Building solar power for off grid application is rather simple, we have touched on the man cave, but for some the image of solar can be a little bit of a mixed bag. Here I hope to put some light on the basics (pun not intended).
The application.
It may seem silly to some, but the application that you are intending solar to be used for is one of the more important parts of the package you will need. Lets say you want a freezer to run from solar and its in the back of the garage or shed.
The freezer will have a “start up current” but will generally run around 200 watts. The start up current which applies to most electrical motors and tools, is the important factor for the inverter selection.
lets say the “surge” is 2000w, thats about 9 amps of power you need to delivery in an instant, a 2Kw inverter would struggle to do this.
The energy use to make the current is stored in the coil, but most modern inverters use a smaller coil and are know as HF ( high frequency) inverters, LF, yes you guess it low frequency have a much larger coil. but can cope with bigger motors.
To cut to the chase here you would want around 4kw of inverter. you could use a 3kw LF inverter. Prices here vary between £350-£750 ( this being a AIO type).
Battery and Voltage.
Ohms law applies to power, meaning that Amps X voltage = Watts. and they can be flipped around, W = V/A or A= W/V.
When it comes to inverters, they tend to have a working voltage, 12v 24v or 48v. this is Nominal and comes from the old Lead acid battery voltages. However the voltage of the inverter will dictate the current (A) that is used to make the AC power (watts).
So lets say you have 3kw. 12v would be 250a. 24v would be 125a and 48v would be 63a
The power draw in A requires bigger cables and different batteries can provide different output currents.
Typically a 12v lead acid battery should draw only 55 amps – that’s just 660 watts,
A 24v LFP battery can provide 100 amps – That’s 2400watts and so on for the 48 volts.
You can parallel the batteries so two 24v batteries will provide 4.8Kw of power, three provides 7.2kw.
What is also true is that you would lower the amps (A) from each of the battery, provided that the inverter power stays the same, and for a long life, lower A is a good thing for batteries.
For example my 3 Kw off gird system has 2 LFP batteries, running one battery draws 125 amps; The two draws 65amps from each of the batteries. I also have a second set which would then put the power draw (strain) even lower at 31.5 amps.
Read more on Maximise your solar battery
Now we have some ideas, lets look at how we should proceed with solar.
While the output is just one part of the solar installation, we have to look at our input.
It is often over looked, or just generalised, but we need to look at the power in two parts. The charge and discharge.
If we have a load on the inverter all night at 300w and for the sake of ease, Ill say Night is 21:00 to 09:00 that means we use 3600w during the night that we need to charge back to the battery.
We also need to think about the day load which is still 300w per hour.
The issue then turns to Sun hours.
Sun hours are the typical duration that the sun is out and fairly good, for the uk this varies from 2 hours to 8 hours.
3600w / 8 = 450w +300.
As you can see here, the charge to the battery has to offer 450w of solar power and we also have to account for the load of 300w. this means that for the 8 hours we would need to make 750w from our solar on 8 sun hour day, on the 4 hour days thats 1500w and 2 hour is 2100w.
However it is wise to note that I have not included the day to night ratio.
So lets say you have four 435w solar panels, that gives you a total solar power of 1740watts
Solar tend to not work as claimed from a number of reasons., you cannot fix a solar array and get the advertised solar production from the panels and you should look at the NOCT data. For the above panels the realistic power would be around 1340w, but you will also have exchange loss, which is around 15%.
So we now come to the compromise.
You can charge the battery from the grid or try to solar panel the power, which is the UK would be impossible or rather impractical. Therefore for the 90 days of low solar, grid or generators may be something to consider.
Cost and return.
The ball park figure for the solar kit would be around £1,500. we then say that we make an entire saving of the power used, 7.2kwh, at 0.22p a kwh is a saving of £1.58 per day repays you in 950 days (2 years 6 months).
This is a fairly low return time compared with other installations, but we like to look at the life picture too. this means that you would save above £2,852 and around £4,267 in total.
The kicker.
There is always a twist with solar, if you do this a different way and had the house on the system and it was grid tied, so we will say add £1000 for a grid tied inverter, and add another 2 panels (£150). we have a different outcome.
The solar panels even with the 4 panels would produce more power than you would use for much of the summer – Between March and October, That’s going to be around 12kWh a day or £2.90, if you are off grid then the power is lost. adding the two panels would give you around 19kwh a day again that’s around £4.40 per day and above £1,242 per year. The return time of this investment is 2.1 years.
So as you can see a little bit more, reduces the return time on the investment. and you would also save money on the house hold bill.
NOT a upsell.
This may seem like a upsell, but this is not the case, you will find that going for the cheaper option as “a test” or a “starting from” is what most people do, they buy a little system and then realise that they could do more with it and save more, and this is how the solar takes over. by having that little bit more, and the next step you are very likely to take, you save yourself £400 of having to buy the off grid inverter to replace this will a hybrid inverter later down the line.
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While off grid works, its when you see the power going to waste hat you could use in your home that will bother you the most. its knowing this option with all things still applies to solar installations, be that off or on gird.
How we did the math.
When it comes to doing math I do like to be a little reserved. there are many factors which effect solar power. So the Kwh price I used was £0.23. For annual solar yield I used 275 days, and I counted the panels as 400w. The life time of a solar kit is around 15 years, but I tend to use 10 years.
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