Finding and valuing overnight baseload

A building's baseload is the demand that remains when nobody is working. Part of it must run: refrigeration, server rooms, fire and security systems, frost protection, and ventilation that keeps a space safe. Part of it need not: lighting left on, air-handling units held in occupied mode by an override, and equipment on standby. Lighting and air-handling schedule faults are fixed in the BMS. Compressed air leaks and a missing frost interlock need maintenance or controls work. Remove a load only when the evidence shows that it is avoidable.

Read the quiet periods

Use interval data at 15 minutes or finer to value the night load. Monthly totals cannot separate night from day. Neither 15-minute nor half-hourly data shows a cycle shorter than the interval. A compressor that runs for 5 minutes in every 10 averages to a flat line at half its running power in every interval. To see the cycle, meter the circuit that feeds it at 1-minute intervals or finer.

Split the data into weeknights, weekends, public holidays and unusual events such as a shutdown or a night of maintenance. Compare like with like. Convert the timestamps to local time before you select the quiet window. If you select it in UTC, the window moves by one hour at each clock change, and the morning start-up falls inside it.

To measure the level, use percentiles. The Lawrence Berkeley National Laboratory load-shape study takes the 2.5th percentile of each day's interval demand as the near-base load and the 97.5th percentile as the near-peak load. Do not use the minimum. One outage or one meter dropout sets the minimum, and it then tells you nothing about normal operation.

The ratio of near-base to near-peak load is a quick test of whether the base load needs an explanation. The study gives the median weekday values for four buildings in Martinez, California, from spring to autumn 2008:

The juvenile hall and the jail are occupied around the clock, so a high night load is expected. Office A has a higher ratio than either of them. An office near that ratio, with no server room or process load to explain it, is worth a walk round at night.

To read the shape, compare it with these patterns:

A flat 20 kW and a load that cycles between 0 and 40 kW at a 50% duty cycle use the same energy in a night. They point to different equipment.

Put a value on an avoidable load

Value only the part that you have shown is avoidable, not the whole night load. Build the hours from the real occupancy, not from a count of nights.

Take an office occupied from 07:00 to 19:00, Monday to Friday. It is occupied for 60 of the 168 hours in a week, so it is empty for 108 hours a week, or 5,616 hours in 52 weeks. Ireland has ten public holidays. If the office closes on one weekday for each of them, the ten days of 12 hours add 120 hours. The total is 5,736 hours, 65% of the year. The day left over after 52 weeks adds 12 or 24 hours more, depending on which day it is. An avoidable 8 kW over those hours:

8 kW × 5,736 h = 45,888 kWh a year

At €0.25 per kWh that is €11,472 a year. A count of weeknights alone, 12 hours on 250 nights, gives 3,000 hours and 24,000 kWh. It misses nearly half of the waste.

Price each hour at the rate that applies to it. The ESB Networks use-of-system tariffs for interval-metered business customers set the night period at 23:00 to 08:00, summer and winter. Take a tariff with a night rate in that band and one day rate for all other hours, and ignore the 17:00 to 19:00 peak band. In the office above, each weekday has 8 empty hours in the night band and 4 outside it, from 19:00 to 23:00. Each weekend day has 9 and 15. Each holiday adds 1 night hour, from 07:00 to 08:00, and 11 day hours. At an assumed €0.15 per kWh at night and €0.25 in the day:

The flat-rate figure overstates this saving by €2,421, or 27%. The FEMP guide to utility rate options explains how time-of-use rates are built. Neither figure includes standing charges, demand charges or the cost of the change. The electricity cost calculator adds the other parts of a bill.

Check reactive energy before you remove a large night load. For maximum demand customers, ESB Networks applies a low power factor surcharge to the kVArh above one third of the kWh in a billing period, which is a power factor of about 0.95. Transformer magnetising current and lightly loaded motors draw reactive power all night. If a change removes resistive load such as lighting or heaters, the kWh fall and the kVArh do not, so the ratio rises.

For a cycling load, use the mean power over complete quiet windows. Do not use the nameplate rating or a single peak. A window that holds many cycles keeps the error from the part cycles at its ends small: a 12-hour window holds 36 cycles of 20 minutes. Take the mean over several matched nights, so that one unusual night does not set the value.

Trace the load to equipment

Start from the whole-building profile. Add metering where it separates one explanation from another: the main boards first, then the circuits on the board with the largest night load. Do not add the reading of a board meter to the readings of the circuit meters downstream of it, because that counts the same load twice. The difference between the board and the sum of its metered circuits is the unmetered remainder. The submetering guide explains how to choose the points.

If the billing meter is on the medium-voltage side of a site-owned transformer, the night reading also includes transformer losses that no circuit meter sees. The no-load loss runs whenever the transformer is energised. The Tier 2 limit in Regulation (EU) No 548/2014 for a new 1,000 kVA liquid-immersed transformer is 693 W, which is about 6,070 kWh a year. An older transformer can lose more. The load loss follows the square of the current. At 20% load it is 4% of the full-load figure, 304 W against the Tier 2 limit of 7,600 W. Count both in the unmetered remainder. They are not a fault.

Check each new meter for negative power on any phase before you use its data. A current sensor fitted the wrong way round subtracts from the total and can hide a night load. The negative power guide explains how to find it.

Check these first:

• Does demand fall when the cleaners or the last shift leave?
• Do the ventilation and heating schedules match the real occupancy, including holidays?
• Is heating or cooling running during a shutdown?
• Was a manual override left on after maintenance?
• Is the standby load many small devices, or one circuit?

On the night walk, look at the air-handling units, trace heating and frost protection without an outside-temperature interlock, hot water heated around the clock, and compressed air.

On a site with compressed air, the night load of the compressor is a leak test. With all the air-using equipment off, the compressor loads and unloads only to replace the air that leaks away. The DOE and Compressed Air Challenge fact sheet on leaks gives the leakage as a share of compressor capacity:

Leakage (%) = (T × 100) / (T + t)

T is the average on-load time and t is the average off-load time, both in minutes. A meter on the compressor circuit gives both from its power profile. A compressor that loads for 3 minutes and unloads for 9 is losing 25% of its capacity to leaks. The fact sheet puts leakage below 10% in a well-maintained system, 20% in a typical plant that is not well maintained, and up to 30% in a poor one. It recommends the test every quarter.

A correlation with a schedule or an event tells you where to look. Before you propose a change, confirm what the equipment is and why it runs. Some night loads are there on purpose: a cold store that holds product temperature, a heater that protects a sprinkler pump room from frost, or a server room that runs a hosted service.

Verify the change

Before the change, record the schedule, the equipment state, the outside temperature or production, and the measurement period. Agree the change with the person who owns the equipment.

Measure the saving on the board or circuit that the change affects, not on the main meter, which mixes in every other change in the building. This is Option B, retrofit isolation with all parameter measurement, in the FEMP M&V guidelines. The guidelines ask for a measurement period that covers the full range of operating conditions. For a night schedule change, we use at least four matched weeks before and after the change. That gives about 20 weeknights and 8 weekend days in each period. For heating or frost protection, normalise for weather with heating degree days, or compare only nights with similar outside temperatures.

Compare whole days, not nights. A lower night setpoint can mean a harder warm-up and a higher peak in the morning. If the night energy falls by 40 kWh and the 06:00 to 09:00 warm-up rises by 15 kWh, the saving is 25 kWh a day. A higher morning peak can also increase a demand charge. The energy savings guide explains how to separate a raw change from an adjusted saving.

Turn a study into monitoring

Set an expected range for each quiet period, and write down the schedule that it assumes. Calculate the mean demand over the quiet window for each of the last 20 matched weeknights, which is four weeks. Set the upper limit at the highest of those values plus a margin, such as 10%. The highest value marks the edge of what the site has shown to be normal, and the margin allows for a night slightly outside it. Use a wider margin for a load that follows the weather.

The method has one weakness. One faulty night in the 20 raises the limit until it leaves the window. Remove nights with a known fault before you calculate. Give weekends their own range. Reset the range after each approved change. If you do not, a load that creeps back to its old level stays inside the old limit.

When the night load goes out of range, send an alert to a named person. The alert gives the circuit, the measured kW, the expected range and the period. An alert that gives only the building total does not tell the reader what to check.

Metering and alerts with ZEM and Edge

ZEM electricity monitors meter the boards and circuits that make up the night load. A ZEM sends its readings over Zigbee, so no data cable runs back to a panel. A qualified electrician fits its current sensors and its voltage reference. It also needs an auxiliary supply of 85 to 480 V AC.

A Gateway running Edge keeps the interval history on site and charts it by circuit. An Edge automation rule on the power of a circuit can raise an alert and send an email. You can limit the rule to a window that runs past midnight, such as 19:00 to 07:00 on selected days. Edge evaluates the window in the Gateway's local time, so it does not move at a clock change. Set the rule to wait longer than one normal cycle of the load before it raises the alert, so that one compressor start does not trigger it. A separate data-quality rule raises an alert when a meter's data goes stale, so a silent meter is not mistaken for a steady load.