions.

There are two basic terms for defining storage, Power and Energy:

Watt: A unit of Power, one Watt is the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V). Items like light bulbs are rated in watts. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energy per unit time. Electrical demand is a power term, usually measured in kilo-Watts (kW).

Watt-hours: A unit of Energy, defined as power over a time interval. One watt of power for one hour is one Watt-Hour. Often used with the prefixes Mega for 1-million (mWh) and Kilo for 1-thousand (kWh).


The suitability of a storage technology is determined primarily by its power and energy capacity and the rate at which these can be stored and delivered. Other characteristics to consider are round-trip efficiency (how much energy is lost from charging and discharging), cycle life (how many times the technology can charge and discharge at a particular depth of discharge [e.g., 80% or 100%]), safety, and ramp rate (how fast the technology can respond to a command).

Other metrics to consider include specific energy and specific power. Specific energy is a measure of energy per unit mass of an energy storage system. Specific power is defined as the power per unit mass of an energy storage system. When specific power and energy are high, the weight of the energy storage system tends to be lower per kW/kWh. See the Figure below for a comparison of specific power and specific energy for each of the many of the energy storage technologies presently available. Optimal characteristics of energy storage technology include high specific energy and specific power, but these features are often costly and not necessary for every application.

 Comparison of Specific Power and Specific Energy

The above graphic covers battery technologies from the point of view of large scale energy storage.  It was extracted from a good report that is worth reading: ENERGY STORAGE TECHNOLOGIES WHITE PAPER by the Port of Long Beach

The basic physics of energy storage

There are three basic ways to store energy; electrochemical, mechanical and electrical.

Electrochemical methods basically change one element or compound into another wherein the process is reversible (the energy can later be recovered). Examples include breaking water into its components of hydrogen and oxygen (H₂O into H₂ and O₂) and lead-acid batteries (PbO₂ + Pb + 2H₂SO₄ into 2PbSO₄ + 2H₂0), more on these later. 

Mechanical methods utilize physical forces such as gravity (pumped hydro), angular momentum (flywheels), pressure (compressed air), and heat.

Electrical storage includes capacitors and inductors. Some energy storage combine elements of these ways.


After APS explosion injures 4 firefighters, Arizona cities enact battery storage laws for utilities, homeowners

There is an interesting, but non-technical, article in the Arizona Republic about batteries in PV systems.

Most of the major Phoenix area cities have documents that detail the requirements for new solar systems and it is likely that the cities will be updating their documents soon with battery requirements.

After APS explosion injures 4 firefighters, Arizona cities enact battery storage laws for utilities, homeowners

First Solar exits the EPC business

SEPTEMBER 20, 2019

Citing the wild success of its Series 6 module, First Solar has announced that it is closing its engineering, procurement and construction business in the United States in order to concentrate on scaling, developing, and selling modules.

There is no further news on the First Solar website, but this seems to be unwelcome news for Tempe.

See the pv magazine USA article for more detail: First Solar exits the EPC business

Understanding your APS Connected Photovoltaic System

If you have a photovoltaic (PV) system connected to APS and do not have other monitoring of the PV system such as that provided by most inverter manufacturers, it is not easy to determine the solar production that corresponds with the monthly electric bill.  APS requires a solar production meter and this data is recorded and made available to the customer, but with a little effort.  The following example is based on the system APS calls RCP, utility speak called 'Resource Comparison Proxy' (note that the APS website was developed by utility personnel using their view of the situation, not the customer view). Under RCP APS purchases all excess PV output at a fixed rate. Some APS customers with older PV systems use the now grandfathered 'Net Metering' (rate rider EPR-6) wherein any excess PV production, measured in kilowatt-hours (kWh) is used to offset energy delivered by APS. For either Net Metering or RCP, the APS bills show only the measurements made by the bi-directional billing meter.  A summary of APS rates is available on our website at: Summary of APS Solar Rate Plans- 2019

Look at the recent example of an APS electric bill and see if you can determine the solar production (not shown):


This APS bill shows that 229 kWh in this case was sold to APS under the RCP rate.  The bill does not show the solar production that was directly used; to determine this, one has to have an APS web account (free) and has to go online to learn more. There is a lot of data available.  Note the 'Meter reading' dates above, this will be used later to calculate the matching solar production. Using a browser such as you are using to view this article, go to https://www.aps.com/

The APS website was recently updated.  The top of the page has a Log In.    

APS Web 1

APS Web 3

Enter your APS username/password then click login.  If you do not already have a username and password, click on 'register'. The following is typical.

APS Web 2

Next select the tab 'Billing and Payment', then click on 'View your billing and usage'.  Be patient, this may take a minute or so while the APS website loads your data.

APS Web 4


This is a very useful page. There are really two related pages, 'Energy from APS' and 'Energy to APS'. The above is 'Energy from APS'.  Further detail is available by positioning the cursor over a specific date, this produces a pop-up with details:

APS Web 5 

 In this pop-up: 

Total Produced = PV energy generated

Total Delivered is energy from APS

and the On-peak and Off-peak components

are shown.

Selecting the 'Energy to APS'  tab shows the solar components:

APS Web 6

APS Web 7 

 In this pop-up: 

Total Produced = PV energy generated

Total received is the daily excess energy sold to APS (if RCP) or net Metered to APS.


Each of these pages has a 'Create Report' button. 

APS Web 8

There are 3 options:

Download graph data: This will provide daily values

Billing and usage data:

Peak usage data: 

The 'Download graph data' option downloads a file named downloadGraphView (y).xls wherein the 'y' is used if there are subsequent downloads.  These files have a header and 35 days of data.  If prior data is needed, simply select an earlier month. Typically the data will look like this (two months illustrated):

(Note: Windows 10 identifies these files "The file format and extension of 'downloadGraphView (2).xls' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway?".  Seems that the actual format is not .xls, but Excel loads the files.

APS Web 9


The values for the billing date range, July 25 to August 23 in this example, need to be added.  This can be done with the spreadsheet program or manually.  In this example the sum is 711.5 kWh. This is not straight forward since the values shown are actually text and Excel can not directly add them.  Use the =value(cell) function in another column to convert the text to actual values.  Now that the actual solar production is known, the below  chart shows the relationship of these values.

Cald 11

The calculation of Home useage is (Solar Production) + (Purchased from APS) - (Sold or net metered to APS).