Building a Solar Power Mains Backup System for Your Home
Foreword: Carrying Out Electrical Work at Home
Although this is an ‘off-grid' project, and as such does not normally require a qualified electrician to make the changes, part of the project requires a connection from your electricity consumer unit fusebox to one of the components of the system.
This means that not only is this project not one which is easily reversed or removed, and as such is a major electrical project in your home rather than a ‘quick green fix', but it also requires a qualified electrician to make the actual changes and connections.
Part P of the Building Regulations specifies that changes to the electricals in your home should be notified to the local council, and inspected and approved by Building Control. However, if you employ a properly qualified electrician, he or she can certify the wiring him- or herself. You should also inform your home insurance company that you have an off-grid solar PV system in your home.
Ready for the Power Cuts?
With climate change constantly in the news over the past few years, many people are considering solar power as a means to cut their electricity bill and reduce their home carbon footprint.
A large, grid-connected solar photovoltaic system will certainly do this, and, with the introduction in the near future of a ‘feed in tariff' for the UK, could pay some good financial dividends to anyone going down the grid-connected route.
But another issue has begun to raise its ugly head in recent months - the possibility that due to a general lack of joined-up thinking in central government, over the next decade or so the UK will not have enough electricity generating capacity to meet the needs of the nation. Old coal and nuclear plants are due to be closed over the next few years, and the UK will be importing gas from increasingly unreliable parts of the world. Russia, Libya and Iran have already decided to form a new ‘Gas OPEC', and the target of building thousands of new offshore wind turbines by 2020, whilst laudable, may not be technically possible in the end.
The upshot of all this is that power cuts may become increasingly likely and commonplace.
That's OK, you might think, even without electricity we will still have heat, because we have a gas boiler. Unfortunately not - most modern gas boilers have electric ignition and an electric water pump. Even those of us with a grid-connected solar PV system will not be able to use the electricity generated during a power cut, because all grid-connected systems are designed to cut out when the grid goes down.
So what's the answer?
Well, there is an intermediate solar power solution which can kill two birds with one stone, although because it is not technically grid-connected, you will lose the advantage of being able to sell electricity back onto the grid. However, this solution will give you light, heat and communications during a power cut, as well as free electricity from solar power to reduce your bills and cut your carbon footprint.
Imagine You Live on a Boat
Imagine you live on a houseboat, permanently moored and taking its power from the shore. It doesn't go anywhere and if there is a problem with the shore power at the mooring, all your lights go out and you have to go down the pub. Not so bad, you might think, but when you get home your houseboat is cold and dark, and your cat is a trip hazard.
But the people in the boat moored next door have an ocean-going yacht, and when the shore power fails you notice that they still have lights, are playing music and watching TV, and seem to be very happy, laughing and joking, watching everyone else scratching around for candles in the dark.
Your neighbours have solar panels on the roof of their yacht, but of course they are no use at night. But what your neighbours also have is an on-board bank of batteries, and something called a ‘power combi' - a clever box which houses a solar charge controller, a battery charger and a power inverter so that they can run all their normal domestic 240V A/C appliances from their 24V battery bank.
It turns out that this very same box of tricks has a connection so that when they are moored, they can just plug it in to shore power and it will charge up their battery bank and run all their appliances directly from shore power.
Normally what happens when they cast off from shore and go sailing is that their fancy power combi box switches over automatically to the batteries once they disconnect from shore power. In the event of a power cut, the very same thing happens: the box of tricks just switches seamlessly over to the batteries.
Now hang on a minute, I hear you say, I don't live on a boat, I live in an actual house, miles from any water! What does this have to do with me?
Identifying Your Essential Services - Heat, Light and Communications
Right, down to the nitty gritty.
Most homes have more than one electric ring main, to serve different parts of the house, such as different floors. Or possibly there is a separate one for the garage. Our end-of-terrace has two ring mains - one serving the kitchen, with all the heavy-duty appliances like the cooker and the washing machine on it, and the other one serving all the wall sockets in the rest of the house.
The first thing you need to do is think about your essential services - the ones you would really like to keep during a power cut. You should disregard the stuff that needs a lot of power, like many kitchen appliances, but the fridge/freezer doesn't use that much power so you will probably want to include it/them in your essential services list. Typically you will be thinking about:
- your fridge/freezer
- your gas boiler/central heating pump
- table lamps with low energy light bulbs plugged into wall sockets
- your television/laptop with TV card
- mobile ‘phone chargers in wall sockets.
Those will be the main things - basically, food storage, heat, light and communications. If you can use your mobile ‘phone as a modem then you can even get internet access with a laptop during a power cut - it's likely that the local telephone exchange will be down and broadband will also be down though, so even if you have a working computer/telephone, you may still not be able to use terrestrial telephone services.
Once you have identified these ‘essential services', you need to find out which ring main in your house they are powered by. You can do this by going to your consumer unit (fusebox) which will probably be with your electricity meter, flicking off each circuit one by one, and seeing which appliances go off.
What you are really looking for is a ring main that doesn't include any high-power appliances but does include the supply for your essential services like your gas boiler and fridge. Once you have identified this ‘essential services' ring make a mark above the trip switch for it on the consumer unit fusebox, so you know which one it is.
It may be simple to identify your ‘essential services' ring main, but equally it may not be. When we first wired up our own system, we thought everything was arranged correctly, but when we put on our dishwasher after hooking up the system, the power combi started to overload and would have cut out if we hadn't switched off the dishwasher.
It turned out that the dishwasher was not on the separate kitchen ring main as we had thought, but was on our ‘essential services' ring main with the other low-power appliances. The dishwasher uses a lot of power, and was pushing the power combi over its 1.5kW capacity limit. What we had to do in the end was have an electrician re-wire the plug socket that the dishwasher was plugged into, to the high-power kitchen ring main.
Another way of doing this would be to use a simple power extension cable, and run it from the back of the dishwasher to a plug socket in the high-power kitchen ring main. Using a power extension cable is the simplest solution, and you can use one or more of these cables to make sure that the correct appliances are plugged into your ‘essential services' ring main. For example, if there are two ring mains in your house, one for upstairs and one for downstairs, and the gas boiler is plugged in downstairs, you could run a power extension cable from upstairs to downstairs, have it plugged into one of the upstairs sockets, and then plug the gas boiler into it downstairs, so moving it onto your ‘essential services' ring main.
You should make sure that the appliances which are plugged into your ‘essential services' ring main do not exceed the power supply capacity of your chosen power combi.
Your Backup Power System
Your mains power backup system for your essential services ring main is exactly the same as the system on the ocean-going yacht described above.
The components of your system will be:
- a power combi - incorporating a power inverter, a battery charger and a solar charge controller
- a bank of deep-cycle (leisure) batteries
- some solar photovoltaic panels.
There are many different makes, models and combinations of the above, so it's probably best if I just describe the setup that I have at home, and you can make your own judgement as to whether any of the different components need to be adjusted for your own situation.
For my power combi I use the Powermaster PM-1500SL 24V:
www.powermastersystems.com/Template_000.htm
This combi includes a 30A solar charge controller which can take up to about 1kW of solar PV at 24V, or around 500W at 12V. As well as being able to include more solar PV, using a 24V setup instead of 12V tends to be more efficient too.
This is an incredibly clever box of tricks and includes all sorts of features which I don't use but you might want to, from a dusk-‘til-dawn activated output circuit for lighting, to a connection to start a backup generator. You probably won't need it all but it's there if you do.
I bought the 1.5kW model - basically it can support appliances running up to a total of 1.5kW, which is more than enough for the ring main in my home that it supports. I can use relatively power hungry appliances like the vacuum cleaner on it without any problems. It does struggle with a hairdryer though, and my very understanding partner now has to dry her hair from a socket in the kitchen, so be warned! But for everything else - desk lamps, computers, hair straighteners, TV, video games console etc - it has no problem at all. Powermaster do a 3kW model as well though in case you need more power than that, and there are probably other models out there with an even higher maximum output. Mine can also support ‘start up' surges of up to 4.5kW, for things that take more current to start but then settle down. So there is plenty of power there.
For batteries, I have 720Ah of no-maintenance, gel electrolyte lead acid deep cycle batteries. The gel electrolyte batteries are more expensive, but have the advantage that they emit little or no hydrogen gas, and do not require topping up with water every so often. Remember that your batteries must be located somewhere with good ventilation, and away from naked flames, to avoid the risk of an explosion. At the same time remember that your power combi must be with your batteries (but not in a cupboard with them), but your power combi should also be as close to your consumer unit fusebox as possible (although this is less important). Bear in mind also that your power combi may make a bit of noise: mine has a cooling fan in it which can be quite loud when you are not used to it, so you might want to put it somewhere where it is least likely to be a distraction.
For solar PV panels, I have three 165W Schuco 24V panels - remember the voltage of the PV panels must match the voltage of the power combi you choose, and if you are using 24V, your batteries must be wired up to give a 24V circuit. You would normally do the latter by splitting your 12V battery bank into two halves, wiring the two halves together in parallel at each end of the row, and the batteries in each half together in a row in series.
If you have roof space on a south-facing roof, that's the best place for your solar PV panels, and you might have to get a professional installer to put them up. Mine are mounted on my south-facing back wall, because most of the roof space is taken up with my solar hot water panel.
It is possible to use the setup without solar panels at all - this would provide backup for the mains on your ‘essential services' ring main in the same way as any uninterruptible power supply (UPS) would do. If you use the system without solar panels, you should nonetheless switch over to the batteries on a regular basis, say once a month, and let them run down and recharge, because otherwise the batteries may not retain their storage capacity, and may not last for very long when you come to actually need them.
Follow the instructions that come with your power combi for wiring up to the battery bank and solar PV panels. Make sure everything is done in the correct order specified in the instruction manual.
Connecting Your Backup System
In completing this step, you should be assisted by a qualified electrician.
Once you have correctly set up your backup power system, it's time to connect it into your essential services ring main.
What You Need
A length of mains power cable, the normal standard for house wiring: The current capacity must be matched to the output of your power combi - in the case of mine, it's 15A or above, but it's 30A in the case of the 3kW model. How much you need depends on how far your power combi is from your consumer unit fusebox - you will need approximately double the distance, but get a bit more just in case.
A mini circuit breaker (MCB), again 15A in my case, but 30A for the 3kW model: This MCB will go in between your power combi and your ring main, and it will prevent damage to your power combi by tripping out in the same way as the trip switch in your normal fusebox does in the event of an overload somewhere on the circuit.
Go to the consumer unit and, for safety, switch the whole thing off with the big main red trip switch. Then find the switch you have already marked as being your essential services ring main, and switch that off as well. You should be safe to work now.
Install the mini circuit breaker near to your consumer unit fusebox. You will need a wooden panel somewhere to screw it onto; there may be room on the existing panel with your fusebox.
Using a screwdriver, loosen the screws holding your chosen ring main cable into the fusebox, and pull the cable out. You then need to wire this cable into the OUTPUT of the mini circuit breaker, making sure each wire is attached to the correct terminal, in exactly the same way as it was wired into the fusebox.
Then you need to take a length of mains cable, and connect the 230V/240V OUTPUT on your power combi to the INPUT on the mini circuit breaker.
Finally, take the last length of mains cable and connect the SHORE POWER INPUT on your power combi to the switch you have marked on your consumer unit fusebox, which you took the ring main cable out of.
Double and triple check all the connections, from the battery bank to the solar panels, to all the mains wiring and the connections into the mini circuit breaker. One wrong connection will at best trip out the circuit or at worst damage your equipment or could even cause death from an electric shock.
PLEASE NOTE: A QUALIFIED ELECTRICIAN MUST MAKE THE ACTUAL CHANGES TO WIRING.
It's Time to Set Your Solar Sails
If you have wired everything up correctly, you're now ready to set sail on the sunny seas.
Make sure the on/off switch on your power combi is set to ‘off', and the MCB is switched off too.
First of all switch back on the big red trip switch on your fusebox. All your power should come back on except for your chosen ring main.
Next, switch your power combi on. The batteries you have bought should come ready charged, so when you switch the combi on, it should go through its setup procedure and then show the battery voltage on its front display. If you have wired up your solar PV panels correctly, and the sun is shining outside, the solar panel logo on the combi should light up to indicate that it is receiving electricity from the solar PV, and there should also be a light indicating that the battery charger is charging the batteries.
So if all is well with your combi, and everything is working so far, the next thing is to switch back on the trip switch on your consumer unit fusebox for your chosen ring main. This will start supplying mains power - ‘shore power' - to your combi. You should see a light appear on the combi indicating that 240V ‘shore power' is present. You are now also charging your batteries from the mains as well as from your solar PV panels.
If all is well here, throw the switch on the mini circuit breaker to its ‘on' position. You should now have power to your chosen essential services ring main, and your batteries will be charged from the mains and your solar panels at the same time.
You might want to leave the system a little while to allow the batteries to be fully charged and topped up.
Casting Off From Shore
Now for the moment you've been waiting for. Time to pretend there is a power cut. Go back to the consumer unit fusebox and flick the big red trip switch for the whole house.
If everything is set up correctly, what should now happen is that your combi realises that you have cast off from the shore, and switches over automatically to take power from the batteries and feed it into your essential services ring main.
Everything not on your essential services ring main will go off - ceiling lights, the shower, kitchen appliances, or whatever. But everything on your essential services ring main should be working quite happily. Check the gas boiler to make sure, and the desk lamps, the TV, the fridge and whatever else you have on the circuit. It should all be working perfectly. And more than this, the power being generated from your solar PV panels will be going directly into the circuit, running your appliances on pure clean green solar energy, and making your batteries last as long as possible.
You are now adrift on the infinite solar sea. Congratulations! When the power cuts come, you will probably have all your neighbours banging on your door demanding to have their mobile ‘phones recharged and asking for you to keep their grandparents warm. The best way to avoid this is to show them how to install a system like this themselves, before it happens. Spread the word!
What is now happening is that you are making the most of the input from your solar PV panels. If you have enough solar PV, you may find that in the summer months you can just leave your essential services ring main to run off-grid, and you should do so because that is the way to use the electricity from your solar. But as the daylight fades, what will happen is that your batteries will run down. When they are near to being empty, your power combi will sound an alarm. When you hear the alarm you should switch the trip switch on your consumer unit fusebox back on, and leave it on until the batteries have fully recharged from the mains ‘shore power'. You can then flick the trip switch off again until the batteries run down again.
With my own system, we find that in the middle of winter without any solar power to top the batteries up, we can last around 48 hours off-grid with 720Ah of batteries. We could probably make it last longer than that by conserving power if we had to. Two days is a long time when you're freezing in the dark, and to be honest, if the power cuts go on for longer than that, we will probably have more to worry about than missing this week's Top Gear.
Power cuts will be just as likely in the summer however, due to increased demand for air conditioning etc, and if like us you have a solar hot water system, the last thing you want is for it to be sitting there on the roof absolutely useless because there is no electricity to run the pump. So if you make sure that is on your essential services ring main too, then you can run the solar hot water system using electricity generated by your solar PV panels.
There's Always a Down Side
There's always a down side, and in this case you have probably already guessed what it is.
It's basically the fact that you have to top up your batteries from the mains every so often, depending on how much the sun is shining and how much electricity you are using on your off-grid ring main.
In the summer months you may not have to top them up very often; in the spring and the autumn it will be more often. In the winter, we just leave the mains switched on permanently, so we are not really getting the benefit of the little solar power we do get in winter but the batteries are always fully charged in case of a power cut.
It basically means a bit more maintenance and attention than you would have with a normal grid-connected PV system. The benefit of course is you have all your home comforts during a power cut.
Another downside is the efficiency of the battery charger. For the Powermaster combi specified above this is a maximum of 85 to 90% which means at least 10 to 15% energy is wasted when charging the batteries from the mains. If everybody did this then much energy would be wasted. For this reason you should only go ‘off grid' with your system during sunny spells and emergencies.
It's worth considering in advance of installing a system like this whether the benefit is worth this small amount of hassle - to us, it's just second nature now. We don't even think about it, it's just routine like watering the plants.
If you do decide to go for it - good luck!
Andy
Photos (c) Solar Bud 2009
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10 Jan 09
