Solar Panel Specifications - Page 1
Forum section: electronics / general

author: bw459205
author_rank: Newbie
timestamp: December 07, 2020, 04:13:31 pm
content: I have a question for a PV power system expert- If I want to run a piece of electronic equipment perpetually in a static outdoor environment, what is the process of determining the specifications of the PV panel and battery? The equipment draws circa 25 watts at 12 volts. I want to run it in the UK, so there’s only about 8 hours or so of daylight in the winter, and it’s usually pretty overcast. My current working assumption is a 250 Watt PV panel, plus a 100 Ah lead acid battery. This is based on a 2X factor for the battery and a 10X factor for the panel. Does this sound about right, or am I overdoing things? Thanks.

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 07, 2020, 04:30:42 pm
content: You need to use a tool like this onehttps://re.jrc.ec.europa.eu/pvg_tools/en/tools.htmlto estimate your solar production and you need to take into account anything on the solar horizon that may effect it (trees etc). If you use the off grid selection it even includes a battery.

author: bdunham7
author_rank: Super Contributor
timestamp: December 07, 2020, 04:48:23 pm
content: http://contemporaryenergy.co.uk/insolation-map/Are you able to orient the panel at an angle (slant) of at least 45 degrees and towards the south?If the panel is flat the math doesn't work in the winter at all. Your panel's 250W rating is at 1kW/m2and so in the winter it will only produce 125Wh/day or so. You need 600Wh/day even if the charger and battery are 100% efficient--and they aren't. If you can orient the panel differently, you can find insolation calculators that will help you determine the average output, but keep in mind that this will vary, so you need a safety margin and you need storage. To be safe, I think you need at least 1kW/h per day minimum, averaged over up to 3 days, which looks difficult give the dire insolation numbers for the UK in December.The battery specification appears to have enough energy to power your unit for 50 hours, but you can't use it like that and expect it to last. Ideally it should be fully charged at the end of each day and use no more than 20% of its capacity overnight. That way its life is longer and you have enough reserve for those times where you have a few days of rain or very hard overcast. Keep in mind that when you fully charge a LA battery the cycle is less efficient because the energy recovered from the last 10-15% of charge--the 'top-up'--is much less. If you make the system so that the battery isusuallyfully charged daily, then if conditions become unusually dire and it is unable to fully charge the battery, the round-trip efficiency will increase and you may get by until things brighten up.I used this site:https://pvwatts.nrel.gov/I used 0.25 and 0.50 as the DC system size (1 and 2 panels respectively) and looked at the energy production for December. It looks to me like you would need 6 panels @ 45 degrees tilt and 180 degrees azimuth plus at least 2 100AH batteries to see it through a difficult December. I tried London, Manchester and Glasgow, not knowing where you are, and got similar results.

author: trobbins
author_rank: Frequent Contributor
timestamp: December 08, 2020, 12:00:23 am
content: Are you going to use a PV regulator or mppt of some kind, or were you expecting to direct connect the PV module to the battery?You will need a low voltage disconnect function if your load equipment doesn't gave a self-disconnect or power down of some kind. The level of EODV will basically determine one aspect of your capacity/availability.Do you have any ability to alarm for a pending loss of power, and hence provide some form of back-up intervention?

author: f4eru
author_rank: Super Contributor
timestamp: December 08, 2020, 01:11:44 pm
content: What availability do you need ? 90% ? 99% 99,9% ?What battery life do you expect?What is the real daily average consumption?location, obstacles, roof slant angle, etc...

author: trobbins
author_rank: Frequent Contributor
timestamp: December 08, 2020, 11:44:43 pm
content: Quote from: f4eru on December 08, 2020, 01:11:44 pmThe OP may be more at ease with identifying how many times per year they could suffer an outage from battery disconnect (MTBF), and how long it would take them to return to site and somehow recover the battery to normal operation (MTTR) or otherwise have an automated reconnect that occurs at some substantial capacity level (assuming the PV is not disconnected).

author: bw459205
author_rank: Newbie
timestamp: December 13, 2020, 03:39:18 pm
content: Quote from: fourtytwo42 on December 07, 2020, 04:30:42 pmThanks very much. I checked out the tool. It looks good. This has opened my eyes to how terrible off-grid PV is in the UK!

author: bw459205
author_rank: Newbie
timestamp: December 13, 2020, 03:46:59 pm
content: Quote from: trobbins on December 08, 2020, 11:44:43 pmYes, MTBF is probably a more practical way of framing the problem. I'm happy to go and 'wipe it's butt' maybe once or twice a year. But on the grounds that something the size of a car battery would only hold the load up for a couple of days, my working assumption is that the PV needs to generate enough power to keep the battery topped up all year round. Which I'm learning very quickly leads to a huge panel area in the UK. Probably, this isn't going to be practical. Surviving November to February looks like it is challenging unless you've got multiple kW class panels.

author: bw459205
author_rank: Newbie
timestamp: December 13, 2020, 03:52:24 pm
content: Quote from: bdunham7 on December 07, 2020, 04:48:23 pmMany thanks for your time and attention with your post. Most insightful. The results (in the UK at least) are much worse than I expected!