How the calculation works
For one 50 watt light used for 12 hours: 50 × 12 ÷ 1,000 = 0.6 kWh. In normal electricity billing, 1 kWh is commonly called 1 unit, so the light uses 0.6 unit per day.
Daily, monthly and yearly consumption
| Period | Operating time | Energy for one 50 W light |
|---|---|---|
| 1 day | 12 hours | 0.6 kWh / units |
| 7 days | 84 hours | 4.2 kWh / units |
| 30 days | 360 hours | 18 kWh / units |
| 31 days | 372 hours | 18.6 kWh / units |
| 365 days | 4,380 hours | 219 kWh / units |
The actual number of nights and switching hours can vary with season, timer settings, photocells and maintenance schedules, so use your measured operating hours where possible.
What if you have many street lights
Multiply the one-light result by the number of lights. Ten 50 W lights running 12 hours use 6 kWh per day, 180 kWh in a 30-day month and 2,190 kWh in a year. Fifty lights use 30 kWh per day, while 100 lights use 60 kWh per day.
| Number of 50 W lights | Daily units | 30-day units |
|---|---|---|
| 1 | 0.6 | 18 |
| 10 | 6 | 180 |
| 50 | 30 | 900 |
| 100 | 60 | 1,800 |
How to estimate electricity cost
Because electricity tariffs differ by country, utility, time of use and billing slab, enter your current local rate rather than relying on a fixed rate in an article. If your rate were 0.20 in your local currency per kWh, 18 kWh for a month would cost 3.60 in that same currency. Replace 0.20 with the rate on your bill.
50 watts versus actual input power
A street light labeled 50 W is normally rated by electrical input or nominal fixture power, but the real draw can differ slightly because of driver efficiency, dimming and power-factor behavior. For an energy audit, measure actual watts with suitable equipment or use manufacturer input-power data. For planning, the rated 50 W figure is a practical estimate.
Dusk-to-dawn and seasonal operating hours
Twelve hours is a convenient planning assumption, but night length changes during the year. A photocell-controlled light may operate less than 12 hours in summer and more than 12 hours in winter. If you know average nightly hours, replace 12 in the formula. A 50 W light running 10 hours uses 0.5 unit per day; at 14 hours it uses 0.7 unit.
Solar street light planning note
For an off-grid solar street light, daily energy demand is only the starting point. Battery capacity, system voltage, allowable depth of discharge, solar-panel output, weather and controller losses also matter. A 50 W light used for 12 hours needs about 600 Wh of usable energy before adding system losses and reserve margin.
Fast way to calculate any light
Use the same method for any wattage: watts × hours ÷ 1,000. A 30 W light for 12 hours uses 0.36 unit per day; 60 W uses 0.72 unit; 100 W uses 1.2 units. Then multiply by the number of lights and billing days.
Frequently asked questions
How many units does a 50 watt light use in 12 hours?
0.6 kWh, which is 0.6 electricity unit.
How many units does a 50 watt light use in one month?
At 12 hours per day for 30 days, it uses about 18 kWh or 18 units.
Does 1 kWh equal 1 electricity unit?
For standard electricity billing, 1 kilowatt-hour is commonly referred to as 1 unit.
How do I calculate the bill for the street light?
Multiply monthly kWh by the electricity price per kWh shown by your utility or tariff plan.
Enter your own values, keep the assumptions visible and use this guide to understand the formula behind the result.