Sunlight Calculator – Another Product You Don’t Need

Home » Blog » Sunlight Calculator – Another Product You Don’t Need

Robert Pavlis

Someone on a social site asked if anyone knew of a light meter that would measure the amount of light in their garden. I burst out in laughter thinking this was a great joke. Then someone posted a link to just such a product; the Sunlight Calculator. I cried. Are people really buying such a product?

Sunlight Calculator, a product you don't need
Sunlight Calculator, a product you don’t need, source: Incremental Tools

Sunlight Calculator – The Claims

The following are taken from the dealers ad.

  • “The lighting conditions of the micro-climates in your garden aren’t always obvious.” Yes they are – just hold out your hand and look for shadows.
  • “This meter measures the duration and intensity of sunlight falling at a given spot over a 12-hour period.” It might measure these things but it does not report them. It only reports sun, part sun, part shade or shade. You get neither duration nor intensity readings.
  • “Though not a panacea for all garden problems, it can help you locate plants where they will grow best.” Not really, since light levels are just one of the parameters to consider.

SunCalc® – How It Works

After you turn it on, the meter measure the amount of light for 12 hours and determines how much sun you have. That is simple enough.

What happens if you turn it on at noon?

Sunlight Calculator then measure light for only part of the day, and will give you the wrong readings. So to make this work you need to go outside and turn it on before sunup. Sorry – I am in bed sleeping.


The Sunlight Calculator gives one of four readings which translate into the following.

  • Full sun: 6+ hours
  • Partial sun: 4-6 hours
  • Partial shade: 1.5 – 4 hours
  • Full shade: less then 1.5 hours

May I suggest that if you go outside 3 or 4 times during a single day, and look up, you can determine these values without a meter. Warning – do not look directly at the sun.

Food Science for Gardeners, by Robert Pavlis
Here is a post that will show you how to make your own sun map: Sun Mapping Your Garden the Easy Way

If you are a visual learner, my video will show you how to make a sun map.

YouTube video

If the above video does not play, use this link:

Is SunCalc Faster Than Pen and Paper?

Added Aug 2023.

This blog post has received more negative feedback than almost every other one I have written. Several people claim that they don’t have time to go outside and use the paper method and the CunCalc is so much more efficient and faster. I decided to put that claim to the test. I compared SunCalc and my paper method for a fairly small garden consisting of 2 beds with a total of 274 square feet.

Pen and Paper: Total time was 60 minutes spread over 1 day.
SunCalc: Total time was 121 minutes spread over 67 days.

You can see the full details of this experiment in this video.

YouTube video

Plants Require Sun or Shade

Some plants prefer sun and some prefer shade. Almost all will grow in part sun/shade. The more I grow things the more I am surprised that so called sun plants do just fine in shade and vice versa. The plants are adaptable.

My shade garden was shaded by two medium sized sugar maples. The two trees had to be removed one winter, and my garden was suddenly a full sun garden. All of the plants did just fine. Some of the shade plants are actually growing better now than before. Admittedly, this garden does get watered a couple of times a season, but it can go several weeks without water.

Accuracy of Plant Information

Some people claim that just knowing sun or shade is not accurate enough and that plants are fussier than that. They might be fussier but plant labels and on line information about plants is not more detailed. Look at plant labels and check the light requirements. They are not more detailed than sun, shade or part shade, and most give a range, “sun to part shade”.

Keep in mind that the same plant label is used over a wide geographic area. We get the same plant label here in Ontario that is used in California, but full sun in the two areas means something completely different. We grow hydrangea in full sun in Ontario, while the south recommends part shade.

Microbe Science for Gardeners Book, by Robert Pavlis

Isolation Level

Two-thirds of the technical brochure deals with something called “isolation level”. There are lots of formulas and technical information on this, but it never tells you how you should use the information.

Here is some of it. WARNING: read this only if you want a nap.

The values are generally expressed in kWh/m²/day. This is the amount of solar energy that strikes a square meter of the earth’s surface in a single day. Of course, this value is averaged to account for differences in day length. There are several units used throughout the world.

The conversions based on surface area are: 1 kWh/m²/day = 317.1 btu/ft²/day = 3.6MJ/m²/day

The raw energy conversions are:1kWh = 3412 Btu = 3.6MJ = 859.8kcal

Do they really believe gardeners are going to do these calculations?

Stop Buying Unnecessary Products

The Sunlight Calculator is not something you need. You can make a sun map following the procedure in my video in less than an hour and you don’t have to buy anything.

What really bothers me about such products is that they are damaging the environment. The product needs to be manufactured, packaged, and shipped around the country – probably half way around the world. That all takes resources including oil. In the process it adds pollution to the environment and increases global warming. You will use it once, and store it in the shed.

If you care about the environment – don’t buy the Sunlight Calculator.

If you like this post, please share .......

Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

84 thoughts on “Sunlight Calculator – Another Product You Don’t Need”

  1. I am an avid gardener and have been for many years. This is a product that I did not know existed, but wanted to create because I have several different areas to my garden. And though I am looking at them several times a day I never stand there all day watching each section to see how many hours each section is actually getting. Yes, you can garden without this but plants are quite fussy and do require specific amounts of sunlight to thrive. Thrive is the key word there.
    I do not think this is a useless tool I have actually been wanting to find some thing like this for many years. No, it is not a necessary tool, but definitely a useful one, Yes, if it actually gave you a number instead of a category that would be more specific, but looking at them this seems to be the best you can get.

    • Your comment is based on the assumption that “plants are quite fussy and do require specific amounts of sunlight to thrive”. That is simply not true. Plants are very adaptable and can deal with different intenisties of light as well as different durations.

      The other important point my article makes is that you can’t even find this type of accurate information for any plant.

      • While plants can adapt the quality and quantity can greatly affect blooms. While I can have a beautiful crepe myrtle plant, too much shade you have poor flowing.

        • Nobody is suggesting using too much shade. Try to find a value for the minimum amount of sun a crepe needs – and for what climate that is for, because 8 hrs direct sun in California is not the same as 8 hours in Ontario.

          If you can’t find that data – and you won’t – it is of little value to have a precise value of the amount of sun in a specific spot.

  2. Jumping on the bandwagon here… My tiny courtyard has at least eight different sun zones. Trees, walls, stairs, all cast different shadows at different times of day. I’ve got a 4×4 raised bed that clearly gets much more sun in one corner. There’s a patch of dirt where the landlord says nobody’s ever been able to get anything to grow — before I go to the expense of making a bed and amending the soil, I’d like to know if I can expect to get anything to grow there either.

    This product you are laughing at might not be the best of its class, but for a lot of people your blithe assertion that you can just go outside a few times to see if it’s sunny is kinda insulting.

    • If you watched the video in the post you would have learned how to do this with just going outside a few times. I not only tell you that you can do it quickly, I even show you how to do it.

  3. Sure is a lot of negative comments here for some reason. Thanks for the content, it answered a lot of questions for me.

  4. Very disappointing article, but what I really came to say was yes, some of us DO work 7 days a week. It’s called having multiple jobs. All best wishes and kind regards!

    • Then you don’t have time to waste on a sun calculator since measuring sun and shade is faster with paper and pencil!

  5. As a beginner gardener, I really like this post. It’s funny, money saving, and easy to follow. I just want to grow my lettuces and peppers, some onions and chives, and maybe a little pansies indoor. I live in a tiny apartment with no balcony with very noisy traffic. Planting is a way for me to connect to some green and find peace.

    Thanks for writing and sharing your knowledge. I can see you have a few resistances there. But to me, you’re like my grandpa who was a farmer. He passed away, but he had the same sass as you 🙂

  6. I have multiple areas in my yard with differing light conditions thanks to surrounding trees and structures that create shade. It would be tedious to do this by hand over a 12 hr period. Getting up before the sun to click some buttons (likely 7am) isn’t that hard.

    This article REEKS of laziness and lacks imagination. This is also very useful for lanscape designers who don’t want to sit and look at a property all day!

  7. You are a self identified look plantaholic, which is not what most people are. It seems to me, this can beat helpful tool for those that just want a nice garden and didn’t have time or inclination to do it the way you suggest. I think it could be particularly useful for people that are on a property where the light conditions are somewhat variable throughout the day due to shadows being cast by buildings, trees other shrubs etc. This is a quick and dirty, somewhat objective way to find out what might correspond with the directions on plant containers.

    • 1) most people have the weekend off.
      2) It only takes one day.

      If you are never in your garden – why are you gardening?

  8. Thanks! I have looked for years for just such a device–where do I find it? With 18 different things to plant and a million places to plant them plus the fact that the angle of the sun shine changes through the seasons it was all just too complicated for me. Trust me, this is a tool of great value to those with little extra time on their hands…I can’t believe I even took the time to write this!

  9. If you take a meter on a full sun day, put it in a spot that you want to plant a rose bush, and then read it when it is done calculating, using the guidelines for full sun (6 or more hours of sun), you have your answer. Plant the rosebush right there. No need to go through any fancy calculations. Do this in any spot that you want to plant flowers and then plant the flowers that need the amount of sun the meter gives you. Then just let nature give you what it will. Just take a deep breath and enjoy life.


Please leave a comment either here or in our Facebook Group: Garden Fundamentals