Electricity use is measured in watts. The power consumption of grow lights is always listed with the product. Knowing how much power one or more appliances consume and multiplying that with number of hours the setup is running then multiplying with the electricity cost gives us the electricity cost.
To calculate how much your monthly electric bill will add up to, you need to know three things:
Power consumption (watts)
Electricity rate ($/kWh)
1- Power Consumption (watts)
All the devices that run on electricity will have its power consumption listed. At a minimum, in a small grow tent of about 2×2’ (61×61 cm), two small fans (5W each) together with a 125W LED grow light could do the job.
This means that every hour, this setup would consume 135W: the lamp consumes 125W and the two fans consume 10W together.
A proper ventilation kit costs about $100-150 and draws 20-40W.
If a portable AC unit is necessary to regulate room temperature, the cheapest ones cost around $250 and draw 800W. This would typically only be necessary in warmer climates or when growing with HID lamps in moderate climate. Depending on the ambient temperature and how much heat your system produces, it could get costly to manage heat.
2- Photoperiod (run-time)
This simply means hours of operation per day. During veg or with autoflowers, 18/6 photoperiod is often used. 18 hours of light and 6 hours of darkness.
During flowering, the photoperiod is usually reduced to 12/12 for non-autoflowers.
If the grow’s veg stage is equally long as the grow’s flowering stage then the average photoperiod would be 15/9 throughout the grow. I.e. 15 hours of use per day of all electrical equipment.
*Running a 135W LED + USB fan system for 15 hours would consume 135*15 = 2025 watts or ~2.0 kWh.
*Running a 300W HID and ventilation at 30W for 15hours would consume 330*15 = 4950 watts or ~5.0 kWh.
*Running a 600W HID, ventilation at 30W and an AC unit at 800W for 15 hours would consume 1130 * 15 = 16.950 watts or ~17.0 kWh.
3- Electricity rate
Electricity is charged per consumed kWh, or kilowatt hour (1000 Watt = 1 kWh). The price is a bit different from state to state and country to country. Speaking for most of US and EU, the cost of electricity will be around $0.1-0.2 (10-20 cents) per kWh.
Search on Google for “average electricity price [state or country] and let the internet give you an idea of what the price is where you live. The price you’re paying should also be listed on your electricity bill.
Let’s assume that the cost for electricity is $0.15/kWh. Now we can calculate the final monthly and yearly cost:
*135W LED @ 2kWh/day = $0.3/day. 60kWh/mo = $9/mo. 180kWh/3mo = $27. 720kWh/year = $108/year.
*330W HID @ 5kWh/day = $0.75/day, 150kWh/mo = $22.5/mo. 450kWh/3mo = $67.5. 1800kWh/year = $270/year
*1430W system @17kWh/day = $2.55/day. 510kWh/mo = $76.5/mo. 1530kWh/3mo = $229.5. 6120 kWh/year = $918/year.
The formula is:
Hourly power consumption in kWh * Photoperiod * Electricity rate = Cost for photoperiod
0.135 kWh (135W LED lamp) * 15 hours * $0.15/kWh = $0.30
The electricity cost of running a 135W (0.135 kWh) lamp for 15 hours when electricity costs $0.15/kWh means the total daily (15 hour) cost will be $0.30/day for this set up.
Same example as above but only 12 hour photoperiod.
0.135 * 12 * 0.15 = $0.24
Knowing how to calculate energy consumption and electricity cost is not only smart for budgeting but also to compare grow lights from one another.
For some, a 125W LED grow light could be a good replacement for a 300W HPS. As we see above, the cost difference of running these two setups (135W vs 330W) on an average of 15 hours a day for a year results in $162 electricity cost difference. Not to mention that the LED doesn’t need several bulbs and the LEDs will run fine for several 10.000s hours whereas HPS bulbs need to be replaced yearly.
There can even be significant differences in power consumption LED vs LED. This is a topic for a different article but a lamp with 0.7 umol/J efficacy would need to draw twice the electricity to put out the same amount of light as a lamp with 1.4 umol/J efficacy. In other words, the lamp with lower efficacy may have a cheaper upfront cost but it will draw more power and cost you more in the long run.