The effect of efficient lighting on HVAC system is too substantial to ignore in payback calculation.

On the face of it, an efficient lighting system will save only the incremental energy requirement of old energy system. Let’s get along this topic with a familiar example. There is a room with 10 fluorescent tubes of 36 watts each. Along with the ballast, the energy requirement is 40 watts for each bulb. Since the inefficient incandescent bulbs and old ballasts are becoming obsolete, we don’t consider them. If we consider 350 days of 12 hours lighting time and 20% additional idle running time then the usage hours of these bulbs will be approximately 5000 hours per annum.

So the total energy requirement of these lights is 10*40*5000/1000 = 2000 kWh. Now let’s see how much we can save by some retrofits. Two very common solutions will be switching to LED lights and installing occupancy sensors. The equivalent replacement of these tubes would 18 watts LED bulbs, which costs around Rs.900 as per latest quotes available.

  • Total Cost of 10 bulbs = 10 * 900 = 9000
  • Energy Requirement by LED bulbs = 18*5000/1000 = 900 kWh
  • Energy savings by LED = (40-18) * 5000/1000 = 1100 kWh

An occupancy sensor would cost around Rs. 2500 and potentially save 20% or even more in some cases.

  • Energy savings by Occupancy Sensors = 900*20/100 = 180 kWh
  • i.e. Energy requirement after installing occupancy sensors = 720 kWh

Now the total investment adds up to Rs. 11,500 and energy savings adds up to 1280 kWh. Considering per unit cost of Rs.7.5, the payback will be 1.2 years, which is quite good but there is much more into it when you consider the effect of efficient lighting on your Air conditioning system.

 

Air conditioning load:

The energy consumed by Air conditioning depends on three particulars.

  1. The temperature maintained inside the room- Higher the better
  2. The temperature at which the heat is rejected at condenser i.e. outside temperature – Lower the better
  3. The amount of heat which gets added into room – lesser the better

 

Let’s focus on the third point. The AC system needs to carry all the heat added inside the room and reject it through condenser. For doing this, it needs to draw power equivalent to heat rejected divided by its Coefficient of Performance (COP), which depends on first two points above. Very obviously, it is wise to minimize the heat sources inside the room. Any electrical equipment inside the room, whether efficient or inefficient, including lights, fans, computers, chargers, and refrigerators etc. adds heat into the room. The amount of heat is equal to the power drawn by the equipment i.e. a bulb of 40 watts will add heat equivalent to 40wh in one hour. To keep this explanation simple recall the law of conservation of energy. All the electricity entering the room ultimately gets converted into heat after doing its useful work in fans, lights and other equipment.

 

Effect on payback of efficient lighting system:

Now let’s get back to the payback calculation. The energy saved by the new lighting system was 1280 kWh and it eliminates equivalent amount of heat being added into the room. If the COP of the AC system is 2 then it saves further 640 kWh of energy by reducing the heat load of AC. So the total energy saved becomes 1920 kWh and the payback reduces to 0.8 years which is even better.

Of course, this is applicable only for the places which need to be maintained cooler than outside temperature. For places which need to be maintained warmer, this calculation would reverse and the effective payback will become higher - 2.4 years. The effect of efficient lighting on HVAC system is too substantial to ignore in payback calculation. This is a hypothetical example and real case payback can vary a lot depending on actual usage. This effect is applicable and worth considering for all improvement measures taken for equipment which are located inside a conditioned area.

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