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--- opinion:frater_secessus:how_much_excess_solar_power [2022/12/14 12:58]
frater_secessus [How much extra solar power do I have?]
+++ opinion:frater_secessus:how_much_excess_solar_power [2023/04/07 15:06]
frater_secessus [calculating power available (W)]
@@ Line 1: / Line 1: @@
+DRAFT
+FIXME
 ====== How much extra solar power do I have? ======
 Yes, we can use [[electrical:12v:battery_monitor|a battery monitor]] to see how much power is going into the battery bank.  And we can look at the [[electrical:solar:charge_controller|solar charge controller]] to see how much power it is making.
-But how can we tell how much untapped power the solar setup leaves on the table because aren't demanding it?  Put another way:  how much more stuff could we be running off the solar?
+But how can we tell **how much untapped power does our solar have in reserve?** Put another way:  how much more stuff could we be running off the solar?
@@ Line 19: / Line 23: @@
-===== empirical testing =====
+===== direct testing =====
+Plug it in and see if the solar can support it [[electrical:solar:offthepanel|without affecting the battery bank]].  Easy peasy.
+The downside is this is a YES/NO answer;  it tells you a specific load will run but does not tell you about how much power is actually available.  If you want to see the Bigger Picture then read on...
+===== prediction =====
+==== looking up daily insolation (FSE) =====
+Hours of Full Sun Equivalent (FSE) is a convenient way to think of the amount of sunlight reaching the ground over the entire day.  4.5 hours of FSE means 4.5kWh of power reaching the ground per square meter.
-===== observation and prediction =====
+To predict daily harvest we will use
+  * FSE
+  * your panel wattage
+  * and your system's overall efficiency
-==== overall system efficiency ====
-You can get a feel for overall system efficiency by
-   - watching your system over time, as described above
-   - comparing real-time harvest to theoretical harvest
+==== calculating power available (W)  ====
-===== average power harvest for a given time/place =====
+Real-time harvest is a bit easier since there is no time component.  We are working in Watts (W) rather than Watt-hours (Wh). What we are doing is comparing the strength of sunlight reaching the ground (perpendicular) vs [[electrical:solar:output#zenith_angle|the cosine of the angle at which it actually strikes the ground]], or strikes a flat-mounted panel.
-power available
+This cosine is [[https://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html|calculable for a given time and place]].
+**Example**: if you have 200w of panels and calculated cosine is .70 then **140w of direct light is landing on your panels** in clear skies.
+==== finding and using system efficiency ====
+For [[electrical:solar:output|various reasons]] your solar setup will not be able to harvest all the power landing on the panels. For systems with PWM controllers the percent harvested will be something like 70%.  For MPPT it might be 83%.  These constants will be placeholders you can use until you figure out your specific system's overall efficiency.
+You can see our system's overall efficiency at a given time:  //actual harvest / theoretical harvest//.  The numbers will jump around but over time you will start to see a pattern.  Maybe yours makes 80% or 86% over time.  Use that  number to **predict how your system will behave** in another time/place:
+    panel wattage x system efficiency x cosine
+>> ...if your mppt controller typically yields 83% after derating, and the calculated cosine is .70 then you might expect ~116w in clear conditions at that time in that location. (200 x .83 x .7 = 116.2).

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