Should I go off-grid?

matelodave

HertsLad said:

matelodave said:

Bear in mind that you have to generate enough power during the day to not only supply your daily needs but also to replensih what you've used overnight from the batteries. You only need a day or two of minimal sunshine and you will deplete the batteries but also have no energy available for your daily consumption. 

3kwh at 240v = 12.5amps which translates to 150a/h at 12v or 75a/h at 24v. As lead acid batteries shouldn't be discharged below around 50% you'd need so store at least twice as much as you think you'll use plus you need to take into account all the inefficiencies that are involved. 

Running a generator to deliver enough energy to provide for your daily consumption could well be more expensive than paying your daily standing charge - especially now petrol is around £1.50 a litre and diesel even more and it's not the most efficient way to charge batteries unless you are prepared to run it every time you want some leccy.  Would you neighbours be happy with one clattering away for several hours to recharge the batteries - I dont think so.

I also noticed that you were thinking of using propane the cost of which becomes disproportionally expensive as the cylinder size reduces, so you'd need to contemplate 47kg cylinders to make that viable although it might work out cheaper to use a propane powered generator rather than petrol.

It may sound like fun to try and beat the system but TBH the cost and hassle probably outweighs any advantage that you might gain from not paying the standing charge.

You make some very good points and have correctly realised that I am having fun trying to beat the system.

You calculate I need 75ah at 24v. So in ball park terms, leaving out inefficiencies, that's presumably what my first two 12v batteries can almost supply, if drained flat (they are 70ah each). But I can only drain them to 50% and that's where the second pair of batteries come in, wired in parallel to the first pair. In drawing 70ah or 75ah from the array of 4 batteries, they will all be around 50% flat. If so, it is not far out. And it gets 'better' because this first array of batteries will only supply half the needs of the house. So they don't need to supply 75a/h but only 37.5ah. As a result, the first array of batteries will be only (roughly) 25% flat.

Is my thinking correct, or am I missing a trick?

I bought a 47kg cylinder of propane when the price was £89. I fear a refill will be far more expensive so am looking into the possibility of using LPG from a filling station. Is there a risk of prosecution or is it only fuel station regulations, one has to worry about?

A lot depends on how you intend to use the batteries - most are rated at the 20 hour rate which means that they'll only have their rated capacity if you drain them at c/20,  If you  drain them faster with a load greater than c/20 then their capacity is reduced by quite a bit - look up Peukerts Law - BatteryStuff Tools | Peukert’s Law - A Nerd’s Attempt to Explain Battery Capacity so all these things tend to conspire against a simple calculation.

Also take into account that running stuff from an inverter isn't always trouble free - some items need a pure sine wave rather than the stepped square wave from a so-call quasi-sine wave inverter. You also need to take into account starting loads, most things with a motor like a washing machine, microwaves or even an electric drill require between 3-5 times their running current to start  them so inverters and generators need to be capable of providing high peak currents for several seconds as stuff starts up - Microwave ovens are particularly brutal in that respect, especially the cheaper ones which pulse whilst operating.

Dragging lots of amps from batteries requires stout cables to minimise voltage drop, that's why solar systems use higher voltages than 12 or 24v as it's easy to lose a lot of power through undersized cables.

QrizB

If we're talking lead-acid, I should mention that a US full-time RVer whose adventures I follow managed three happy years of off-grid life from a pair of GC2 6v, 225Ah flooded lead-acid batteries, with regular maintenance and topping-up:

https://mouse.mousetrap.net/blog/2021-09-25-last-stand-of-the-GC2-.html

He has since switched to lithium.

Sadly GC2 batteries are more than $100 each in the UK; a pair could be £300:
https://www.ebay.co.uk/itm/153716666865

matelodave

This shows how the capacity is dependent on the sort of load - taken from QrizB's link for GC2 batteries

Voltage 6 Volts

Capacity (C5) 185 Ah

Capacity (C10) 207 Ah

Capacity (C20) 225 Ah

Capacity (C100) 250 Ah

The article written is also quite useful as it shows that you can make your batteries last a longer if you look after them and understand what is going on.

Conversely you can shorten their life dramatically, to a year or less if you don't.

Undercharging lead acid batteries or letting them go flat will finish them off in no time.

Likewise overcharging will consume the electrolyte so you do really need to make sure you can keep them in good nick with the proper charging equipment and a decent maintenance and charging regime., The same goes for Lithium, they need the correct treatment to ensure a long and happy life.

I'm not trying to turn you off, but after 35 years in the telecoms industry where they use a lot of batteries for stand-by and solar charging I've seen a lot of batteries destroyed by incorrect charging and lack of proper maintenance etc. BT could make exchange batteries last 25 years or more whereas some customers could finish them off in less than a year.

HertsLad

QrizB said:

He has since switched to lithium.

Do you know if Lithium capacity is subject to similar rules as lead acid batteries, if the capacity is used quite quickly?

Verdigris

You can get 95% DOD (depth of discharge) and more than 6000 full cycles from LiFePo.

mikb

Although there is an American bias to the content, you could spend forever researching and running numbers with the people at http://diysolarforum.com/ -- there is a lot of general advice and diving down deep into technicals and safety aspects too.

As to comments earlier about buying LiFePo4 cells from China direct, or via Alibaba/Aliexpress -- definitely read the above site and work out if you want to take that gamble.

Behind door 1: The cells you want arrive and it all goes well.
Behind door 2-5: Anything from  -- nothing arrives, wrong cells arrive, damaged, are woefully under-spec (relabelled smaller cells rated in Marketing-Amp-Hours)

HertsLad

Chris_English said:

What are you actually wanting to achieve here?...You are upset at a standing charge increase of about a pound per week; a completely negligible amount.
What’s driving you here? 

I don't regard it as minor or negligible. My monthly direct debit for gas and electricity has been £22 per month. In 5 days time it will go up to £55 per month for using a mere 550kWh per annum of gas and about 1100kWh per annum of electricity. It's too much. What's driving me, and what I seek to achieve, is a reduction in those costs. What's wrong with that, especially given that this is a money saving forum?

My latest idea, and I agree it's a bit ambitious, is to build my own wood fired power station, modeled on large coal fired power stations. So the main elements will be a boiler, steam turbine and alternator. If I succeed, the burning of wood won't be good for the environment. Many other people will increase their use of wood burning stoves. It's partly the Government's fault. They should have built several more nuclear power stations before the existing plants wear out.

Verdigris

This thread's definitely a wind-up. Time to move on.

QrizB

@HertsLad here's an illustration for you. I've got almost 3kWp of solar on my roof. Here's my generation day-by-day from the start of November until now.

If I needed 3kWh/day (ie. 1kWh for each kWp of panel) I would've struggled through November, December and January. It's only now that we're into February that I'm managing to generate that much on most days.

And the peaks are high enough (over 8kWh on the 1st and 2nd of November, and again on the 11th of Feb, for example) that I'd either need a lot of battery capacity or I'd waste the power.

Even with 9kWh of panel - 3x what I've got - I still wouldn't have managed to consistently generate 3kWh/day through December and early January.

(You'll find my energy stats for the last nine months summarised in this post, if you're interested.)

HertsLad

A 12 panel system was predicted by EON to generate 3,659 kWh per annum on my roof. Even if it's a bit optimistic, it's still quite good.

If I use 1659 kWh (i.e. more than my present 1050kwh), mainly when the sun shines, then 2000 kwh might be exported to the grid. If Octopus pay 10p per kwh, then the credit could be £200. But it seems more likely for receipts to be 5p per kWh long term so the credit becomes only £100. Or even less if my figure of exporting 2000kWh to the grid is too high. Or if the Octopus standing charge is even higher than EON's. Furthermore, I fear Octopus are not registering new customers at present.

An approved grid tie system costs around £4000 including scaffolding, labour, costly electronics and, of course, profit.

I can buy 12 panels for £1500. 6 controllers and inverters (1 each per pair of panels) might cost £500 so the price difference is £2000 between DIY and supplied/approved.

The payback period seems too long and the initial outlay of £4000 is too high. My view is that it's more interesting to go for the DIY option, with less than 12 panels. That way, the initial outlay will be lower and less electricity will go unused. Maybe 2000kWw would go unused if I have an off grid system with 12 panels. I know I could use 'surplus' electricity to heat water for showers and washing clothes but I already have solar thermal tubes which will do that.