Solar charge setup

[KirstinS]

Hi all

I have inherited a 40w Maplin solar panel. I am also moving house in two weeks and the new place has large shed with sunk bike hoops in it.

Perfect for an entirely off grid charging I thought. Stick panel on roof, connect to 12v 80ah leisure SLA leisure battery. Just need to buy an invertor 12v to 240v and job done , right??

It's a south facing garden in sunny shoreham (nr Brighton).

I've no idea how much I can expect the panel to generate - especially in winter. Or how much I need to charge my 9ah battery

Anyone point me in the right direction or help with this ?

I'd love to be able to take my ebike(and so my entire 14m round trip commute off the grid)

Thanks
Kirstin

 

[NRG]

A rough estimate, a 40w solar panel in full working order, in summer, would need to be in bright sunshine for 7-8 hours to generate 280 to 320Wh, ignoring any losses. Your 9ah battery, assuming its 36v has a capacity of 324Wh so you will need 8 hours continuous sunshine to put into your 80ah SLA the energy that can be stored in your bike battery.

With inverter losses and charger losses you would really need to put in 430 to 480Wh at a guess.

How many 8hr sunny days do you get in Brighton? In Winter you may generate zero energy.

I think you'll need a considerably bigger solar panel!

 

[Old_Dave]

Reports on the internettywebby are not very complementary about this panel, output on a good day is about half of the expected.... And nuthing when it's cloudy.

The good news is that it was free, lol


Sent from my iPhone using Tapatalk

 

[Pro-connected]

You can estimate the monthly output from a photovoltaic panel using the US National Renewable Energy Laboratory ‘PVWatts’ online calculator. It has World-wide solar data built-in including London Gatwick.

Using Gatwick data gives the following estimated monthly outputs from a 40 Watt panel:

Jan - 1,280 Watthours
Feb - 1,520 Watthours
Mar - 2,280 Wathours
Apr - 3,480 Watthours
May - 4,040 Watthours
Jun - 3,680 Watthours
Jul - 3,960 Watthours
Aug - 3,920 Watthours
Sep - 3,040 Watthours
Oct - 2,320 Watthours
Nov - 1,360 Watthours
Dec - 800 Watthours

Taking NRG's calculated capacity of 324 Wh and an overall charging efficiency of 60%, each charge will require 540 Wh, so your 40 Watt PV panel will give you enough energy for 58 charges per year. Unfortunately in the winter months you'll have to wait a long time for the battery to charge: December would typically only fully charge the battery once.

BTW - The electrical output of a PV cell is dependent upon the intensity of the light to which it is exposed. So PV cells will tend to generate more electricity on bright days than when skies are overcast. However, photovoltaics do not need to be in direct sunlight to work, so even on overcast days a PV cell will be generating some electricity.

 

[Deleted member 4366]

You need a minimum of 110aH 12v battery. You shouldn't take them past half-way, which means 55aH.
36v 9aH = 12v 27aH. You need to make very generous allowances for inverter and charger losses. If you only charge a couple of times a week, the panel might be OK. A smaller 12v battery might be OK if you don't fully discharge tour bike battery. When you do the economics, it's no viable. You need a panel, inverter and battery compared with about 5p a time to charge your bike. 3 years at 2 charges a week = £15.60. To get payback will take about 50 years if your 12v battery lasts that long. I'd be surprised if you get 5 years out of it.

 

[Jeremy]

As an idea of what you can just about get, I have four 50Wp monocrystalline panels on my electric boat, arranged on a fairly horizontal canopy. I have a maximum power point tracker circuit that gets me about 15 to 20% more from the panels than I would if charging directly with them. I use this array to charge an 80Ah LiFePO4 battery that has a fully charged voltage (at the point that the BMS kicks in) of 14.6V.

The boat motor uses around 100 to 120W when cruising and the panels can usually provide around half of the power the motor needs, a little more on a very bright sunny day. As long as I only run the boat for around half of daylight hours each day it just about stays fully charged.

The losses in my system are fairly low, as I'm charging the battery/running the motor directly from the solar panel MPPT, with no intervening inverters or chargers to waste a bit of power. Even so, I only manage to get around 500 to 600 Wh or so per day in summer from a 200Wp rated array.

With a 40Wp panel and the extra losses from the inverter, battery charging etc I suspect that you'd struggle to get more than around 50 to 60Wh per day in reasonable weather, a fair bit less when it's cloudy or in winter.

 

[KirstinS]

Thanks everyone , really useful albeit disappointing info !

Ah well, forget it then. It was a hopeful thought given I have the battery and panel for free. I have no desire to spend large chunks of cash on this ......id rather spend it on ebikes

Solar doesn't really pay in uk is the message I'm taking away (aside from those who got amazing early feed in tarrifs on their properties as my father in law bangs on about.

I shall stick it on the shed anyway and hope for rather lesser outcomes - like running some led lights, a stereo and maybe charging the odd mobile device

 

[Jeremy]

If you want to do it on the cheap, keep a lookout for cracked panels being sold off cheap by the PV installers. More often than not these cracked panels work as well as new ones, the only probable issue being that they may fail prematurely (although I know people who've used them for years without problems). They don't look great, so the installers can't use them, but they usually have a layer of plastic film under the glass that still keeps the water out OK.

Well worth going on the scrounge amongst any local PV installers, as a friend picked up six panels (~120Wp each) for a total of £50 just before Christmas. The nice thing with these is that you can get a high enough voltage to run the bike charger directly. 99.9% of ebike chargers work over a very wide voltage range at the input (often around 90V to 250V) and they will work as happily on DC as AC. THis means you can knock up a PV array that will run at, say, 200V nominal, and then run the ebike charger straight off it, with no need for the losses associated with the lead acid battery, inverter and charger.

 

[oigoi]

I live off grid and I can confirm that in this country in winter solar is virtually useless. I have 800W of panels charging a big bank of lead acid batteries and it does not make enough in the middle of winter to charge my bike every day. A couple of times a week maybe. About 9 months of the year it is fine the middle 3 months of winter it is hopeless hence my wanting to have a wind turbine as well

 

[Scimitar]

What the Maplins panel is useful for (and was probably sold for) is topping up a leisure battery on standby. That's about it. For that, they work well enough.