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electrical:solar:output [2024/11/04 12:23]
frater_secessus [where and when] |
electrical:solar:output [2025/06/18 22:30] (current)
frater_secessus [panel temperature] |
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| The location and season both have profound effects on solar harvest. [[opinion:frater_secessus:panelsizesforinsolation#representative_areas|For example]], Phoenix AZ has **over 8.5x the amount of solar power** available in summer as Seattle WA in the winter. For any given location, winter harvest can be about half of summer harvest. | The location and season both have profound effects on solar harvest. [[opinion:frater_secessus:panelsizesforinsolation#representative_areas|For example]], Phoenix AZ has **over 8.5x the amount of solar power** available in summer as Seattle WA in the winter. For any given location, winter harvest can tiny compared to summer harvest. |
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| For more specific numbers you can estimate solar harvest using [[electrical:solar:pvwatts|models like PVwatts]]. | For more specific numbers you can estimate solar harvest using [[electrical:solar:pvwatts|models like PVwatts]]. |
| ===== panel temperature ===== | ===== panel temperature ===== |
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| Solar panels are dark in color and get very hot. Unfortunately, voltage (and therefore power) output //decreases// as panel temperature //increases//. This is the reason an air gap between the panels and the camper's roof is recommended to allow cooling airflow. | Cell temperature affects the voltage part of the panel's I/V curve; warmer temps drive voltages down and very cold temperatures drive it up. This is why Voc+20% or similar rules of thumb are used when assessing how much voltage "headroom" the solar charge controller has -- a cold snap could, for example, put a 48Voc rated array over a 50v controller input limit. |
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| | For this reason the array's real-world Vmp is rarely the Vmp given on the lab rating. It doesn't matter much to PWM but you will observe your MPPT finding a Vmp that does not match rated Vmp. And since with MPPT ''Vmp * Imp = Mpp'' lower Vmp means lower harvest. Hence the panel mounting airgap; it allows cooling airflow underneath. |
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| | ==== high cell temperatures ==== |
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| The data below, derived from [[http://digivation.com.au/solar/tempderate.php|this calculator]], show power from a 100W mono/poly panel dropping off as ambient temps rise: | The data below, derived from [[http://digivation.com.au/solar/tempderate.php|this calculator]], show power from a 100W mono/poly panel dropping off as ambient temps rise: |
| So a [[camping:snowbirding|snowbird]] who "chases 60" will be losing ~10% of panel output during the warmest part of the day. Snowbirds chasing 70 will be losing ~13% of panel output. | So a [[camping:snowbirding|snowbird]] who "chases 60" will be losing ~10% of panel output during the warmest part of the day. Snowbirds chasing 70 will be losing ~13% of panel output. |
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| Note: sub-freezing temperatures can push Voc/Vmp above their rated values. If you will use solar in sub-freezing conditions leave plenty Voc headroom in your solar charge controller spec. | |
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| Note: that [[hvac:insulation#heat_from_solar_panels|radiated heat from the underside of panels can raise temperatures inside the camper]]. | Note: that [[hvac:insulation#heat_from_solar_panels|radiated heat from the underside of panels can raise temperatures inside the camper]]. |
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| | ==== low cell temperatures ==== |
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| | Sub-freezing temperatures can push Voc/Vmp above their rated values. If you will use solar in sub-freezing conditions leave plenty of Voc headroom in your solar charge controller spec. +20% margin is traditional. |
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