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Old 08-05-2020, 11:02 PM   #1
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Default Solar panel - How many W*h

How many Watt-hour per Watt of installed solar panel could you expect on average per day?

I heard 4 W-h per day per W on sunny day. So a 100W panel would generate 400 W-h per day.
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Old 08-05-2020, 11:17 PM   #2
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How many Watt-hour per Watt of installed solar panel could you expect on average per day?

I heard 4 W-h per day per W on sunny day. So a 100W panel would generate 400 W-h per day.

That is pretty close to what most would get with moderate efficiency panels and a decent controller. Some days, maybe a bit more, cloudy, rainy, full shade will give a whole lot less.
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Old 08-06-2020, 02:51 AM   #3
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So for those of us who get confused continually..........

What does 400 w/h/day mean in terms of charging a battery or bank of batteries? Say, for example, you had a single 100AH battery and it was depleted to 70%. What does 400 w/h/d mean in terms of charging that battery back up to 100% Or if you had 200AH of battery? It seems these are vital bits of knowledge for deciding solar needs vs battery capacity.
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Old 08-06-2020, 03:49 AM   #4
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So for those of us who get confused continually..........

What does 400 w/h/day mean in terms of charging a battery or bank of batteries? Say, for example, you had a single 100AH battery and it was depleted to 70%. What does 400 w/h/d mean in terms of charging that battery back up to 100% Or if you had 200AH of battery? It seems these are vital bits of knowledge for deciding solar needs vs battery capacity.
400 Wh/d (not w/h/d) means the solar panel give you enough energy to run a 400W appliance one hour every day (or a 200W appliance two hours every day).

Assuming a voltage of 12.8V, 400 Wh/d would be 400 / 12.8 = 31.25 Ah/d. Thus the solar panel will give you 31.25Ah every day.

To recharge a 100Ah battery with a 70% state of discharge, it would take (70% * 100Ah) / 31.25 = 2.25 days to recharge it. Twice a long for a 200Ah battery.
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Old 08-06-2020, 12:03 PM   #5
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So for those of us who get confused continually..........

What does 400 w/h/day mean in terms of charging a battery or bank of batteries? Say, for example, you had a single 100AH battery and it was depleted to 70%. What does 400 w/h/d mean in terms of charging that battery back up to 100% Or if you had 200AH of battery? It seems these are vital bits of knowledge for deciding solar needs vs battery capacity.

There is a bit of a disconnect between the watt hours per day from the solar and the actual charge you replace in your battery because of the charge efficiency, which varies between battery types and brands plus it varies over the range of a single charge cycle.



Nearly all the energy from the charging gets stored in most batteries when the SOC is low (under 70%, maybe) so charge efficiency is nearly 100%. By the time you get to the top end of charging with over 95% full batteries, that charge efficiency drops a lot, probably under 50% of the energy gets stored. This is for AGM. Lithium is nearly 100% all the time.


Because of this, you can't really predict a consistent and really accurate amount of power you will get per day, especially if you are going to full or near full on the SOC.



The old standard 30 amp hours of recovery per day that has been around is probably a good place to start for planning, as it is on the conservative side for high sun conditions and maybe a bit low for low sun times of the year. We have seen as low as 5 amp hours per day in clouds, rain, shady times.


All of these are for a typical install 100 watt panel.
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Old 08-06-2020, 03:34 PM   #6
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Nearly all the energy from the charging gets stored in most batteries when the SOC is low (under 70%, maybe) so charge efficiency is nearly 100%. By the time you get to the top end of charging with over 95% full batteries, that charge efficiency drops a lot, probably under 50% of the energy gets stored. This is for AGM. Lithium is nearly 100% all the time.

The old standard 30 amp hours of recovery per day that has been around is probably a good place to start for planning, as it is on the conservative side for high sun conditions and maybe a bit low for low sun times of the year. We have seen as low as 5 amp hours per day in clouds, rain, shady times.


All of these are for a typical install 100 watt panel.
Thanks, booster. I have a single 100w suitcase that I deploy at sites. I tend to only think that it's beneficial if it's in direct sun, but from your comment it would appear that there is some gain even if it's not. Also, one of the reasons that I bought the suitcase was that I don't like to camp in the sun but again roofmount would give me some advantage even there. Things to consider.
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Old 08-13-2020, 08:07 PM   #7
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Hi,
The NREL PVWatts program will allow you to do a good estimate of watt-hours per day from a given size panel. It takes into account your location, time of year, average weather and panel orientation.

To use it, put in your location, your panel size (watts) -- just use 1000 watts as a round number, the panel tilt (mostly 0 for RVs). It will give you a table that shows KWH per month for each month of the year. Divide the month number by 30 to get daily output in KWH.
Bear in mind this is for average weather, so some days worse, some days better.

There is a fair bit of variation from place to place in the US, and a whole lot of variation from summer to winter - especially in the North.


Gary
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