Sunlight Calculator – Another Product You Don’t Need

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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.

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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.

Soil 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.

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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. Who knew such a wee thing could be so divisive…I was thinking of getting one as I am notorious for killing house plants and am about to get a house with a garden so feeling a smidge nervous I might just kill it all. However, your blog convinced me otherwise. I have no strong feeling on who gets these, though I think your comment about purchasing low cost items like this is pertenent and should be considered. Equally, if people feel like they will be better gardeners with it, go for it. If it lowers the barrier of entry into gardening in their mind, then overall things will be better for it.

    One thing I do have some feeling over* is the correct terminology though; I believe you mean insolation rather than isolation? Insolation is the incident solar radiation or incoming solar radiation relative to an object upon which it lands. Though as a sometimes introvert I am intrigued by the concept of measuring the incoming energy per square meter from isolation…!

    *meant in jest and good nature

  2. I go to work under the cover of darkness and come home from work under the cover of darkness. How would you suggest I measure the sunlight in any given area of my property? I want to pick berries when I get home for a fresh snack, but need to know the areas best suited to grow.

    • The comments on this post are ridiculous. It takes seconds to look at Google Maps and work out what direction your garden faces, and therefore light levels. Then a few more seconds to work out which way the shade falls from established trees and plants. Basic stuff. I’ve been gardening for 30 years and never needed a device to tell me what areas are full sun, part shade, dappled shade or full shade in my garden. All you need is a sense of direction and some common sense. How the hell do people think gardens were grown before these devices existed? It wasn’t by relying on a bit of plastic that’s for sure. I do wonder if this post has been targeted by negative comments by the manufacturer of these devices because I cannot believe people who are reading gardening blogs are this out of touch with their gardens.

  3. This article was written by someone who lacks the imagination necessary to recognize that there are people out there whose circumstances are far different than their own.
    For example, people who simply aren’t home all day, but still want to have plants in the yard.
    Or people who have sporadic tree cover, and don’t trust their guess about whether it averages out to six hours of direct sunlight or not.
    Or people who want to determine exactly where in an irregularly-lit yard there actually is enough light.


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