How to calculate solar power energy?

[Brovashift]

Hi all, I'm new to off grid living and currently in the investigatory stage, but I'm trying to find out how to calculate required power, based on power draw, and recharge rate, number of batteries etc. Ideally during winter months, so short cloudy days, working to the worst case scenario so that longer summer days should be covered.

 

I have been looking at a 6000w solar panel kits just to experiment with, and seeing how long a 600w light will last on a single battery charge, and how long it takes to recharge, simply because I have a 600w light and ballast. 

If I had a 100ah battery would the light last 10hours? Pure guesstimation...

I was thinking of doing some trial and error, but I'd prefer an easier and cheaper way of calculating this stuff from people more knowledge than myself, if possible. Thanks 

 

TIA

[Marvin]

I think your about to get a blast of experience any minute.... (not just from me!)

 

Trial and error is ok but there are calculators out there that you can use for free to get reasonable figures. Have you used this one:

 

https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html

 

M

[TonyT]

Look at your current energy bills to see what kind of consumption you have.

That’s a good starting point

[Dillsue]

18 hours ago, Brovashift said:

Ideally during winter months, so short cloudy days, working to the worst case scenario so that longer summer days should be covered.

Remember that in summer your usage is likely low and potential generation is high. In winter its the reverse so if you spec a system for worst case winter load, it'll massively overproduce in the summer. 

Are you properly off grid as in no grid connection available  or just want to be more independant?

[JohnMo]

Couple of things worth noting

 

Panel angle, near horizontal great summer production and rubbish winter, near vertical lower summer production (but you will have loads anyway), but pretty good winter.

 

Several small arrays spanning east to west will give more production than a single large array pointing in one direction.

 

 

[Brovashift]

Thanks for the tips guys. I'm currently looking at that PVGIS tool shared above, looks very useful thanks, but just working my way through the "Getting Started" guide as it seems pretty complicated at first glance.

 

This post was actually the result of a conversation yesterday after a few beers down the pub, just making the most of that sunshine before winter kicks in properly lol.

 

I have been looking into off grid living i.e. completely off grid, no mains, and exactly what that would entail regarding; heat, water, waste, gas, and of course electric... and if its really feasible to live off grid with all the mod cons, and not turn into a complete Hill Billy lol. No offence to any Hill Billy's reading this post 😅

 

I'm in North Wales and so would look to buy land, assuming it is a realistic project, somewhere in the NE and surrounding areas.

 

You posts having given me a lot to think about. Regarding system spec I have no idea at the moment, I am right at the beginning of a very long journey, still in the investigatory stage. I assumed that if I need X amount of power to run everything for 24hrs, and have a solar panel big enough to produce more than I need, or as mentioned small arrays, which I assume is several smaller solar panels? As well as adequate battery storage, and an automatic sun tracker to follow the sun for best angle. I still wonder if this would be enough, even on trickle charge days, to not have to rely on an expensive backup generator.

 

Obviously if we were in a sunnier climate I'd have more confidence. This is the UK after all. 

[SteamyTea]

6 hours ago, Brovashift said:

I have been looking into off grid living i.e. completely off grid, no mains, and exactly what that would entail regarding; heat, water, waste, gas, and of course electric... and if its really feasible to live off grid with all the mod cons, and not turn into a complete Hill Billy

Start by getting accurate measurements of what you currently use and when you use it.

 

Most off grid people have a generator tucked away.

[Dillsue]

By all means tinker with PVGIS but it's output is meaningless if you don't know what your consumption is.  You need to work out your daily demand. Assuming you're not currently living in a doorway, get yourself one of the basic plug in energy meters and start to understand what each bit of electrical kit consumes over a day. Then you can add everything up and have an idea of how much you consume. If you're on your own metered electric supply, take daily readings and youll know exactly now much you are using per day.

 

Generators are cheap to buy if you want to be fully off grid, but why? For a couple of hundred £ a year standing charge the security of a grid connection is a bargain. You don't have to take any electricity from the grid but it's a nice bit of insurance to have. Obviously if there's not already a supply where you're living there's a connection charge.

 

 

[MrTWales]

I'm one day shy of a full year with panels and it's amazing how generation varies. Not just by month but by day. I know it's kind of obvious but one day can be three times the previous day very easily. My system is 12*410w panels at only 10 degrees on a flat roof, so bad for winter but it was an easy install and I can clean myself etc. 

 

The chart shows the amazing May and crappy July/August that we had!

 

[sharpener]

@MrTWales' bar chart is very characteristic and similar to what we have experienced over 12 years with our first 3.7 kW of panels, July and August have often been poor compared with May.

 

Since last year we have a total of 6.9kW installed PV capacity plus 10.65kWh in Pylontech batteries and a 5kW Victron inverter/charger but are very far from self-sufficient in winter (except when we are away, when there is a standing house load of about 200W which is mostly MVHR on setback, and so the batteries will store about two days' worth).

 

Analysing existing energy consumption figures is a must. Also a significant amount is used for cooking so much depends on whether you cook by electricity. We have an oil AGA running 24/7 in the winter which helps a lot, even so we needed to upgrade with a 3rd battery module charged on E7 to cope with winter evenings.

 

Ppl who are wholly off-grid often have 20kW installed solar and 25kWh or more in battery storage, often thermal storage as well bc this is cheaper per kWh. Very expensive in terms of capital and hard to justify on purely economic grounds. Our more modest setup has a predicted payback of 6 yrs. You could achieve almost the same results for less with a cheap Chinese inverter but we wanted the Victron for its 2hrs of whole-house backup.

 

I don't want to be disloyal here but this forum is more specialist and an afternoon's browsing there will give you a lot of interesting case studies of what ppl have done and what you can get away with!

[TabWheeler]

replying to follow the topic as I am also looking at going completely off grid. Also wanted to know if you're allowed to do the full electrical install DIY or if you still need a registered electrician for a completely off grid home?

[SteamyTea]

2 minutes ago, TabWheeler said:

wanted to know if you're allowed to do the full electrical install DIY or if you still need a registered electrician for a completely off grid home

I think Building Control will need to see the electrical certificate before sign off.

 

Regarding electrical usage.

It is quite easy to not be drawing any electric power. Parasitic loads are storage killers. MVHR and sewage treatment pumps are a bit of a problem, but they don't have to re run 24/7.

It really is a matter of sizing correctly and accepting that sometimes you just have to turn things off, or not have them working all the time.

Cooking is an interesting one.

It is not compulsory to use every job, and have an oven on for 2 hours every day.

If you are low on battery storage, boil some pasta or rice, heat a jar of sauce up, supper sorted.

[joth]

49 minutes ago, TabWheeler said:

replying to follow the topic as I am also looking at going completely off grid. Also wanted to know if you're allowed to do the full electrical install DIY or if you still need a registered electrician for a completely off grid home?

You could charge 12 or 24V DC batteries from PV and use that to power everything you can. So long as there's no mains (240V) in the house, and all less than 50V, then it's not covered by building regs. Basically, it's same as car or caravan or narrowboat electrics.

If the house has any installation 240V then it needs to comply with relevant regs for RCD protection, consumer units, wiring etc regardless of where the electrons are coming from.

[sharpener]

3 hours ago, TabWheeler said:

replying to follow the topic as I am also looking at going completely off grid. Also wanted to know if you're allowed to do the full electrical install DIY or if you still need a registered electrician for a completely off grid home?

 

You don't need a registered electrician and you can get your LA Building Control to inspect your own DIY installation, which is what I did with my Victron storage system. IME they do not know a lot about electrics but in some places they contract it out to a third-party inspector who does.

 

Put everything you can think of on the initial application so you can come back later when you add the EV charger, heat pump or w.h.y. without paying again.

 

Get hold of a copy of the IEE Code of Practice for Electrical Storage Systems, it is incorporated into the Building Regs and there is a lot of stuff to comply with, including earthing which is a complicated subject on its own.

[Hastings]

Unless you have acres of PV panels you will likely need a generator or wind/water turbine to get you through the winter months.

 

The biggest issue and the one that 'gets' you in the winter is the total background demand ie. the total of all bits of gear that are on all the time. These can include water filters (40-80W for UV steriliser) which could be multiple (iron and manganese filters are common for borehole sources). The DC-240V AC inverter : even a smallish 4kVA Victron consumes 25-30W continuous. You can install an electric-free septic treatment unit or add on another 50-100?W for the normal (and much cheaper) air blower ones. So that takes you into the range of 70-200W. My 6x380W mono-crystaline (better in overcast conditions) PV array sometimes produces a total of 40W in the middle of a dull rainy day in November.

 

Much harder to calculate is the demand from things turning on and off automatically. Heating systems, water pumps, fridge/freezer. Off-gridders with no turbines usually resort to simple wood burners.

[Hastings]

Be very careful of performing calculations using average or mean weather data, which is what is mostly available. It is the minimums that you need to plan for.

 

It is too many years ago that I used the PVGIS tool but I think it may use averages. A more useful resource is pvoutput.org where PV array owners upload their live and historical power production levels. It will show you that a 4kW array in Glasgow produces almost no significant power in Dec/Jan.

[Marvin]

9 minutes ago, Hastings said:

Be very careful of performing calculations using average or mean weather data, which is what is mostly available. It is the minimums that you need to plan for.

 

It is too many years ago that I used the PVGIS tool but I think it may use averages.

 

Yes, the basic uses averages compiled from hourly records over many years.  You can use the PVGIS tool to produce hourly recorded data over 10 years for each hour of each day. You just export an Excel file once you have chosen your PV location etc data. Using that information made me realise how pointless it would be for us to attempt to go off grid! 

 

Below is a data output for our setup if we had it up and running for 2011 to 2020 for each month (across the top) and year down the left side. 

The figures are in kW hours.

 

Comparing the best year to the worst year production is about 4% difference either way from the average of the years.

 

Comparing the best in a month to the worst in the same month is at the bottom.

 

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
2011	179	241	600	800	856	773	780	690	569	420	248	180	6,337
2012	221	286	612	689	775	702	812	647	585	310	258	185	6,083
2013	182	269	426	695	764	801	920	782	522	393	251	182	6,187
2014	222	353	647	679	795	886	931	744	573	382	228	188	6,627
2015	237	274	546	787	710	843	777	624	662	401	174	155	6,190
2016	207	312	538	707	826	706	805	785	528	460	272	190	6,337
2017	236	264	474	736	795	852	771	727	501	372	267	171	6,167
2018	191	380	431	635	883	879	927	685	611	470	289	147	6,528
2019	235	379	514	731	862	807	911	800	582	327	225	197	6,570
2020	195	286	576	816	971	821	806	737	620	341	235	184	6,588
Average	210	305	536	728	824	807	844	722	575	388	245	178	6,361
spread of KWh production over each month.
High	237	380	647	816	971	886	931	800	662	470	289	197	6,627
average	210	305	536	728	824	807	844	722	575	388	245	178	6,361
Low	179	241	426	635	710	702	771	624	501	310	174	147	6,083
	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
hi/mid percentage	13%	25%	21%	12%	18%	10%	10%	11%	15%	21%	18%	11%	4%
Mid/ low percentage	-15%	-21%	-21%	-13%	-14%	-13%	-9%	-14%	-13%	-20%	-29%	-18%	-4%

 

[Hastings]

I recommend you concentrate on your autumn/spring period. It's just about do-able, whereas winter isn't (in the UK).

 

Then with the over production in summer connect up a hydrogen generating machine to store hydrogen for winter heating!

[TabWheeler]

On 01/11/2023 at 18:10, joth said:

You could charge 12 or 24V DC batteries from PV and use that to power everything you can. So long as there's no mains (240V) in the house, and all less than 50V, then it's not covered by building regs. Basically, it's same as car or caravan or narrowboat electrics.

If the house has any installation 240V then it needs to comply with relevant regs for RCD protection, consumer units, wiring etc regardless of where the electrons are coming from.

Fab thank you. I was going to put a 240ac inverter in only because it felt like a useful thing to have around. the only specific uses I could think of for it were a washing machine and dishwasher but there must be 12/24/48v versions for boats and motorhomes. If I can avoid the need for Building Regs then I will! - unless anyone wants to tell me a really obvious reason for needing 240ac that I've not thought of!!

[JohnMo]

Mains powered equipment is as cheap as chips, other voltages and DC equipment not cheap, even the light bulbs are expensive. Every electrician understands it, DC is a different animal that people are not used to.

 

DC cabling tends to be quite big, so costs more to install. You can't go the Screwfix or similar place on a Sunday morning to pick something up.

 

Complying with building regs is easy, not expensive and you know it's safe. 

Edited November 5, 2023 by JohnMo

[Dillsue]

11 hours ago, TabWheeler said:

Fab thank you. I was going to put a 240ac inverter in only because it felt like a useful thing to have around. the only specific uses I could think of for it were a washing machine and dishwasher but there must be 12/24/48v versions for boats and motorhomes. If I can avoid the need for Building Regs then I will! - unless anyone wants to tell me a really obvious reason for needing 240ac that I've not thought of!!

If you buy an inverter with a 13a socket built into it and plug an appliance directly into it then there's no building control or part P spark needed as its all factory assembled/tested and deemed safe. The moment you start installing 240v cabling/sockets then it comes under BC/part P

[joth]

Of course, the fact it's not controlled by building regs doesn't automatically make it safe. Low voltage wiring means higher current, for the same delivered power, and high current can easily get super hot and start a fire if buried in a wall.

 

[SteamyTea]

16 hours ago, joth said:

Of course, the fact it's not controlled by building regs doesn't automatically make it safe. Low voltage wiring means higher current, for the same delivered power, and high current can easily get super hot and start a fire if buried in a wall.

 

Yes, a very important point.

The electrical shock risk is reduced, but not the fire risk.

 

All switching has to be capable of dealing with the DC currents as well.  Not a case of assuming that ordinary switches can cope.

 

Edited November 7, 2023 by SteamyTea

[Hastings]

I chose to install a 240V inverter with my off grid 2-storey house refurb due to the considerable extra cost of heavy gauge DC wiring that is needed to run wires further than a few metres.

 

Even 48V, which is my battery bank system voltage, can only be run the length of an artic lorry using fairly standard cable. 12V works cost-effectively around a car etc.