DIY PV System to Save Money AND for SHTF

[Tarrel]

I've been racking my brains as to what kind of solar PV setup we need at the cottage. On the one hand, I want to be able to generate power when the grid goes down. On the other hand, it would be good to have a grid-tied system that saves us money on the electricity bill. (I try to work it so that money invested in preps also has a payback during BAU).

At first sight these are incompatible. Commercial grid-tie systems (such as your average joe has on his house to get the feed-in tarrifs) have an islanding device fitted to the inverter, meaning that if the grid goes down, the system shuts off. This is to prevent power you've generated being fed down the line and zapping the poor fella who's trying to fix the problem.

The other problem is we live in a listed building, and even then, the only viable roof area is partly shaded.

So, I thought about the following system as a project over the winter:

8 x 80W 12v panels off eBay (£80 each), wired as 2 x four-panel strings, each generating 48V nominal.

These mounted on a wheeled carrier (timber construction, around 2m x 2m, angled to around 45 degrees but adjustable). This would be placed in a sunny spot in the garden, which happens to be near the shed which has mains power.

Connect the panels to a Soladin 600 plug-in grid-tie inverter, which would be plugged in in the shed. This would feed in a maximum of 0.6 kW, which would be enough to cover a lot of our base load requirements for most of the day, maybe more.

Totally separately, set up a bank of leisure batteries to be trickle-charged off the mains (say, 300 Ah), and have available a 500W inverter to produce mains elec from these batteries if needed.

So, in a BAU situation, the solar PV would be contributing to our power needs. Assuming an average of 4 hours power generation per day, all year, gives 1460 hours of generation. Assume the system delivers 500W, gives an annual generation of 730 kWh. At 15p per kWh, the saving is £110 per year. I estimate total costs for setting up the system would be around £1300, giving an annual return of 8.5%, which compares favourably with bank interest. (Here's the politically sensitive part; we have an old fashioned meter that I believe will run backwards, so all the power the PV system generates will be made use of, whether we are using it or not).

In a short-term power-outage situation (not uncommon up here), we would use the batteries to provide power for the essentials (e.g freezer and low energy lighting).

In a post SHTF situation, or prolonged power-outage, I would rewire the panels and connect them via a charge controller to the batteries and use this as an off-grid setup.

This system doesn't give me the benefit of the feed-in tarrifs, as it is not professionally installed by an MCS installer, but it does give me the flexibility to design it as I want, and the capital cost is lower, meaning I don't need such a big return for it to make commercial sense.

Any holes in the plan, or blindspots I haven't thought of?

[Skean Dhude]

Sounds good similar to a system I was looking at. The only flaw I can see is that wiring that into the mains may have legal issues because it should be certified. A task an ordinary electrician may have an issue with.

Your 8 x 80W gives 1600 and you scale down to 500. Figures I have seen indicate this might not be scaled down enough particularly in the winter months. However, you are best trying it and seeing what it produces at certain locations and months,

Besides that sounds good.

[Paul]

Like the plan but 600 watts as an absolute max won't go far.
Plugging into the shed could be an issue.
Depending on the cable run, quality, rating, you could have greater line losses than you think.
I always down rate current capability by 1/3 when running though (can I say dubious) wiring.

Earthing could also be an issue.
If your shed wiring is correct and up to code it'll be fed through an RCD.
That RCD will be referenced to the house earthing and in urban locations that's usually PME earthing.
I've had issues with external generator earthing fighting PME and lets face it, solar is just another form of generator.
You trundling round panels on long wires won't help either.
Copper losses are something to take into consideration.

Also, inverters are pretty good at dealing with the occasional surge but hate too much rapid inductive switching i.e. motors and their start up chokes even simple ones like central heating pumps.
PV systems use the mains to buffer this electrical noise, but as a stand alone system it's going to be hammered.
It's because of this I don't directly drive my mains preferring to use the PV inverter to run a separate 500 watt UPS.

Have you thought of providing a totally separate LV system?
Solar is not bad at keeping batteries topped up even in winter (with a much greater markdown in performance in reality to the book figures).
We've got a simple 12 VDC loop in our house powering PIR triggered LED light sources.
Walk anywhere in the dark round the house and it triggers a 6 X LED light.
Total load with everything on? 228mA.
(Took SWMBO and I running frantically round the hovel to keep everything on to test this)
Average, 60mA. That's cock all load.

Two great advantages doing this.
It leaves the UPS to run the essentials i.e. boiler and horrible inductive water pump, leaving emergency capacity for the VERY important camping kettle (120 watts)

I work slightly differently though to conventional PV systems though and during a power out, CH and other essentials are all swopped over to a separate circuit.
Bit of a bind but safer all round.

[Tarrel]

Thanks for the thoughts about the earthing. I hadn't really considered this.

The Soladin 600 is a plug and play system which plugs into a 13A socket, so there shouldn't be any compliance issues. It has all the certifications for a grid-tie inverter.

I need to check the current draw for the freezer (start up and continuous). It's one of the new A+ rated ones. To be honest, the essentials for us are low energy lighting, laptop charging (although this may become redundant post-SHTF) and being able to charge my electric chainsaw for firewood production. The charger for this takes 65W and charging takes about 90 minutes. It's a 36V li-ion model. We have a wood fired Rayburn which gravity-feeds a thermal store. It provides space heating to a large part of the house, so no electricity required for heating, hot water or cooking.

I did think about a dedicated 12V system, but the running of separate circuits was a pain. I might consider keeping in store the materials to put this in after SHTF, when such issues as aesthetics will be less important! For me the important thing is to get hold of the equipment to generate the power now, while it's available, and, hopefully, get some return on the investment during BAU.

I'll definitely look into the earthing issue.

[NorthernRaider]

Guys I know almost nowt about PV systems but I was wondering if you have cost/ legal issues with plugging your DIY PV system into your house, could you not simply make and store a 12V DC lighting system to roll out if needed?

[Paul]

Keeping it simple is always best NR.
My PIR's are battery backed and I've got a simple £5 solar charger for the AA cells they take.

Besides that, I've got two hurricane lamps full of kerosine and 3 bags of tea candles for when all the clever bitz go 'pop'.

I may be a bit of a geek but I ain't stupid!

[Skvez]

8* 80W panels is 640W 'rated'.
You work with a figure of 730kWh / year.
My tests suggest a value of 220kWh/year would be closer to typical for 640W of solar in the UK.
You also don't consider the efficiency losses of your inverter, the spec sheet isn't clear about this but it suggests it's possibly 91% efficient although from some of the other figures they give I calculate 86% … who really knows. And in either case that’s the efficiency at full power, lower power is probably less efficient.

Certainly a viable system, I'd be tempted to charge the batteries from the solar and only switch to the inverter when they were topped up (rather than bother with the AC charger).
I'd also very seriously consider DC lighting (as others have suggested).

[Tarrel]

(31 October 2012, 19:45)Skvez Wrote: 8* 80W panels is 640W 'rated'.
You work with a figure of 730kWh / year.
My tests suggest a value of 220kWh/year would be closer to typical for 640W of solar in the UK.
You also don't consider the efficiency losses of your inverter, the spec sheet isn't clear about this but it suggests it's possibly 91% efficient although from some of the other figures they give I calculate 86% … who really knows. And in either case that’s the efficiency at full power, lower power is probably less efficient.

Certainly a viable system, I'd be tempted to charge the batteries from the solar and only switch to the inverter when they were topped up (rather than bother with the AC charger).
I'd also very seriously consider DC lighting (as others have suggested).

Wow, that's quite a reduction! I thought I was being fairly conservative by assuming an average of only 4 hours per day generation.

Your figures would indicate an inadequate return from grid power savings to justify the outlay. In that case, I'd probably be better going for a (cheaper) fully off-grid system of panels, controller, batteries and non grid-tie inverter, and take the advice from here to go for 12v where possible.

I'd see 12v powering; lighting, laptop, ipad, phones, and 240v powering freezer and chainsaw charger. We do have a three way camping fridge that runs on 240v, 12v or propane (not a freezer though). We've had this for a few years, and it seems to run most effectively on propane.

[Skvez]

Using figures from a professional solar installer suggests that a 4kW system would provide about 3300kWh/year with 'typical' UK sunlight.
Scaling would suggest your 640W system would provide a littler over 500kWh/year.
Either they're being optimistic or my test system was performing below par.

Solar still isn't cost effective compared to buying electricity generated from burning oil and gas but the cost of fuel continues to creep up and the cost of solar continues to creep down … some day it will be.