There is no screen you can buy that reads BACnet and tells the story. Poll five to eight meter objects over BACnet/IP every 60 seconds into a small store, render a summary page that refreshes every 5 minutes, and size the panel at 20 ft of viewing distance per inch of capital height. In a 25 ft lobby that is a 24-inch-class 16:9 screen.
A real time energy dashboard in a school lobby is not one product you order. It is four small parts that meet in a single screen: a written list of meter readings from the building management system, a poll that fetches them every 60 seconds, a stored summary that refreshes every 5 minutes, and a panel sized to the lobby's farthest reader. None of it needs a controls engineer on staff. The controls bidder supplies the object list, the poll and the summary page are a day of scripting, and the screen is an ordinary lobby display. This page gives the number for each decision and a worked example for a 25 ft lobby.
The short answer
Decide three things: how the data leaves the building system, how often the screen updates, and how big the screen is.
The data path is a poll. Read five to eight meter objects over BACnet/IP every 60 seconds into a small store on the school's network. Use the BMS's own dashboard page on the screen only if that page needs no login and already tells the story; many sit behind a login, because they are built for the maintenance team.
The update is slow on purpose. A campus energy figure moves little in five minutes, so render the summary page every 5 minutes during school hours and print an 'as of' time beside each figure.
The screen is set by the room, not by the data. Plan 20 ft of viewing distance per inch of capital letter height for walk-past reading, and a 16:9 panel comes out around 24 inches diagonal for a 25 ft lobby. Our lobby and reception displays are built for flush cut-outs in portrait or landscape; the details to confirm in the quote are listed at the end of this page.
What to read from the building system
BACnet is the data protocol building controllers speak, published as ASHRAE Standard 135; BACnet/IP carries it over the building's ordinary Ethernet. Every reading on the system lives in an object with a type and an instance number, and the live value is the object's present value. The simplest client is a poll: ask for the present value on a schedule and store what comes back.
Five to eight objects carry the whole story:
- Building-wide electricity, in kW, from the main meter
- Today's consumption, in kWh, from the same meter
- Last week's kWh, for the comparison the screen repeats
- Solar output, in kW, if the campus has an array
- Outside air temperature, in degrees F or C, so viewers can connect a cold morning to a high figure
Poll the set every 60 seconds. A building total barely moves inside a minute, and six read requests a minute is a light load on the controller. Get the object list in writing from the controls bidder during bidding, with the type, the instance number and the unit of each present value. If you want to see the network before the bid, ask the controls bidder which BACnet discovery and testing tool they use; it walks the devices and shows every object and its properties.
Ask IT for a read-only path. The display's poller must have no way to write a value into the building system, and the same isolation pattern applies whenever a screen pulls web data next to controls; our article on displaying web weather data without exposing the PLC shows how to keep the two networks apart.
Three ways to get the data on the screen
| Path | What you need | Screen refresh | Where it wins |
|---|---|---|---|
| Show the BMS dashboard page | A dashboard URL the school already uses, with no login | Whatever the page refreshes | Fastest to ship; nothing new to maintain |
| Poll BACnet/IP into a small store | The object list, a 60 second poll script, a small database, a summary page | Every 5 minutes | You control the story, and the archive supports week and month comparisons |
| Require a data feed in the controls bid | A read-only feed of the object list written into the controls contract, plus a custom page | Every 5 to 15 minutes | Cleanest long-term setup; every bidder prices the same scope |
A web page on a wall screen is a solved problem: the same steps that put a web OEE dashboard on a factory TV put the BMS page on a lobby screen, and the article on showing a web OEE dashboard on a factory TV walks through them. The login is the usual failure; a page that asks for a username will sit at its login prompt in front of every visitor. How a signage player handles a page that updates itself is covered in the article on displaying a live dashboard on a signage screen.
Pick something else when the simple path fits. If the BMS page is login-free, readable from the back of the lobby and already tells the story, show it and skip the poll. If the school later wants room-by-room drilldown, that belongs on the web dashboard, and the feed path in the bid is the one to grow into.
Sizing the screen for the lobby
Letter height decides the screen, and viewing distance decides letter height. The planning figure used for highway signs, published by FHWA, is 30 ft of viewing distance per inch of letter height. Lobby reading is walk-past reading, so use the safer rate: 20 ft per inch. Lines sit at 1.7 times the capital height, center to center. The full method is in our guide to screen size and viewing distance.
Two formulas carry the whole calculation:
- Capital height in inches = viewing distance in ft divided by 20 (divide by 30 if people stand and read)
- For a 16:9 panel, diagonal in inches = panel height x 2.04
Worked example: a 25 ft school lobby
The farthest reader stands 25 ft away, at the door to the corridor.
Capitals: 25 ft / 20 ft per inch = 1.25 in for the data lines. The title runs larger at 2.0 in and the 'as of' line smaller at 0.63 in.
Line pitch at 1.7 times capitals:
- Title: 1.7 x 2.0 in = 3.4 in
- Three data lines: 1.7 x 1.25 in = 2.13 in each, 6.39 in for the three
- Timestamp line: 1.7 x 0.63 in = 1.07 in
Text block: 3.4 + 6.39 + 1.07 = 10.86 in. Add 0.6 in of margins: panel height 11.46 in.
Panel size at 16:9: width 11.46 in x 16 / 9 = 20.4 in; diagonal = the square root of (11.46 squared + 20.4 squared) = the square root of (131.3 + 416.2) = the square root of 547.5 = 23.4 in.
The decision: a 16:9 panel of about 23.4 in diagonal, the 24-inch class, carries this layout for walk-past reading at 25 ft. The lobby range runs to 23.8 inches, so it fits; the display sizes and options go into the quote with the cut-out dimension. If readers stand still, the standard FHWA rate gives 25 / 30 = 0.83 in capitals, the same block scales to about 7.2 in, and the screen lands at 15.9 in diagonal, the 16-inch class. A lobby deeper than about 30 ft pushes capitals past 1.5 in and moves you to a quoted wall model from the current range. Two checks before ordering: match the brightness option to the glazing, because tall windows can wash out a standard panel for part of the day, and confirm orientation and power in the quote.
What the numbers should say
Three to five figures, each labeled with its unit, beat a wall of charts. A school lobby reads well with: kW right now, kWh so far today, kWh this week against last week, and outside air temperature. kW and kWh are different animals: kW is the rate at this moment, kWh is the total consumed, and a figure without its unit teaches the wrong lesson.
Put an 'as of' time beside each figure so the age of the data is on the wall. Keep the archive behind the page: a week-to-week line tells a fairer story than the moment's number, because weather and bell schedules move a single day around.
What to ask for in the bid
The project is bid with the controls, so write both halves of the scope into the documents.
From the controls bidder:
- A written list of five to eight objects: BACnet type, instance number, and the unit of each present value
- Read-only BACnet/IP access on a network segment IT approves, with no write path to the building system
- Confirmation that a 60 second poll of that list is acceptable load on the controllers
From the screen vendor:
- The screen size worked from your lobby's farthest reading distance; send the distance in ft
- The brightness option matched to the glazing in the space
- Flush cut-out dimensions, orientation and power, confirmed in the quote
Ask for the same object list from every bidder so the quotes price the same scope.
Recommendation
Recommendation: Poll five to eight meter objects over BACnet/IP every 60 seconds into a small store, render one summary page that refreshes every 5 minutes during school hours, and size a 16:9 screen at 20 ft of viewing distance per inch of capital height, which puts a 25 ft lobby at the 24-inch class. Take the BMS dashboard-page path only if its page needs no login and already tells the story, and write the read-only object list into the controls bid so every bidder prices the same scope.
Questions
Does the lobby energy display need touch input?
No. A lobby energy display is read from across the room, so touch adds cost and a glossy surface for no benefit. Touch belongs on a kiosk people stand at arm's length from, like a wayfinding screen. If students later want to dig into per-building numbers, put that detail on the dashboard's web page and keep the lobby screen to three or five figures.
How much data does the BACnet poll collect?
Six objects polled every 60 seconds produce 1,440 readings per object per day, 8,640 stored values in total, and about 3.2 million values a year. With timestamps that is a few tens of megabytes, so the school's smallest server holds years of history. Keep the archive: it is what lets the screen show this month against last month, and this year against last year.
What happens when the link to the building system drops?
Build the page with an 'as of' timestamp beside every figure, and design it to hold the last stored values if the link drops. The timestamp shows each number's age, so a stale reading is visible instead of silently wrong. The poller resumes on its next cycle once the network is back, and nothing on the display needs touching.
Can the screen show energy use per student?
Yes, and it is often the figure students remember. Divide the day's kWh by enrollment: a campus using 12,000 kWh in a school day with 600 students shows 20 kWh per student. Print the two inputs in small type beside the result so the figure stays meaningful when enrollment or the bell schedule changes.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.