<p>Technically (at least in solar energy industry):</p>
<ul>
<li><p><strong>Irradiance</strong> is the instant power of solar radiation per unit area, it is measured in <strong>W/m2</strong> (it is instantaneous, therefore no time dimension in the units).</p>
</li>
<li><p><strong>Irradiation</strong> is the <strong>quantity</strong> of solar energy per unit area, measured in <strong>Wh/m2/time</strong> (day or year or so; time dimension here is relevant).</p>
</li>
</ul>
<p>Practically, the terms are often confused.</p>

CC BY-SA 4.0
2023-02-22T12:56:44.457Z
W = J/s, so I don't understand your statement of no time dimension in the units...?
AtmosphericPrisonEscape
CC BY-SA 4.0
2023-02-23T02:15:25.753Z
@AtmosphericPrisonEscape right, very true correction.  But I think what he's getting at is that the second one actually is a measure OVER a time.  Similar to how velocity at any instant has the units of time (and tends to change over time of course typically)... but distance is actually more intimately linked to time because it's an integral over a chosen time period?  I always have gotten too confused with W vs kWhr and such myself :-p
JeopardyTempest
CC BY-SA 4.0
2023-02-25T02:56:41.217Z
Wh/m2 has no meaning unless you include a time period, such as per day, per hour, or per year.  But, of course, once you do that there is no difference between the two definitions.
Mark
CC BY-SA 4.0
2023-02-27T07:55:16.637Z
Thanks for all very relevant comments so far. Answer edited accordingly. Well, I still have things to study more here.
jurajb
CC BY-SA 4.0
2023-02-28T20:42:08.123Z
The definitions are now identical, except for a constant which depends on the value you choose for the time period in the second one.  If you choose 1 hour, the definition becomes Whr/m2/hr - the hours cancel, leaving W/m2, which is the same as the first definition.
Mark
CC BY-SA 4.0
2023-03-06T10:40:12.023Z
@Mark correct. But (at least in solar energy segment) besides hourly you often have daily, monthly, yearly aggregation, long-term average yearly totals, average daily totals or other time aggregations. In such cases 'time period' becomes useful and I believe that such data are easier to understand for the end-user.
jurajb
CC BY-SA 4.0
2023-02-22T09:34:55.117Z
2023-02-27T07:55:54.243Z
2023-02-27T07:55:54.243Z
jurajb
<p>Solar irradiation is a measure of the energy delivered by the sun per unit area, watts or joules per square metre for example.. It is often recorded over a specific time period, so it is very much to do with solar radiation, but not exactly the same thing.</p>

CC BY-SA 4.0
2019-07-18T23:01:20.893Z
that already explains a lot, but is irradiation and irradiance also the same? regarding [https://www.quora.com/What-is-the-difference-between-irradiation-irradiance-and-insolation] it seems that irradiation is not very much used. So what would you say is the actual difference between solar irradiance and solar radiation? in your answer you only refer to irradiation, it would be really nice if you could compare the terms if that's possible
Marcel H.
CC BY-SA 4.0
2019-07-21T12:54:54.657Z
I'm sorry, that link didn't show up correctly: https://www.quora.com/What-is-the-difference-between-irradiation-irradiance-and-insolation
Marcel H.
CC BY-SA 4.0
2019-07-21T13:02:18.627Z
Solar radiation is the radiation emitted by the sun and without which there would be no advanced life on Earth. Nearly all living things depend on it.
Michael Walsby
CC BY-SA 4.0
2019-07-21T13:08:41.043Z
okay, that helped a lot! thank you very much, so I guess solar radiation is just a term describing the radiation coming from the sun, whereas solar irradiation is a scientific term describing a power per unit area ratio
Marcel H.
CC BY-SA 4.0
2019-07-21T13:56:04.093Z
That's right, you've got it!
Michael Walsby
CC BY-SA 4.0
2019-07-15T10:15:43.797Z
2019-07-15T10:15:43.797Z
Michael Walsby
<p>I currently read a few papers and one of them stated to forecast solar irradiance using measures of solar radiation.<br>
I hope this question is not trivial as I'm actually from germany, but if there is a specific difference between these two terms I would be glad if someone could explain it.</p>

<atmospheric-radiation><solar-terrestrial-physics>
Solar irradiance/radiation difference
Marcel H.
# Solar irradiance/radiation difference

(Asked by: username_0 on 2019-07-15)

I currently read a few papers and one of them stated to forecast solar irradiance using measures of solar radiation.  
I hope this question is not trivial as I'm actually from germany, but if there is a specific difference between these two terms I would be glad if someone could explain it.

**Tags:** atmospheric-radiation, solar-terrestrial-physics


---

## 2 answers

### Answer by username_5 on 2019-07-15. Score: 4 | Accepted answer

Solar irradiation is a measure of the energy delivered by the sun per unit area, watts or joules per square metre for example.. It is often recorded over a specific time period, so it is very much to do with solar radiation, but not exactly the same thing.

> **Comments:**
> **username_0**: that already explains a lot, but is irradiation and irradiance also the same? regarding [https://www.quora.com/What-is-the-difference-between-irradiation-irradiance-and-insolation] it seems that irradiation is not very much used. So what would you say is the actual difference between solar irradiance and solar radiation? in your answer you only refer to irradiation, it would be really nice if you could compare the terms if that's possible  
> **username_0**: I'm sorry, that link didn't show up correctly: https://www.quora.com/What-is-the-difference-between-irradiation-irradiance-and-insolation  
> **username_5**: Solar radiation is the radiation emitted by the sun and without which there would be no advanced life on Earth. Nearly all living things depend on it.  
> **username_0**: okay, that helped a lot! thank you very much, so I guess solar radiation is just a term describing the radiation coming from the sun, whereas solar irradiation is a scientific term describing a power per unit area ratio  
> **username_5**: That's right, you've got it!  

---

### Answer by username_1 on 2023-02-22. Score: 6

Technically (at least in solar energy industry):

* **Irradiance** is the instant power of solar radiation per unit area, it is measured in **W/m2** (it is instantaneous, therefore no time dimension in the units).
* **Irradiation** is the **quantity** of solar energy per unit area, measured in **Wh/m2/time** (day or year or so; time dimension here is relevant).

Practically, the terms are often confused.

> **Comments:**
> **username_2**: W = J/s, so I don't understand your statement of no time dimension in the units...?  
> **username_3**: @username_2 right, very true correction.  But I think what he's getting at is that the second one actually is a measure OVER a time.  Similar to how velocity at any instant has the units of time (and tends to change over time of course typically)... but distance is actually more intimately linked to time because it's an integral over a chosen time period?  I always have gotten too confused with W vs kWhr and such myself :-p  
> **username_4**: Wh/m2 has no meaning unless you include a time period, such as per day, per hour, or per year.  But, of course, once you do that there is no difference between the two definitions.  
> **username_1**: Thanks for all very relevant comments so far. Answer edited accordingly. Well, I still have things to study more here.  
> **username_4**: The definitions are now identical, except for a constant which depends on the value you choose for the time period in the second one.  If you choose 1 hour, the definition becomes Whr/m2/hr - the hours cancel, leaving W/m2, which is the same as the first definition.  
> **username_1**: @username_4 correct. But (at least in solar energy segment) besides hourly you often have daily, monthly, yearly aggregation, long-term average yearly totals, average daily totals or other time aggregations. In such cases 'time period' becomes useful and I believe that such data are easier to understand for the end-user.  

---

