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How do you build a Home Nanogrid? Let me show you.

This is a new idea, for me. Something that I have been thinking about since I installed an ETS and switched to Time-of-day rates with the local utility. This is where you can find everything that I am thinking about, and how we can make this work.

I have created a newsletter that focuses on this subject, the place that you will get my less refined thoughts on the subject.

Below are the latest articles that I've published. Over time you will find everything you need to build your own microgrid at home, safely and securely.

Time of Day Calculations with VTScada

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Someday I hope that it will be standard for utilities to provide a API that customers can use to calculate the current rates for energy, etc. Today however, we have to do it by hand.

My last post was a straightforward energy rate with a base charge, but I have a time of day rate at the house, so my rate is dependent on the month, day, hour and whether or not its a holiday.

This solution uses some advanced scripting.

Note, this is my first pass, I am actually testing to make sure it works as I write this. Also, I wasn’t consistent with my thinking, but it works. If you have suggestions on how to clean it up, please let me know in the comments at the end.

Different Tag Types and Functions Used

To make this work I used a few different tag types and built in VTScada functions. Make sure you have the server clock set to 24hr to make this work as written.

Tags:

  • Totalizer
  • Calculation
  • Context

Functions:

What’s the active rate?

For this task I started at the end and worked backwards. I created a Calculation tag that would record the current month cost, and a totalizer tag to calculate the current energy cost.

The calculation tag is straightforward, it sums the energy cost and base charge.

The totalizer tag multiplies the current power by the active rate

The active rate is divided by 3600 to get KW-S

The active rate tag is where all the magic is. The [ActiveRate] determines which rate should be used for calculating energy cost at any moment.

There are three rates,

  • [Standard]: This is the rate you would be charged with a straight KWHR only charge
  • [Off-Peak]: This rate is used for holidays, weekends, and between 11pm-7am.
  • [On-Peak]: This rate is used during the winter only and between 7am-12pm and again from 4pm-11pm.

Calculating the Active Rate

Before creating the expression for the [ActiveRate] I created my triggers. I was originally going to use trigger tags for the various times, but decided on calculation tags and using the built-in time and date functions.

Need to update Holidays every year

With the tags here I can calculate the [ActiveRate]

What Month is it? [Month Peak]

Using the Date() function I can grab the month number. If its 12 (December) or (that’s the || ) January or Febuary, then return 1, else return 0.

You can use TRUE/FALSE, X/Y, etc for this difference, but for this type of calculation, and for how I do [Holiday] I prefer to use a number.

What day is it? [DayofWeek]

I do something similar for getting the day of the week. What I want to know is whether or not its a weekday. If its a weekday I will return a 1.

What time is it? [TimeBlock1] …

The three time blocks are similar. There are four timeblocks, but the fourth isn’t calculated. I chose to leave out the one from 11pm to 7am, it is assumed that if non of the others are true, it must be this time block.

Is it a Holiday?

To determine if its a holiday or not, I created a Holiday tag and then added all the holidays together.

I could have OR’d them together to get the same result, and may change that in the future.

Something that I want to improve is have a static [Holiday] tag by adding an expression that figures out the year and concatenates that on the reference tag. By doing that I can just add 2021Holidays, 2022Holidays, etc. and never look at it again.

The Holidays that count for [Off-Peak] rates are on the NS Power Tariff page:

I create a calculation tag for each and manually add the month day for each –

Another Way to Handle Holidays

There is another way to do this where I change the [2020Holidays] context with just [Holidays] and then OR all of them together, but I decided that was too “messy”.

This keeps the tag structure cleaner and sometimes cleanliness of code is more important than simplicity. Always think about what Future You will think about Past You. I know I question Past Me and my decisions when looking at code and designs all the time.

Putting the [ActiveRate] together

Now I have everything I need to put the [ActiveRate] together.

For expressions like this I use Notepad++ or something similar. You can format it nicely so you get all the commas, brackets, etc in the right place. When adding it directly expression box in VTScada it starts to get messy.

This expression is two nested IfElse statements. The first checks to see if its [On-Peak]. On-Peak is only valid when [Triggers\Month Peak] is true AND its in time block 1 or time block 3.

If that expression isn’t true, I check to see if its [Off-Peak]. This is any holiday (or the Monday following) OR the weekend OR the missing time block.

Finally, if its not [On-Peak] or [Off-Peak] it must be [Standard].

Looping back to the start

The active rate flows back into the totalizer we started with and we are calculating the energy cost based on the time of the day.

Learn more about VTScada

As part of the  Home Energy Management course I will have a full Getting Started with VTScada course included. Nothing in that course will rely on a paid version of VTScada, but will rely on the VTScadaLight version.

If you want to have access to this course when it is released sign up for my newsletter.

Calculations in VTScada

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You already know I like VTScada as my HMI/SCADA application of choice, and being able to add functions and calculation to tags is another reason I like it. And did you know they have a FREE (as in beer) version that you can download and work with? Its what I use for my Home Energy Management system.

Calculation and Context tags don’t count towards your tag limit, so all these tags that I’m adding won’t add towards your 50 tag limit.

Calculating cost of energy

The example I’m using is part of the energy management system that I’m putting together at the studio. This is calculating the power using the NS Power Domestic Service Tariff. I will be adding the TOD Service Tariff as part of a course I’m developing.

The domestic service tariff is:

(Monthly usage in kwh * Base Rate) + Service Charge
The Base Rate is currently 15.805 cents/kwh
The Service Charge is $10.83 / month

Step 1: Add Constants

First lets add calculation tags for the Base Rate and Service Charge. This way when they change (increase) you only have to change a single tag.

You will have to click “All Tag Type” and scroll down to Calculation

Next type in your name. I have organized my tags such that this is in a Calculations/Tariffs structure. This is using the Context tag type, something that helps with organizing tags.

Then go to the Calc tab. We are using a Constant for this tag.

Everything in dollars

To make sure that I don’t mix up dollars and cents, I’m making everything in $. The reason I chose dollars is to make the Engineering Units look clean in the Display tab

Now do the same for the monthly base charge.

Step 2: Calculate kilowatt-hours

If you have a meter that you are reading KWH directly, and it is resetting for you, you can skip this step. To keep the number of tags to my SCADA as few as possible, I am only reading kW and am using a totalizer tag to calculate my kwh.

First I create a Totalizer Tag to calculate the kw-s.

The tag type is in the header

I have the totalizer looking at the Power tag as the source.

This tag resets every month.

Step 3: Convert kWs to kWh

Then in another calculation tag I do my first expression for this example, and call it kW-HR. In this I select expression and the input box on the right.

This expression is very simple, I divide the [kw-S] tag (which is in the same “folder” as the kW-HR tag I’m creating) by 3600, the number of seconds in an hour.

When you select OK, the value will show up in the Calculation tag box. At this moment we have used 687 kWh this month.

Step 4: Putting it all together

Now for the last step, calculate the current cost of power this month.

For this we create our last calculation tag of the example:

I have placed this tag in the Monthly Power context and had to reference up a couple levels in the tag hierarchy to get to the right one, [Calculations\Tariffs\Standard] and [Calculations\Tariffs\Standard Base Charge].

The result of this equation, as of today, is: $120

Trending and adding it to a screen

I like to trend the value, and when it peaks is the total cost of energy for that month. The studio is on TOD rates, and we shift as much energy to the off-peak rates as possible, that represents a “wiggly” trend. We store heat and do laundry, dishes, etc at night increasing the rate of change, and then during the day we go back to normal.

As for adding it to a screen, I have it shown on my Overview display below the 10 minute power and voltage trends:

Where to go now?

As I work on the Home Energy Management course, I will continue to add short how-tos to the site.

As part of the Home Energy Management course, I am developing a separate course on VTScada. This course is aimed at people that already have some equipment that talks one of the communication protocols that comes with VTScada AND wants to get started with the free 50 tag version. Its the Getting Started with VTScada course, and it will be released before the Home Energy Management course.

If you want to have access to this course when it is released sign up for my newsletter. You can also sign up to be a Skills Member and get 25% off all courses as they are released, or as a free member and receive 10% off.

If you have any questions leave a comment below, you have to be signed in as a member. Its free.

Home Energy Management

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One of the great things about volunteering with IEEE is that you get opportunities to share some of your expertise with your peers. In December a colleague and I submitted a proposal to fund an Energy Management Workshop.

My office temperature

My System at the Office

A few years ago I set up an energy management system at the office. I tried a couple of the market solutions at the time, but everything was locked up in the “cloud”.

I finally decided to go the custom route and found a great company that built energy monitors called GreenEye by Brultech. Around the same time Trihedral released VTScadaLight.

With these tools I can do just about anything.

What did I do?

Back then we used Excel to connect the energy monitor to VTScada, this has some drawbacks, but it has worked well for a few years now.

GreenEye Energy Monitor

The greeneye is overkill for a lot of reasons, 32 CT channels, pulse counters, and temperature monitors.

I have 8 channels set up to monitor the following groups:

  • Heat
  • Major Appliances
    • Water Heater
    • Dishwasher
    • Stove
    • Dryer
  • Lights
  • Everything else

With this information I am able to determine how we use our home, how home upgrades affect our power usage, etc. But to do that analysis we need a recorder.

VTScada

This is where VTScada comes in. With VTScada I’m able to make all kinds of graphs showing the mains voltage, instant power, etc. I can also use the included historian to look at any combination of information that I may like.

The VTScadaLight version that I use for this has a couple of limitations, but nothing that gets in the way for a system like this. The biggest is the 50 tag limit and limit to one web connection.

If either of these get in the way for your installation, I have 200 tag licenses for sale here.

200 tag VTScada HMI Development Runtime License

Allow for two business days after purchase for delivery of license via email.

$1,600.00
Shop now

What has this system given me?

With this historical information we have been able to make the following major decisions and changes in the way that we heat the home to save operational budget in business speak.

The first upgrade was installing a heat-pump, the next was installing a electric thermal storage (ETS) so we can get on Time-of-Day (TOD) rates.

Heat-pump

The first major change was to replace our pellet stove with a heat-pump. We always thought that the pellet stove was a cheaper way to heat the house, but when we looked at the cost of pellets and electricity for the stove a heat-pump was way cheaper to heat the home.

Time-of-Day Rates

The bigger decision was switching to time-of-day rates. Even though we use the entire space all day, we would be able to save about 12% a year in electricity charges. The NSPower tariff structure is here – https://www.nspower.ca/about-us/electricity/rates-tariffs/domestic-tod/time-of-day-rates

Graph built on python

To get the TOD rates we installed an ETS that heats the studio during the off-peak hours to keep the office warm all day. We did a little additional insulating at the same time. You can see the results in the graphs above. There is a big continuous load from 11pm till early morning.

Normal Habit Changes

We made similar changes to our normal habits, including only washing dishes at night, early morning showers, clothes drying at off-peak, etc.

We will have our first bill in the coming weeks, I will make another post with our Year-over-Year changes.

Where am I going from here?

There are a few things that don’t work the way that we were hoping, one is the hot water heater. We need a new one, and I want to “tune” the heater schedule.

After this, the majority of the work needs to be done with the SCADA system. I want to be able to estimate the energy cost using calculation tags in VTScada, that don’t count to the 50 tag limit, so we aren’t surprised when the bill comes from NSPI.

Time is a fickle thing when it comes to programming.

Updating the connection

A really big change that I am working on is how we pull the data from the GreenEye into the SCADA. I want to get rid of Excel as the middle-man and use something that is more robust and “standard”. For that I’m looking at a couple of solutions:

  1. Build a custom driver for VTScada to pull the data directly.
  2. Convert the Greeneye to Modbus on the server

I’m pretty sure I will be focusing on the second option for the workshop and then working on getting the first option figured out for future installations. There are a couple of GitHub projects that I think will be able to make it work, jkeljo/greeneye-monitor and riptideio/pymodbus. One gets the GreenEye data into a python script and then I can convert that to modbus that will connect with VTScada.