Solar Panel Daily Wh Output per Solar Watt

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mattnj

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www.red-data.co.uk
What are the rule of thumb values for watt hours per day per solar watt in the south of England? more specifically if I needed say 10amp hours per day (12v system) between 1st Mar > 30th Oct what size panel(s) do we think I need?
 
The rule of thumb I know is for the three summer months only, giving the average daily yield for a panel mounted flat in mid to northern European latitudes (rather similar in summer due to the longer days up north): 25 percent in Ah of panel wattage.
So 40 W in your example. To get the same in March and October, my guess is you would have to double the panel size.
 
mattnj said:
so, have I got this correct, we think say a 200w panel would do 25ah a day ish during Mar-Oct?
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Yes, but that is only my guess, as March and October are outside the rule of thumb that I am familiar with.
The average for the full 6 months would be pulled down quite a bit, I think, by the lower yield at the beginning and the end of the period.
 
mattnj said:
so, have I got this correct, we think say a 200w panel would do 25ah a day ish during Mar-Oct?
Click to expand...

You can't have the same figure for all of those months. Some Summer months i get 100ah per day, in November i get about 16ah per day.

If you wanted to get 10ah per day during the worst two months i'd say you need 100w. But this would give you much more in the best Summer months. Make sure you have enough battery capacity to smooth the yield out over a couple of days, at least.
 
mattnj said:
What are the rule of thumb values for watt hours per day per solar watt in the south of England? more specifically if I needed say 10amp hours per day (12v system) between 1st Mar > 30th Oct what size panel(s) do we think I need?
Click to expand...

You specifically asked for Wh per W solar installed but a rule of thumb probably isn't available to cover the large differences in output over the entire summer. The easiest way is probably just to look at PVGIS http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php

PVGIS indicates that in an average year with unshaded panels lying flat in Portsmouth area, you can expect following per 1W installed:

Wh/day during these months
Jan 0.59
Feb 1.16
Mar 2.36
Apr 3.60
May 4.13
Jun 4.51
Jul 4.25
Aug 3.46
Sep 2.61
Oct 1.49
Nov 0.78
Dec 0.46

You then mention Ah/day and can get a good enough idea by dividing PVGIS Wh/day by something like 13.8 to get Ah/day.

June is 4.51 Wh/day for 1 W, making 40W about 180 Wh/day or approx. 13Ah/day.
October is 1.49 Wh/day for 1 W, making 40W about 60 Wh/day or approx. 4.3Ah/day.
 
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I find this site interesting for an indication of what's possible:
http://www.newquayweather.com/wxsolarpv.php
Obviously for a boat you usually need to scale for the size of the panel and also the less useful aiming of the panels.
It would be interesting to have similar web pages for elsewhere in the country (and beyond).
 
Looks like a 100w panel will give you your 10Ah per day but, given that it isn’t much, what is it you need it for?

I use about 100Ah per day, and find I get it from 500W of solar between April and September. Have to use a genny October to March.
 
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