Solar electric system 101

We're investigating building in some resiliance and looking at practicalities of what to do with our home electricity system. I found this video a nice introduction to solar power, inverters, chargers, batteries, generators and so on. Maximus Ironthumper lives off grid and is UK based, so talks about our 240 Volt system rather than 110 Volt appliances. The principles will be the same though.

Here in the US, my own Schneider XW+ 6848 provides split-phase 120/240V AC, which I need to power my submerged well-pump.  120VAC goes to the standard wall outlets.

I also have their smaller Conext SW4024 inverter for the workshop system.  It also provides 120/240VAC, though all of my power tools so far consume only 120V.

Have you come up with an itemized list yet of what it is you need to power.  At my own homestead, with lights, TV, computer, and the refrigerator running 24/7, I find I need ~3.5kWh of power per day.  More if there are construction projects or whatever.

I could suggest that as a starting point.  You'll need to come up with a system that will provide enough power for the winter months, which will be far dimmer than the summer months.  How many gloomy, cloudy days do you get between sunny ones in the winter?  That will be your breaking point.  I could suggest to you two alternatives.  First, design a solar-only system that can provide your needed power over the winter, or Second, a smaller solar array with a generator backup.

I went with the former, because solar panels are now dirt-cheap in the western US.  We have a local seller's venue here called "Craigslist" that I routinely shop for panels.  Typically you get the best deals on high-voltage residential sized panels (grid-tie style).  With a MPPT charge controller, you can wire strings of grid-tie panels together, run the raw high voltage solar electricity back to your home, and let the MPPT controller transform the high-voltage down to battery charging voltage.  The extra voltage gets transformed into extra charging amps.  At my own cabin, I'm running arrays of four 30V panels in series to get 120VDC.  This gets transformed down to 50-something volts to charge my 48V battery bank.

The 6848 inverter then takes the 48V battery power and converts that to US standard 120/240V 60Hz AC.

I'm at 35 degrees North, and I'm finding that in the winter, in the rain, I get about 0.5 sunhours (sh) of power in December.  On sunny days I'll get 3.0sh. I'd suspect you get at bit less if you North of 55 degrees, and you'll need ~6000W of panels to make 3+kWh of power in winter.   Won't know for sure until you actually have the system wired together.  I'd suggest starting at 6000W of solar, but plan on upgrading if your production lags behind what I've experienced.

Alternatively, go with a smaller 48V system with 3000+ Watts of solar, but incorporating a backup generator that charges the batteries in the winter time.  Both my 6848 and 4024 inverters have ACin terminals to accept 240VAC directly from the generator to charge the batteries.  So, no separate charger is needed.  With a 5000-8000W generator, you can start that up on gloomy winter days and top off the batteries that can't be fully charged.

Thanks for your reply Michael. It's all a bit daunting; to know where to start and how to clarify requirements. At the moment I'm thinking of a several stage project:
1) To introduce a simple battery back up system that will power "essentials" such as lights and computer for a few hours. We've done this already at our shop which seems to be successful in keeping us able to sell things over the till for a few hours if we lose power there.
2) Introduce a renewable system that will provide some power to direct use when available. At the moment we're actually leaning to a very simple waterwheel generator rahtre than solar, mainly because we already have bits to do it (other than the smart inverter, which could be that purchased for #1). The disadvantage is that this is unlikely to give us surplus power for water heating in summer. Solar would give us more in summer (we get a good 20 hours of some sort of daylight in June/July) meaning we wouldn't have to run the stove for hot water for baths or pots of tea.
3) Introduce further renewables - solar and/or wind. The surplus from this would be dumped into water heating. The aim is to have a system that keeps a days supply in reserve (currently (if I remember correctly) we use about 10kWhr a day).
4) if 3) is successful, consider going off grid. It seems crazy, but most of our electricity charge at the moment is standing charge for our connection, we pay slightly more than most in the UK due to our highlands location. If we have sufficient internal redundancy, then this final stage is the only one that would actually give us a quick payback financially.

Nancy Reading wrote:Thanks for your reply Michael. It's all a bit daunting; to know where to start and how to clarify requirements. At the moment I'm thinking of a several stage project:
1) To introduce a simple battery back up system that will power "essentials" such as lights and computer for a few hours. We've done this already at our shop which seems to be successful in keeping us able to sell things over the till for a few hours if we lose power there.
2) Introduce a renewable system that will provide some power to direct use when available. At the moment we're actually leaning to a very simple waterwheel generator rahtre than solar, mainly because we already have bits to do it (other than the smart inverter, which could be that purchased for #1). The disadvantage is that this is unlikely to give us surplus power for water heating in summer. Solar would give us more in summer (we get a good 20 hours of some sort of daylight in June/July) meaning we wouldn't have to run the stove for hot water for baths or pots of tea.
3) Introduce further renewables - solar and/or wind. The surplus from this would be dumped into water heating. The aim is to have a system that keeps a days supply in reserve (currently (if I remember correctly) we use about 10kWhr a day).
4) if 3) is successful, consider going off grid. It seems crazy, but most of our electricity charge at the moment is standing charge for our connection, we pay slightly more than most in the UK due to our highlands location. If we have sufficient internal redundancy, then this final stage is the only one that would actually give us a quick payback financially.

Hi Nancy,
It being the winter and i was idle today I punched your location into my solar modelling software. Congrats that is the first time my software returned a null result for a 3 month period!!! So the graph shows daily production for a 1kW solar array as well as monthly production. A 1 kW array is quite small array but it gives you an idea. The software adjusts for Where you are and adds in local weather conditions (hence the no results in summer).
Cheers,  David

David Baillie wrote:Congrats that is the first time my software returned a null result for a 3 month period!!!

Well you made me laugh!
I don't quite understand why it would be returning 0 for the summer months but generating (a bit) in December. That doesn't seem right to me (I could accept the other way around though!). Is this based on actual weather (for Oban)? We had quite a good summer here till into July last year, Oban is a hundred miles or so further South and will have very slightly different weather to us.

Nancy Reading wrote:
4) if 3) is successful, consider going off grid. It seems crazy, but most of our electricity charge at the moment is standing charge for our connection, we pay slightly more than most in the UK due to our highlands location. If we have sufficient internal redundancy, then this final stage is the only one that would actually give us a quick payback financially.

I suspect that this is going to only get worse as lines companies and grid operators start to deal with the costs of maintenance and upgrades to infrastructure that's out on the fringes of their service areas. The first thing they will do is try to shift costs onto the customers out on the periphery, so that "slightly more" could turn into "astronomically more" with the stroke of a bureaucratic pen. One thing that might help (but probably not in your case) will be all the wind and solar generation that's being deployed at the edges of many transmission networks. Britain has a very centralised grid backbone because when it was built the main generation came from coal and the big plants were in the industrial heart of the island, but now there's all that wind power coming online and that's mostly coastal or offshore, or in sparsely populated moorlands.

At some point, it might make sense for the residents of places like Skye to "cut the cord" and run the local grid for the island itself, with the generation handled by residential solar excess in the summer and wind the rest of the time, some battery storage, and maybe a thermal backup powered by biomass from coppice forestry.

Phil Stevens wrote:At some point, it might make sense for the residents of places like Skye to "cut the cord" and run the local grid for the island itself, with the generation handled by residential solar excess in the summer and wind the rest of the time, some battery storage, and maybe a thermal backup powered by biomass from coppice forestry.

There is quite a bit of wind power potential on Skye, and mixed local feelings towards it (as you might expect) Various schemes are in concept and are dependent on upgrading the distribution network. I'm not sure how things may pan out in future in this case.
You may be aware that Eigg, also part of the inner Hebrides, is not grid connected, but has managed to set up their own island grid as you suggest with wind, water, solar with back up diesel generators and a 24 hr battery storage bank. It seems to work, but certainly wasn't an easy task to set up. You can read more here : http://isleofeigg.org/eigg-electric/ and I found this nice video as well:

Nancy Reading wrote:
1) To introduce a simple battery back up system that will power "essentials" such as lights and computer for a few hours. We've done this already at our shop which seems to be successful in keeping us able to sell things over the till for a few hours if we lose power there.

This is easy.  A small 12V system will supply this.  Let's say you consume 125W watts of light electricity for about 8 hours in the winter.  That's five highly efficient compact floursceent bulbs.  That's 125W X 8hr = 1000Wh of power.  Say your TV consumes 50W, and you watch 2 hr per day.  Another 100W.  It takes at least 25Wh of power keeping the inverter turned on.  For 8hr, that's another 200Wh.  Total for an on/off 12V system, 1300Wh.  You don't want to drain your batteries less than 50%.  So, you need (1300Wh X 2)/12V = 216Ah battery.  Two 6V golf-cart batteries, such as a Trojan T-105 fit this size.  Let's say you only get 2.0sunhours (sh) of light in the winter.  That might be optimistic?  To get 1300Wh of power with only 2.0sh you should install 1300Wh/2.0sh = 650W of solar panels.  Maybe three 240W grid-tie panels.

Nancy Reading wrote:
2) Introduce a renewable system that will provide some power to direct use when available. At the moment we're actually leaning to a very simple waterwheel generator rather than solar, mainly because we already have bits to do it (other than the smart inverter, which could be that purchased for #1). The disadvantage is that this is unlikely to give us surplus power for water heating in summer. Solar would give us more in summer (we get a good 20 hours of some sort of daylight in June/July) meaning we wouldn't have to run the stove for hot water for baths or pots of tea.

Keep in mind that 20 hours of daylight does NOT mean 20 sunhours.  A sunhour is the amount of sunlight that keeps the panel at FULL POWER.  A 6am the sun may have risen, but the sun might actually be shinning on the back side of your panels if the panels are facing South, and the sun has risen at East-Northeast.  Maybe by 7-8am, the sun has shifted far enough South to actually be shining on the front of the panels, but at almost a right angle, solar output is only 5%.  Let's say that maybe you get 7-8 sunhours of power in summer.  As your system grows, 12V becomes more and more problematic.  Just too many amps flowing through the wires to get substantial power.  Practical limit is maybe 1000-1500W.  After that the current draw could potentially start getting dangerous.  At this point you are better off switching to 24V.  With a 24V system, loads up to able 3kW are easy to manage

Nancy Reading wrote:
3) Introduce further renewables - solar and/or wind. The surplus from this would be dumped into water heating. The aim is to have a system that keeps a days supply in reserve (currently (if I remember correctly) we use about 10kWhr a day).

Solar by far is going to be the easiest to implement.  I personally would start with solar, and once you have a system up and running, then start to experiment with hydro or wind.  I honestly don't have any practical experience with either, but I don't see many success stories with either.  With solar, the success stories are everywhere.

Nancy Reading wrote:
4) if 3) is successful, consider going off grid. It seems crazy, but most of our electricity charge at the moment is standing charge for our connection, we pay slightly more than most in the UK due to our highlands location. If we have sufficient internal redundancy, then this final stage is the only one that would actually give us a quick payback financially.

With what I've got planned out in section two, I'd say you'd be ready to go off-grid.  Your single biggest problem is going to be switching electronics.  Going from 12V to 24V means at least swapping out a new inverter, and buying more batteries.  Maybe it might be more cost-effective in the long run to start out in section 1 at 24V instead of 12?  Most MPPT charge controllers can handle at least 12 and 24V, but the better quality ones can be used for 12V, 24V, 36V, and 48V systems.  An entry level charge controller that can work at either 12V or 24V is Epever's Tracer 4210AN.  It can handle up to 40A at either 12V or 24V.  That means as much as 1000W if you chose 24V.  A somewhat better choice would be their 6420AN, or even their 8420AN controller.  More amps and more volts for future expansion.  24V inverters cost about the same in side by side comparisions with 12V ones, but you would need two more 6V batteries, and maybe two more 240V panels.

So, in the short-term, you'd need to spend more money to buy two more batteries, and two more panels, but in the long run, you'd save money because you can grow the system without having to replace all the electronics.  Over time, you can upgrade to a larger battery size/number, or upgrade the number of panels.  With the 8420 controller you are likely to expand up to 2400W of solar.  I would say that was money well spent.

Nancy Reading wrote:

David Baillie wrote:Congrats that is the first time my software returned a null result for a 3 month period!!!

Well you made me laugh!
I don't quite understand why it would be returning 0 for the summer months but generating (a bit) in December. That doesn't seem right to me (I could accept the other way around though!). Is this based on actual weather (for Oban)? We had quite a good summer here till into July last year, Oban is a hundred miles or so further South and will have very slightly different weather to us.

Not knowing your exact location I punched in a generic "isle of Skye" location. If you are comfortable send me an approximate long/lat and ill adjust. The software uses the closest weather station to its long lat position. It uses historical weather not any individual year. If I had to guess the zero production it would probably be no noticeable production during the key daylight hours. I agree it seems odd ill have to refine a bit. On a tangent I assume you have checked out out Hugh Piggott's site: https://scoraigwind.co.uk/
As I'm sure you know he literally wrote the book (many books now) on homescale wind... I had his book but eventually discarded wind for my location.
Cheers,  David

David Baillie wrote: It uses historical weather not any individual year. If I had to guess the zero production it would probably be no noticeable production during the key daylight hours.

It may be that at 57 degrees North the sun is not intense enough, but I would have thought that was even worse outside of summer hours. There are a few solar installations locally and as far as I know they do generate something. I'm expecting that it would definitely be worth getting the latest technology to get good generation in lower and overcast conditions. You could try Benbecula (57.4458 N -7.31964 W) I think the weather there is closer to ours than the mainland tends to be.

On a tangent I assume you have checked out out Hugh Piggott's site: https://scoraigwind.co.uk/
As I'm sure you know he literally wrote the book (many books now) on homescale wind... I had his book but eventually discarded wind for my location

Wind is probably the most potential for my location, but the most difficult to get right. It's something I've looked at in the past, but not so much recently. Our average speed is something like 15mph over the year, but gusts can be up to 70 most winters (although we've not measured it here ourselves recently). I'll have to do a lot more research before starting to put something together (it won't be this year!)

A quick thank you Michael for your in depth calculations! I'll have to read and digest before commenting further.

Nancy Reading wrote:
4) if 3) is successful, consider going off grid. It seems crazy, but most of our electricity charge at the moment is standing charge for our connection, we pay slightly more than most in the UK due to our highlands location. If we have sufficient internal redundancy, then this final stage is the only one that would actually give us a quick payback financially.

Gosh yes, Nancy!

I've now got solar panels, so in summer the standing charge is double the amount I pay in electricity from the grid.

At 53.8 degrees North, with 6 1.5 kW solar panels, between May and September I used 1kW hour each day from the grid and exported 6 to the grid.

The last three months, as you know, have been spectacularly free of sunshine, so with the short days as well I've been importing 3kW/day and exporting one.

NB I've got gas central heating, hence these figures. I'd like to get a device fitted to the panels to transfer excess power to the hot water tank to reduce the gas consumption (further).

Helen - I've got one of these: Paladin solar HW diverter. Don't know if there is a UK equivalent, but our mains here are 240V 50 Hz as well, so you could get one of these. Works a treat.

Nancy Reading wrote:I'm expecting that it would definitely be worth getting the latest technology to get good generation in lower and overcast conditions.

Unfortunately the tech you are looking for is older and all but abandoned due to politics and geography. You can find amorphous panels if you look hard enough, but they are outside the economies of scale that are pushing current centralized shipping based production- unless the UK/EU has found a way around sunbelt engineering effects, you will be incentivized to save money by burning more carbon to go "green".

Thanks for 'bumping' the thread Coydon. By coincidence, in my efforts to declutter my paperwork I came across an article I kept from at least 15 years ago with some links to solar calculator sites. Some of them are of course defunct now, but this one seems pretty good (see chart it generated below). It showed I can expect to generate a reasonable amount of power in summer at least; above 100kwhr/month from April to August inclusive from a 1m2 south facing array. Very little (less than 25kWhr/month) between November and February
Yearly PV energy production [kWh]: 644.54
Yearly in-plane irradiation [kWh/m2]: 865.14
Year-to-year variability [kWh]: 17.22
It seemed to make remarkably little difference for different angles however, possibly because the day length is so long in summer? At the moment I'm thinking of the house gable end which faces almost due South, but a roof mount due West or East is another possibiliity.

Maybe if it is old technology I am looking for I can find some old panels that someone is upgrading? No hurry anyway, something will turn up.

Is this to be for personal/off-grid use? General calculators are likely to assume a fixed angle is being done to produce maximum total power to feed back into the grid. For personal use it would be beneficial to angle the panels for max output around winter solstice. This will reduce summer output a good deal with less of an increased return in winter, but usually summer generation is still more than plentiful. I'm not sure if places get enough fog/clouds in summer to affect that, or if wildfires will start to mimic supervolcano eruptions, but that is the conventional wisdom now.

Similarly, having all panels centered on a North-South axis will give the greatest overall production, but having half pointed a bit east and half a bit west will give you better production early and later in the day when you might otherwise already be drawing on the batteries.

The thread with Helen's radical simplicity mentions the winter angle thing...

Hi Coydon - good points. This is for personal use. The feed in tariffs just don't make sense now financially - you are better off using your electricity yourself as Phil Stevens alluded to above.
For myself, I think that relying on just one form of renewables is not robust enough and I would prefer to have a degree of redundancy (especially if we do go off grid). My husband still favours water power, I quite fancy wind (Skye is quite windy!)

isle of skye average wind speed (km/hr)

For reference 16 km/hr is about 10mph.
Both of those options involve moving parts, which of course have downsides.
My personal thought for us is that wind and water will give us more in winter, solar more in summer so they balance out. I would quite like to have excess PV in summer to use for water heating, and potentially cooking as Maximus in the original video does. Sometimes we just don't want to ramp up the woodstove for hot water and having the sun heat it by electric makes the plumbing a whole lot easier! Having more than one panel at different angles is certainly something to consider (although the fewer holes in the roof the better!)