Running a Grow Light? Day and Monthly Electricity Consumptions

Determining the running cost of a grow light and how much electricity is consumed is paramount if you do not want to ending up spending too much money over the months. Here the numbers

The cost of running a grow light depends on a) the wattage of the growth light (in Watts) b) the number of hours is kept on and c) the electricity cost ($ per kWh). For instance, a 600W growth light can cost up to 40$ a month or around 1.3$ a day assuming a 19p per kWh electricity cost.

However, you need to know where to find such information, understand what this really means, and finally, based on that, adopt some tricks that might help you save money in the long run. Let’s dive in!

Find The Right Information Is Key

To quantify the cost of running a grow light you need, at hand, three pieces of information:

  1. The envelope of your grow light or (if any) its user guide. If not available you can find an online version of it or even some information available online like in Amazon or any other online shop (once you know the name/brand of the lamp);
  2. A copy of your most recent electricity bill;
  3. The number of hours per day you are planning to keep grow light ON. This depends on your herb.

So, let’s see what each piece of information tell us.

Electricity Bill: A Jungle of Numbers

If you are not familiar with the details of your electricity bill, do not worry, you are not alone. Below you can find an example of one of my electricity bills (United Kingdom). The only number if you have to look for is the “Cost for Electricity”. My bill is more complex than a typical one as I have two different tariffs, one for night time and one for daytime hours. If this is your case as well you should look at the daytime tariff as this is when your bulb will be ON.

Example of UK electricity bill: the cost for Electricity (first orange line from top) is the cost for kWh

In my case, as you can see from the picture below, such price is of 18.39p (quite pricey, but that’s because is referred to daytime kWh). This means that if I have any generic appliance (grow light included) of a wattage (on this “wattage” more detailed later) of 1 kilowatt it will cost 18.39p to run for an hour. Do not forget the 5% of VAT that makes the cost of 19.31p.

Below you can find a bill from another UK supplier. In this case, again, you have to look at the price associated with each kWh without getting confused by other charges. This information under the section “Electricity charges” and the column Price. Again the 15.56p should be increased by the 5% VAT becoming 16.34p.

Another UK electricity bill: Electric charges – Price report the cost of 1 kWh

In case of a bill produced by a USA supplier (in Washington state) as the one shown in the photo below you can clearly see, in the “electrical usage information” section, that each of the 103888 kWh (probably an industrial customer, but the same concept applies to domestic bills) costs $0.08. Hence, once including taxes (around 10%) the costs per kWh raise to around $0.09.

Example of a USA electricity bill – Source: US Depot of Defence

In the UK there is a large variety of suppliers, so it is not a surprise that you can find large differences in the way the bill is designed. However, you should be able to identify the cost per kWh quite easily as such cost is always multiplied by a large number (the number of kWh that for a typical house can be many hundreds per month). Moreover, do not forget to include any extra expenditure expressed as a percentage of the total amount to pay (as referred to the amount of kWh) like VAT or city council tax.

The Grow Light Wattage

Now that you know how much a kWh (the energy to run any of your appliances) costs, it is time to understand how many kWh you need to grow your plant. For this you need two info: 1) The “Wattage” of your grow light 2) the amount of time you are planning to keep the grow light ON. Now we are going to focus on the Wattage.

The Wattage is the power absorbed by your grow light to work properly. Higher this number, higher will be the amount of kWh the grow lamp is going to use. As explained in this article, having a higher Wattage value will allow you to grow more plants as the surface area will be larger, however, higher also will be the electricity bill.

This value is always expressed in Watts (written as W). You need to divide this by 1000 to obtain the kW you need for the calculation. For instance, if the Wattage is 500W this equates to 0.5 kW for our purposes.

In the photo below you can see an example of a snapshot taken from Amazon. The two grow lights shown to have a wattage (highlighted in yellow) of 600 and 1000W respectively (the equivalent of 0.6 and 1 kW). If you know the name of your grow light you can just google it or look for it in any online shop where you would always find associated the wattage in the title itself.

It is important to highlight that this wattage does not require any further adjustment. This value should be taken as reported independently from the technology (LED or fluorescent). Indeed, 1kW of fluorescent-based grow light is the same as 1kW of LED-based grow light.

The Number of Hours Your Grow Light is ON

This strongly depends on the light needs of your plant although it is a good rule of thumb to assume 12 hours per day. You can also go for longer hours (24) as a few herbs will be fine with that, however, given the cost that this will cause and the overall benefit might be marginal, I recommend to stick to the 12 hours pattern, ideal for the majority of cases.

How Much Does It Cost Running a 1000W, a 600W?

Hence, now that you know the cost of your electricity (kWh) and the Wattage of your light, suppose that you want to quantify you’re daily/monthly/yearly electricity expenses. How can you do that?

For the purpose of this guide, I will refer to the two grow lamps in the Amazon snapshot of the previous photo (1000W and 600W) adopting the first UK bill introduced in the article. However, the same calculation can be performed for any grow light and country of billing reference adopted.

Running Electricity Cost for a 600W Grow Light

A 600W grow light (LED or not) kept for 12 hours a day will cost around 41$ per month. If the 600W growth light is kept for only 8 hours the cost will reduce to 28$ a month.

  • Cost per kWh=19.31p (including taxes)
  • Wattage (in kW) = 0.6kW (600W)
  • Number of hours=12

Running Cost= Cost per kWh * Wattage (in kW) * Number of hours = 

Daily Running Cost= 19.31 * 0.6 * 12=139p=$1.39

Hence the monthly and yearly costs will be:

Monthly Running Costs = Daily Extra Bill Cost *30 = 1.39 * 30=$41.7

Yearly Running Costs= Monthly Extra Bill Cost *12 = 41.7 * 12=$500

Running Cost For a 1000W Grow Light

The running electricity cost of a 1000W grow light (LED or not) kept for 12 hours a day is around 70$ per month. If the 1000W growth light is kept for only 8 hours the cost will reduce to 49$ a month.

  • Cost per kWh=19.31p (including taxes)
  • Wattage (in kW) = 1kW (100W)
  • Number of hours=12

Daily Running Cost= Cost per kWh * Wattage (in kW) * Number of hours

=19.31 * 1 * 12=232p=£2.32

Hence the monthly and yearly costs will be:

Monthly Running Cost= Daily Extra Bill Cost *30 = 2.32 * 30=£69.6

Yearly Running Cost = Monthly Extra Bill Cost *12 = 69.6 * 12=£835

Running Cost: Lazy Way To Assess Them

Suppose that you want to quantify the running cost in a fast way without you getting “dirty” with math. In such a case I realized for you two easy graphs to quantify, in a glance of an eye, the yearly running cost of your grow light assuming a common 12 hours daily cycle.

Graph to quickly quantify the yearly running cost of small-medium size grow lamp
Graph to quickly quantify the yearly running cost of medium/large size grow lamp

The results will not be accurate to the single dollar, but accurate enough to have a good idea on what you are going to spend to operate your lamp.

Two graphs, one for small grow lights (up to 60W) and the second one for the larger ones (typically for the more experienced gardeners).

How to use them: 

  1. In the horizontal axis pick the electricity price for your kWh (from your bills). Any currency applies. If you do not find the exact price of your kWh cost, is not a problem, take the closest one. Once done, from there, imagine tracing a vertical line until crossing the colour line (purple in our case) corresponding to the size of your grow light (1 in the photo below). 
  2. Once crossed, imagine to trace another line, this time horizontal, until you met the vertical axis. The number you find there corresponds to the running cost of your grow light.

An example of step 1 and step 2 applied for the UK case (19.31p kWh cost) for the 600W grow light is shown in the photo below.

Example on how to use the graph

Define Your Daily Budget And I Will Tell You The Bulb Size

The graphs previously adopted can be used in the opposite way to quantify what is the highest bulb size that you can buy considering the maximum extra bill cost you can tolerate. That’s pretty easy:

  1. Suppose your cost per kWh is of 0.19p (again from the electricity bill);
  2. Suppose your yearly budget is £120 (let’s say £10 per month);
  3. Trace a vertical line starting from your kWh price as shown in the figure below;
  4. Every time you cross a colour line, check the horizontal value if it is still lower than your maximum budget;
  5. Once you overcome your budget check the colour line immediate below, this is the size of the bulb you can afford;

As shown in the figure below, with a budget of £120 the coloured line immediately close to such value is the one associated with a 100W grow light (blue line). This is then the highest size that in this example you can then afford.

How to find the largest grow light size based on your budget

Are LED Grow Lights Cheaper To Run?

Lots of buzz on how LED grow light are cheaper but not for the reason you might think. What does it mean? If you buy a 1000W fluorescent light and 1000W LED light which one will be cheaper in terms of running costs? 

None! They will both have the same identical running cost! Indeed, the wattage, independently from the technology, it is the factor that dictates the electricity price in your bill.

Then why LED light is defined as cheap? Because for the same Wattage (hence the same running cost in your bill) they can grow much more herbs as they use energy more efficiently by producing more (higher intensity) light that allows larger grow area

Example of a LED – Photo by F.D. Richards in Flickr

Hence, if you are aiming to grow a certain number of plants you can opt for a lower wattage LED-based grow light compared to a fluorescent one. Although the quantification is a bit more complex as affected by the spectrum features, not only coverage area, a good rule of thumb is that fluorescent light can be replaced by a LED light with half the wattage (source).

This implies that, if for instance, you need a 600W fluorescent grow light to make your herbs thrive, the same can be achieved with a LED light of the only 300/400W. This means that the running cost is almost halved.

However, the LED grow lights are more expensive. Nonetheless are still more convenient than fluorescent light as they can last up to 10 times a fluorescent lamps, making them way more economically efficient. In a future article, I will investigate and compare the full life cycle of fluorescent and LED lights to provide more accurate figures.

Tips: How Can You Reduce Running Cost

Now that you have all the insights regarding the cost of running a grow light is important to understand if there is any change to reduce your running costs. Here my tips:

  1. Use the (limited) available natural light: Do you really need 12 hours of light? If you live in an extremely north (or south) country with very few daylight hours, you can still leverage those 3-4 hours of full light in which you may simply turn off your grow light. Even 3 hours less a day implies a reduction of 25% in your grow light running costs. This can be set automatically with a simple timer.
  2. Change supplier or electricity contract. Supplier companies are popping out in the market very frequently (at least in the UK) with enticing offers to acquire more customers.

    Hence, change suppliers or simply reviewing your contract is nowadays a simple operation (does not require anyone coming to your home or install any wire) that sometimes can be done online or with a phone call. This move can save you lots of money as this will also affect the cost of running any other appliance (usually more expensive).
  3. You can switch to day/night tariff with a lower price at night time and give the 12 hours of light to your herbs during nighttime (the electricity price can be even half of the daytime). In the end, the growing cycle of the plant, as totally indoor, is defined by you, so it should not be an issue in which hours of the day.

    I never tried it myself to be honest, but this is something you might think of.  Of course, in this case, grow light should be far away from your bedroom!

Related Questions

What is the cheapest grow light for beginners? The cheapest grow light is the fluorescent daylight type of a few watts that can be bought for around $5 in many supermarkets/online shops. This is suitable for 1-2 small herb pots.

Can you reduce the wattage and the cost of a grow light? For the majority of grow light, it is not possible to change the wattage of the light produced if not indirectly by changing the mode of the growl light from flowering to vegetative that.

Are there other costs associated with grow light? In case a large grow light is in place a ventilation system, sensors for temperature and tend(s) to control the growing environment might be required. All of those tools have a running/maintenance cost.

Further Readings

6 crucial aspects to look when choosing a grow light: beginner guide

Best grow light for beginners

21 Tips to grow massive basil

18 Herbs that can grow without full-sun

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