Optimising solar array orientation for lead acid battery charging

I have been thinking a bit about how best to orient solar arrays lately. I know that the optimum angle (for maximum total energy generation per year) is quite low, because most of the energy arrives in the summer, but if you want to get consistent power year round then a steeper angle is desirable, to maximise the slender solar in winter. There are also issues around shedding snow in some places (steeper is better) and around making best use of bright diffuse cloudy light (lower angle is better for places that do not see much sun at all).

I had read that it makes sense to split the array and put one part facing east and another facing west (some buildings force you to do this) and in the end you can get almost as much energy per day that way, losing maybe 15% compared with a south-facing array. The east array works well in the morning and the west works well in the evening and so you get more consistent power output without the high peak in the middle of the day, which is likely to be more than your battery can actually absorb in some cases.

So that started me thinking about lead acid batteries and how you charge them and I realised that they take a lot of charge at first, but later in the day they need less because they have arrived at the absorption voltage and the current tapers downward during the absorption phase. During this downward taper, the charge controller is rejecting much of the solar power so it is effectively irrelevant whether your array can produce high power output (unless you have a diversion load such as water heating off the surplus solar which everyone should have but very few actually seem to bother with).

So the logic took me to a conclusion that I have not heard or seen before surprisingly and I thought I’d share it here. If you face your array south east then you can get about 50% higher power in the morning, thus getting the battery charged quicker. Having reached the absorption voltage in the middle of the day, the array needs less power in the afternoon so it does not matter that this is less than you could have had available using a south facing array.

Here are some charts of solar power availability hour-by-hour to 2 arrays sited where I live, one facing south and the other south east (each at 45 degree angle) showing how the available power varies over the hours of the day in each month of the year. I got this data from https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html
The red curve is south facing and the blue one is the south east facing array.

It may not look like it, but the blue curve is about 50% higher power than the red one in the mornings which I reckon will bring the battery up to its absorption voltage quicker and allow it to absorb more power in the course of a day than it could with the red curve, even though the red curve offers more energy overall. That’s because you can’t actually use that extra afternoon energy to charge a lead acid battery because it cannot absorb power at such a high rate during the latter part of the charging cycle.

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4 Responses to Optimising solar array orientation for lead acid battery charging

  1. Andy F says:

    Your proposal makes a lot of sense for most of the year, however around the end of December solar PV in the UK is very challenging and you need all the power you can get just to break even. The difference between the red and blue curves represents power lost to the system. The high latitude means solar has a dramatic fall in output in winter compared with summer.

    If you have wind or hydro as a second source this may not be so serious but I find the week either side of the winter solstice with PV really difficult. The period in 2018-2019 I ran the generator quite a bit but managed to get through the same in 2019-2020 without powering it up at all.

    I’ve 10 KWH LiFePO4 rather than lead acid which I can deep cycle without damage to the cells and gives me a lot more useful capacity.


    • hugh says:

      hi Andy,

      Yes it’s a hard one to juggle, and you are right it does fall over in December in this version. Our December 2019 was dreadful in terms of generators usage (we did most of a year’s hours in a few short weeks). However there is so little solar in December here that even a drastic percentage change is not going to make much difference to the result.

      I’ve got LiFePO4 too now, but I am a long way from recommending it as the best option for customers. Just for learning. I like it, but am not sure it is worth the cost, nor whether it is as sustainable an option as lead acid.


  2. Bob Johnson says:

    Hugh, So I have been working with solar for the last 3 to 4 years, that said it has become obvious to me, that if you want the most bang for your buck you must track the sun! But because I leave in Northern Minnesota, my Latitude lets me do this relatively easy 😉 , Percentage-wise the sun here starts in the East @ lets say @ 90 degrees ( 0 being North)
    and it tracks mostly East to West this happens Starting now in March and April, at the peak of the summer solstice, the sun rises here at about 45 degrees, and sets @ about 315 degrees, What I’m saying is the orientation of my arrays are East-West, Not facing the South! Yes, they are in deep winter but most of the year they are East-West! To that end, I have made “A” frames to pivot the arrays on. Anyway, it’s hard to explain pictures would do wonders.

    ps I just bought your book last week and I am looking forward to getting her in Warroad MN 🙂

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