Watts to Megawatts for Street Lighting Inventories

Adding Up a Street Lighting Fleet in Megawatts

A council's lighting asset register is a very long list of very small numbers: tens of thousands of lanterns, each rated in tens or hundreds of watts. On its own no single fixture is worth discussing. Multiplied by the inventory it becomes a megawatt-scale load that lands in the authority's carbon report, its energy budget and its unmetered supply declaration — and getting from one to the other is a single move of six decimal places.

Conversion factor: 1 W = 0.000001 MW, so divide the watt total by a million. Twenty thousand lanterns averaging 100 W of circuit power draw 2 000 000 W while they are lit, which is 2 MW — the figure that belongs in the report, not the twenty thousand.

What Goes Into the Inventory Total

The GIS asset register

A lighting audit walks every pole and records position, lamp wattage, lantern and column type and condition. The wattage column of that database is what the megawatt figure is built from, so its accuracy sets the accuracy of everything downstream.

Circuit watts, not lamp watts

A 150 W sodium lamp on a magnetic ballast pulls closer to 171 W from the supply. LED drivers are kinder but still take a few percent. Use the circuit figure or the fleet total lands low by roughly a tenth.

Unmetered supplies

Street lighting is rarely metered lantern by lantern. The supplier bills from a declared inventory multiplied by a switching regime, which makes the wattage column a financial document as much as a technical one.

Part-night and dimming regimes

Many authorities dim to 50 or 70 percent after midnight, or switch side roads off entirely. That changes the energy, not the connected megawatts, so keep the two figures in separate columns.

Turning a Lantern Count into a Reportable Figure

Total first, convert once. Working fixture by fixture buries you in leading zeros and invites a slipped decimal on every line.

Multiply count by circuit watts, per lamp type

Group the register by lantern type — 8 400 sodium 70 W, 3 100 sodium 150 W, 6 500 LED 60 W — and work out the watts each group draws before anything is converted.

Enter the grand total in the watts field

Type or paste the summed figure; the megawatt value appears beside it as you type. Spaces in a pasted number are ignored, and a comma is accepted as the decimal mark for European spreadsheet exports.

Reverse it to test a reduction target

When a decarbonisation plan sets a ceiling in megawatts, press the swap button (↔) for MW → W and see the watt budget it leaves you: a 1.5 MW cap is 1 500 000 W to share out across the inventory.

Lift the clean number into the report

The copy button hands over the digits alone, with no unit and no spacing, so the figure drops straight into a spreadsheet cell or a carbon-reporting template. Ctrl + C inside a field does the same.

Very small values switch to scientific notation: anything under 1 W lands below 0.000001 MW, so a 0.5 W control node comes back as 5.000000e-7 MW. That is correct but unreadable on a schedule — for single components stay in watts, and convert only the fleet total.

Lantern Wattages and the Citywide Load They Add Up To

Circuit power per lantern including ballast or driver losses, with the megawatt total for a small town's five thousand columns and for a city fleet of twenty thousand.

Lantern type	Circuit power	5 000 units	20 000 units
High-pressure sodium, 250 W	283 W	1.415 MW	5.66 MW
High-pressure sodium, 150 W	171 W	0.855 MW	3.42 MW
Mercury vapour, 125 W	137 W	0.685 MW	2.74 MW
High-pressure sodium, 100 W	117 W	0.585 MW	2.34 MW
High-pressure sodium, 70 W	83 W	0.415 MW	1.66 MW
LED main road, 100 W	105 W	0.525 MW	2.1 MW
LED collector road, 60 W	63 W	0.315 MW	1.26 MW
LED residential, 30 W	32 W	0.16 MW	0.64 MW

Read the two right-hand columns against each other and the case for a retrofit writes itself. Swapping twenty thousand 150 W sodium lanterns for 60 W LED heads takes the same streets from 3.42 MW down to 1.26 MW — a saving of 2.16 MW, or 108 W per column, from a change that looks trivial on any single pole.

What Helps While You Work Through an Asset Register

Type into either column of the audit

Both fields accept input and the other keeps up, so you can push a watt subtotal in one moment and pull a megawatt target apart the next without resetting anything.

Swap when the target arrives in MW

One press turns the page around to MW → W, the direction a corporate reduction target has to be read in before it can be divided across lamp types.

kW for the feeder pillar in between

Searchable dropdowns on both sides carry every power unit, so the circuit behind one feeder pillar can be read in kW while the whole authority stays in MW.

Eight decimals for fractional megawatts

A lighting total is nearly always a fraction of a megawatt, and results carry up to eight decimal places before the display falls back to exponent form, so 0.00016 MW stays legible.

Street Lighting Audit Questions

How much does an LED retrofit take off a town's lighting load?

Published schemes cluster around a 50 to 60 percent cut in circuit watts per lantern, and the table above sits squarely in that range. Twenty thousand columns moving from 171 W to 63 W shed 2.16 MW. On a dusk-to-dawn regime of roughly 4 100 burning hours a year that is close to 8 900 MWh saved annually, before any night-time dimming is counted.

Why does the invoice not fall by the same percentage as the megawatts?

Because a bill is energy, and load is only half of it. Energy is power multiplied by burning hours, and lighting hours are seasonal — a winter night runs to fifteen hours while a midsummer one barely reaches seven. Standing charges, availability charges and the fixed cost of the unmetered supply agreement do not move at all when the lanterns change, so a large cut in connected load usually shows up as a smaller percentage on the invoice.

Is it worth counting the half-watt a photocell or control node draws?

At fleet scale, yes. Half a watt across twenty thousand columns is 10 000 W, or 0.01 MW — small against the lit load, but drawn 24 hours a day rather than only after dark, so it accumulates about 88 MWh a year. Central management systems that keep a radio node awake around the clock deserve their own line in the register rather than being folded into the lantern figure.

What is a normal lighting load for a town of a given size?

A useful sanity check is one column per six to ten residents, depending on how much rural road the authority owns. A town of 100 000 people therefore carries somewhere near 12 000 lanterns; still on 150 W sodium that is about 2.05 MW, and fully converted to 60 W LED heads roughly 0.76 MW. If your total sits far outside that band, the inventory or the assumed wattages are usually the reason, not the town.

Should the inventory be totalled in watts or converted row by row?

Keep the working columns in watts, because that is the unit on every product datasheet and every asset record, and convert once at the very end. Converting each row instead gives you a page of numbers like 0.000083 MW that are hard to check and easy to mistype, and anything under a watt drops past the display threshold into exponent form. A single conversion at the bottom of the sheet keeps the audit trail readable.

Watts to Megawatts