Small scale "backup" solar PV (ie not FiT)

[RunningOnEmpty]

First post here (been reading a while!) so hello all.

I'm thinking of putting in a small non-grid-tied PV system to allow us to be able to use radios, charge mobile phones etc in the event of a power cut (an ocurrence that is not commonplace at the moment, but is only going to get more so as time goes on).

I should say at this point that I'm not really interested in trying to run my house off PV; my goal here is to have a minimum provision of mains power to run essential items - including, potentially, the pump of a potential solar water heating system. No point in a solar hot water system to reduce reliance on "the grid" if it doesn't work in a power cut!

Roughly speaking I was thinking of the following:

- ~250W PV panel - £500
- charge controller - £160
- large "deep cycle" battery(ies) - £150
- inverter of around 0.5Kw capacity - £150
- optionally, a mains battery charger

Given that this is much smaller than a "normal" FiT PV installation, I'm hoping to be able to achieve it for around £1000.

Has anyone else here tried anything like this, and if so how did you go about it? I'm also thinking that I could spread the cost slightly by buying the battery(ies) and inverter first, along with a mains charger, to give the benefit of a backup system, and then add the solar equipment later on.

Any observations, musings, thoughts and suggestions welcomed.

Cheers
Calum

[clv101]

Hello and welcome!

It's important to think about what you are trying to do, what you are trying to mitigate. If the plan is to maintain a basic electrical service during powercuts, then you don't really need to generate anything. Just keep the batteries charged from the mains, ready to use during the powercut. I expect you'd get a significantly more (approx. 3x as much using your numbers) energy storage by just buying more batteries instead of the PV panel.

Using the PV to generate only becomes useful if you are expecting extended powercuts of weeks. Cuts of just a few days can be covered by battery storage of grid electricity.

However - I quite understand if the geek in you wants build a complete PV system, I'm sure it's a very satisfying thing to do. Forum member mikepepler has done just that and may be along in a bit.

[RunningOnEmpty]

You make a fair point There's two things in my head here:

1. "Wow, you can make electricity from sunlight" (which I still basically think of as this -> <- close to magic, even though I understand the basic physics involved)

2. An awareness that the infrastructure that 'keeps the lights on' is tremendously fragile and that we're likely to experience that fragility much more often in the not-too-distant future - but it's not likely to disappear completely for a fair while to come.

To address point 2, you're quite right - a trickle charger into a battery bank is by far the best way to do it.

My thinking in adding a PV element to the system is that as power interruptions become longer and more frequent, the ability to charge from non-grid sources becomes more valuable. There is also the fact that the kind of highly sophisticated manufacturing and supply chains that make this stuff available to us won't be around for ever - and that the prices won't keep dropping either.

I fully acknowledge that these are more medium - long term concerns, though.

[emordnilap]

Welcome to PS, RunningOnEmpty. Enjoy.

[mikepepler]

Hi RunningOnEmpty

Funnily enough, I installed almost exactly the system you're describing back in April. Details and photos on this link (there's a part 2 at the end of it covering the electrics):
http://peplers.blogspot.com/2011/04/eco ... -grid.html

I don't list costs on there though, so here they are approx:

Main system:
- 6x40W Yingli panels (12V nominal) £600
- mounting rails and brackets (you'd not mentioned these on your list) £300-400
- MPPT 15A charge controller £200
- 40m total cable (you'd not mentioned this - it's not cheap) £100
- second hand batteries 2x180Ah 12v £100
- assorted connectors and fuses (you'd not mentioned these) £100

Secondary system (because there was space and I had the battery and controller before we bought the house)
- 1x40W panel as above (mounted on same rails) £100
- 30A PWM charge controller £60 (end of line, about 5 years ago)
- 80Ah 12V Rolls battery (new) £140

For actual power supply, I've got:
- one step-down and two step-up DC-DC converters, about £20 each
- numerous 12V car-socket adaptors for phone, camera, etc. £5 each
- couple of 12V AA/AAA battery chargers £10 each
- Victron 180W pure sine inverter (I've had this for ages) £100
- Victron 350W pure sine inverter £130
- Siverline 700W modified since inverter £45

For battery maintenance:
- top-quality 12V battery charger that can vary voltage so it can do reconditioning and equalisation (runs off mains, but could easily run off one battery/PV with inverter while equalising the other) £100
- hydrometer and bottle for monitoring and safe topping up of batteries £30
- distilled/deionised water - £2/litre?

As you can see, the ancillary components, mounting kit and maintenance stuff can cost more than the panels! Obviously this means bigger is more cost effective, as some costs are roughly fixed. However, some of the stuff is useful for other things - I've used the battery charger on our car, and have also lent it to two friends in the past few months.

Our system runs these full-time, though I do turn them all off at night:
- solar thermal pump/controller
- wireless router
- cordless phone
- phone/camera/AA charging as required
- LED night light for our pet birds

When it's sunny I also plug in a laptop or two, and from Apr-Sep there was surplus power to run the fridge and freezer in the day. In a grid-down situation, it could probably run the fridge and freezer for a few days, but if I thought it was going to go on for long, I'd probably turn the freezer off pretty quickly and use the power for other things (heating, solar thermal, lighting, etc).

Probably also worth considering the ability to charge 12V batteries for friends - that way they're onside rather than jealous in a power cut! You'd want to do this without disconnecting your own system, so either a DC-DC charger, or mains charger running from an inverter is probably the answer.

[adam2]

For a basic 12 volt only system, you may find this thread to be of interest
http://www.powerswitch.org.uk/forum/vie ... php?t=8246

A simple 12 volt only system is easily installed in a day or two without any speciailst knowledge.
For a larger system or one useing an inverter, it would be well to gain experience first, or to engage a specialist.

[RunningOnEmpty]

Thanks for all the replies folks. I've spent the evening reading the "simple 12V" thread, a number of links off that, most of the pages in your blog and a fair few data sheets from various suppliers and manufacturers websites. A lot more to consider than there at first appears (as with all projects of this type). Yes, I forgot to add in cabling and of course with lower voltage comes higher current and thicker cabling.

I also need to go back to the drawing board and decide (with the not-at-all-technically oriented wife) decide what we really want to achieve before I spend any money or drill any holes.

So, I'm sure I'll have a few more questions here in due course


Cheers for now,
Calum

[kenneal - lagger]

Welcome Callum.

Doesn't Andy Hunt live up his way? He has a blog about his house and PV system which might be of interest. http://greencottage.burysolarclub.net

[adam2]

As others post, if you fear only short term power cuts, then battery storage of grid electricity is the way to go.

If however you fear long term power cuts of more than a few days, then PV generation looks more worthwhile.

A battery charging PV system seldom makes economic sense.
Any such should be considered to be insurance against disasters and emergencies, not as an investment.

If a battery charging system is installed as insurance, then it may as well be used whilst times are normal in order to reduce grid power used and therefore save a little money.
Take care that whilst times are normal, that the battery is kept nearly full, in order that most of the capacity is available in case of power failure.

Batteries should not be regularly fully discharged, though VERY OCAISIONAL deep discharge should not reduce the life much.

[RunningOnEmpty]

Well, where we are in Stockport (south east Manchester) we get little in the way of really severe weather - we're protected from the east by the Pennines, but not up into the peaks enough to be exposed to the kind of stuff they get in Buxton. As a result we don't get downed lines because of snowfall, but we do get interruptions from time to time when a part fails in a substation or someone cuts a cable when repairing the road, or whatever.

My guess is that as time goes on, faults that are fixed in hours now may take some number of days to be resolved - and that we will sleep walk into a generating capacity issue, as well (topic for another day!)

So immediately, it's about providing light and ability to charge mobiles / run a radio during short power cuts. Longer term I'd say we're looking at the ability to run a fridge for a few days to prevent spoilage of food. None of this is (directly) about saving money for me - it's much more about having a resilient household when the normal utilities aren't working. Of course, if power cuts become much more commonplace than they are now, all this kind of gear will become much more sought after (and in truth, this should be a concern for early adopters - I'd expect things like PV panels to become a target for theft in years to come, I'm afraid).

Setting that to one side though, this is very interesting reading (found through the Green Cottage blog) - depends how far one wishes to go (both in changing one's house - and lifestyle - and spending money).

I think what seems sensible is to work out a way of starting small that still provides benefits (eg - mains charger, one battery and an inverter) and then augmenting it as and when funds, time and spousal consent allow.

[emordnilap]

Thanks for that link ROE, some worthwhile reading there.

[rue_d_etropal]

Suggest you have a talk with Andy, as it is his house in Bury. He is a member of this forum and I think his set up would suit you.