Buy output lumens, not power consumption in watts

Solar street light

Buy output lumens, not power consumption in watts

April 9, 2022
5 mins to read

Lumens vs Watts

Power is the amount of energy transferred or converted per unit of time, the power consumption in watts tells you how high your electricity bill will be, it calculates how much electricity is consumed and how much you will pay for it. More power higher power bills, less power is more energy-efficient and cost-saving. 

The output lumens measure how much light you are getting from a luminaire, and tells you how bright your light will be, output lumens refer to the amount of light you can see. Fewer Lumens means it is a dimmer light; more Lumens means it is a brighter light.

The brighter a street light is, the higher its output lumens must be. But the higher the power of the light, it will not necessarily be brighter, and the sure thing is that you will definitely be paying more for its electricity bill. Why is this so?

It is because of the light efficiency! Yes, with a luminaire we must pay attention to the output lumens, but more importantly, is the luminous efficiency. For the same brightness of a luminaire, higher luminous efficiency will bring a lower power output, which is more energy-efficient. Instead, a lower luminous efficiency will result in more power consumption, which will increase a lot of extra money on the electricity bill.

Lightgogo 4 plus 120 watts application in village
Lightgogo 4 plus 120 watts application in village

Efficacy is a key factor

Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of output lumens to power, measured in lumens per watt (LM/W). Higher luminous efficiency means a brighter light, it's more energy-efficient, and most importantly it is a money-saving solution.

Let’s show by case

A few weeks ago, we received a tender file from a client in Mauritius, the requested product is solar street light of 70 watts with 100 lm/w. Therefore, we did a simple calculation to explain the big differences in solutions between 100 lm/w and 180 lm/w.

As per requested of 100 lm/w, power 70 watts

Totally lumens requested is 70 watts x 100 lm/w = 7000 lumens. Their requested battery capacity is 475 WH, let’s calculate the max. DOD (depth of discharge) as 80%, then 475 WH x 80% = 380 WH, then its useful power each day is 380 WH.

(LiFePO4 batteries life cycles depends on its DOD: 2000 cycles @ 100% DOD which is min. 5 years; 3500 cycles @ 80% DOD which is min. 9 years; 5500 cycles @ 50% DOD which is min. 15 years; 8000 cycles @ 30% DOD which is min. 21 years). 

So, useful storage power each day is 380 WH ÷ 70 watts = 5.4 Hours. This means this solar street light will work for 5.4 hours each day in full power under a full recharge of its battery. Things won’t be that good if we take into consideration factors like cloudy days, and dust-covered on the solar panel, these make the real charging value a little bit lower. So this solution gives the backup time 0 days, the system will be down if meet any cloudy days.

A super high-efficiency solution of 180 lm/w 

To reach the requested 7000 lumens, the system power needs to be 7000 lumens ÷ 180 lm/w = 39 watts in total power.

Battery calculated as 475 WH with its 80% of DOD, then its working hours in full power will be 475 WH x 80% ÷ 39 watts = 9.7 Hours. So this solution makes sure its working time is 9.7 hours in its full power to deliver min. 7000 lumens of lights on the ground, which gives them exact same brightness of the above 70 watts, but its working time is 1.8 times longer.

If to make the working time is 4 hours in full power, and take consideration with built-in microwave motion sensing and the smart power technology, the working time will be at least 2 days for only one charge. But there is also MPPT super high-efficiency charging which makes sure good charging too even on cloudy days. The system won’t be down even in harsh weather conditions If taking all these factors into consideration, which makes sure 365 days of good quality lighting by solar only. You can check Lightgogo 4 solar street light on the products page to know more about the advantages of a smart lighting system.

Lightgogo 4 solar street light installation in France
Lightgogo 4 solar street light installation in France

Super high-efficiency is a game-changer 

In the above example, we see the magic result through improvements in light efficiency from 100 lm/w to 180 lm/w, which reduced the power consumption by almost fifty percent, thereby making the whole system work almost twice longer. This will also bring an even greater change in battery life span. In general, a LiFePO4 battery will last 5 years at 100% discharge, when the DOD is reduced to 50% it will last 3 times longer which is up to 15years. If we consider the ROI (return on investment), then a 180 lm/w system will be 3 times greater than a 100lm/w system. So we often say that the super high efficiency will be a real game-changer.

The lumens to light what liters are to petrol. The lumens are the amount of light you want. Therefore, when you buy a new luminaire, consider the output lumens, not the power consumed.
Luminous efficacy is to lamps what petrol consumption is to a carper hundred kilometers driven, it is the usage cost and the most important issue we should be concerned about. Higher luminous efficacy is the more energy-saving and cost-saving solution.
For lighting products, all we should focus on is higher brightness, which means higher output lumens, not a higher power.
For solar street light, we should focus more on higher luminous efficacy, because it provides a much longer life span, and also brings a much greater return on investment.