Solar for emergencies and … fun

Half my home runs from solar power now, and it was relatively cheap to do (ie less than £1000). It was time-consuming, though, and there are plenty of gotchas you need to know (or be prepared to find out!)

This guide is meant to give you enough information to buy and set up a simple system to keep some lights on, phones and iPads charged, and music running in the case of a power cut.

First, some FAQs:

[EXPAND I already have solar panels on my roof, feeding into the grid. Can’t I use them in an emergency?]

No – or at least not without some major purchases and a fair bit of re-configuring. If you have solar panels installed professionally for FIT (feed in tariff) use, they’ll be connected to a grid-tie inverter. This inverter takes the variable power from the panels, converts it into mains-style AC voltage, and pushes it back out to the grid. Although on the face of it it seems you could just wire the output from the inverter to your fusebox and let the solar panels power your house, it won’t work for a number of reasons:

1. Solar power is unpredictable and highly variable – a single cloud in front of the sun can reduce the power you generate by an enormous amount just for a moment. This is no good for running things like TVs and computers which need a constant clean supply. So, your house is never supplied directly by the solar power; it always uses power from the grid. Any power your solar panels generate is fed back to the grid separately. It’s two separate systems.

2. The inverter is designed to work only when it detects the mains voltage and phase coming from the grid. Partially, this is for safety; if an engineer decides to cut power off to your street to work on the street cabling, it would be dangerous for your house to be pushing voltage out onto those wires.

3. There’s a bit of misinformation going round that suggests you could use a small generator to fool the inverter into thinking the grid is present, thus letting you use the solar power generated. However, the inverter is designed to generate as much power as possible from the panels and push it out to a big grid. This only works because the grid acts like a big sponge – it can absorb as much or as little power as you can generate. You may fool the inverter into thinking there’s a mains supply, but any power it generates that you don’t use there and then will end up being pushed back into the generator – which has nowhere to put it. The generator will overheat and die.

That’s not to say the solar panels already on your roof are useless, but you’d need to buy something to store the power (batteries) and a solar charge controller to use instead of the grid-tie inverter. Note that you’ll probably need a more expensive charge controller than the ones mentioned in my shopping lists, as professionally installed solar panels tend to be wired up to produce a much higher voltage (and lower current) than the 12 volt panels mentioned on this page.[/EXPAND]

[EXPAND Can I use an ’emergency’ solar setup to cut down my electricity bill as well?]

By all means! There are limitations, though. The big problem with solar power is that you normally need the most power when the solar panels aren’t generating any. You need lights round your home, but you need them to work at night, when the panels aren’t doing anything, rather than in the daytime when the sun is out. So it’s all about storage. The big advantage of the grid-tie approach (feeding power back to the grid) is that the grid is big and absorbent enough to soak up every last bit of power you can generate. If you want your system to work off-grid, you have to handle the storage yourself – which means having batteries. Batteries need a certain amount of monitoring and care to get the best life and use out of them.

With a reasonable sized battery bank (say, 2 or 4 big car batteries) you can store enough power during the day to feed low energy lights and gadgets at night when you need them. Even so, you still need to be sensible and frugal. You can certainly light your house well enough with solar power, but every light left on when you don’t need it will discharge the batteries needlessly. Most people have at some point found their car battery flattened because they left the sidelights on overnight. They’re only little bulbs – but because they’ve been left on for hours, they’ve eaten through all the juice in the battery.

At my house I use some tricks to stop this happening. Most of the lights in the house are fully automatic; I’ve built some little sensors that fade the lights on automatically when you enter a room, and then switch them off when they detect you’ve left the room. It’s convenient, and kinda fun – you never need to look for a light switch – but more importantly, it stops lights being left on by mistake.

In addition to this, I’ve installed low-voltage lighting everywhere round the house. Mostly LED strips, as they’re cheap, convenient to stick under cupboards and behind radiators, and they run directly off the car battery voltage (12 volts). They’re extremely efficient, and they tend to run for ever – no bulbs to burn out.

So, yes, you can cut your electricity bill, but it’s the combination of free solar power PLUS a more efficient approach to using electricity, that ultimately saves money. It could still be 10 years or more before it pays for itself, but the other benefits – independence from the grid, mainly – make it worthwhile.[/EXPAND]

[EXPAND Will I be able to run mains-powered stuff?]

Yes – but with caveats. You won’t be able to run energy-hungry appliances like kettles or washing machines unless you have a huge solar setup (and a massive battery bank). However, you can use a car inverter (like these sort of things) to run smaller things – laptop chargers, portable TVs etc. You’ll need to keep an eye on how much juice they’re taking to make sure you don’t flatten your batteries. See more on monitoring below. [/EXPAND]

[EXPAND How much power / how many solar panels / batteries do I need?]

Ahh – the ‘How long is a piece of string’ question. It’s impossible to predict without knowing what loads, lights, gadgets you want to run, and how long you’ll need them to run for. The only way to really find out is to set up a small system, and monitor it. Much of it comes down to human behaviour – if you want a lot of light round your house you may need three times the size of our setup here. As a rough guide, me and the missus have 5 (80w) panels and two big car batteries (around 250 Ah), and that gives us enough light, music, phone charging etc to stop us needing to use mains lights at all. Our batteries do get a little flatter than I’d like, though, so we may double up for safety. The 5 panels (400 Watts total) seem to do fine for us. At some point I may wire our main TV to the solar power, in which case I’ll probably need another panel or two and another pair of batteries. The TV gets a fair bit of use(!)[/EXPAND]

[EXPAND How much power will one car battery hold?]

Car batteries (or SLA – simple lead acid batteries) are marked with a capacity in Amp-hours. This means, in principle, that a 12 volt 50 Amp-hour battery should be able to deliver 1 Amp at 12 volts for about 50 hours. Or 2 Amps for 25 hours. (As an idea of real-world use, a phone charger may use up to 0.5 Amps, so theoretically you should be able to charge it for 100 hours. A 5 metre strip of LEDs will use around 1.5 – 2 Amps, so you’d expect around 12 hours of light).

However…

Car battery capacity is often overestimated – you can usually expect to get 70-80% of their capacity at most.
Also, the capacity varies depending on fast you try and get the power out. If you were to try and take 50 Amps from our 50 Amp-hour battery, it would be flat in a fraction of an hour. Big currents – running lots of lights and stuff – make the battery less efficient, and it’ll lose power as heat, leaving less for you to use.
And: car batteries are really not designed to be discharged very much. The normal life of a car battery involves taking a lot of current for a very short time (as you start the car), perhaps discharging it by 5-10% of its capacity, and then straight away you recharge it (by driving around). So a car battery in good health is never usually discharged more than, say, 20%, before it’s being topped up again. Car batteries hate being less than 100% charged. Their chemistry means that whenever they’re not fully charged, lead sulphate – similar to scale in a kettle – starts to build up on the electrodes, permanently reducing the battery’s performance. This isn’t a problem for cars that are driven regularly, as the alternator in the car is constantly keeping the battery topped up. For our needs it’s not so good – we want to be able to use all the power we’ve generated. One thing that helps is to buy deep-discharge (sometimes called marine or leisure) batteries. They’re built to withstand heavier use, and conversely they’re not so good at starting cars. Swings and roundabouts.

It’s a trade-off – the more car batteries you connect up, the less they’ll each be discharged each night, and the longer they’ll last before needing to be replaced. But it costs more.

The only solution is to learn how to monitor your batteries so you can be sure you’re not flattening them too much. Learning how to interpret the battery voltage is the key thing, but it’s pretty easy to pick up, and once you have your setup running smoothly you don’t need to do much other than check the voltage every day or so to make sure all is well.[/EXPAND]

[EXPAND How do I monitor how much power I have left in my batteries?]

Keeping an eye on the battery voltage is the simplest approach. A fully charged battery should read around 12.6 volts. As you use the power, the voltage will drop. As per the previous question, car batteries don’t like being less than fully charged, and the official line is that once the voltage gets to around 12.4 volts you should be recharging the battery. That’s likely to be only around 25% of the battery used, mind. In rough figures:

Battery voltage Capacity left
12.6V 100%
12.4V 75%
12.2V 50%
12.0V 25%

The voltage reading will depend a bit on what load is on the battery at the time, too. It’s normal for the voltage to sag (drop) when there’s a big load connected, and then when the load is switched off, the battery will recover a little and the voltage will come back up a little. To get the most meaningful reading, switch on a reasonable load (say, a few amps) for 30 seconds, switch the load off, wait 30 seconds for the battery to recover and then measure the voltage.

It’s essential that you don’t let the batteries get so flat that they read below 12 volts for very long, otherwise the batteries’ life will be drastically reduced.[/EXPAND]

 

 

Example solar kits

Here’s three setups. I’d go for the middle one to get started, really.

h’s utter cheapskate emergency solar kit – £250-£300

[EXPAND Click to expand:]

Gather

Store

    • 10A Solar battery charger (eg: ebay link or, if you can afford it, an MPPT charger like this. I’ll discuss the advantages shortly)
    • Car battery (60-100Ah)
Use

    • Lighting – LED strips with tails (eg: ebay link – this is a 5m length; if you need shorter lengths you’ll need very basic soldering skills. Oh, and a soldering iron)
    • Car cigarette lighter socket (eg: ebay link)
    • Car USB charger for phones, laptops etc (eg: ebay link)
Connect

    • Fuse holder plus fuse (eg: ebay link)
    • Speaker cable (or mains flex – anything chunky will do)
    • Terminal blocks (for joining wires up and splitting power between multiple things – buy from hardware store or this sort of thing)
    • Crocodile clips for connecting to the car battery (eg: ebay link)
Essential tools and bits

    • Voltmeter
    • Screwdriver
    • Blade for stripping wire

So: what will all this give you? Enough to provide usable emergency light for a few rooms in your house, charge phones, run a car stereo, a CB. It probably won’t replace the lights and mains chargers you use now, but it’ll be a godsend if there’s a power cut.
[/EXPAND]

h’s better emergency solar kit – £400-£450

[EXPAND Click to expand:]

Gather

    • 2 x 80 Watt, 12 V solar panels (eg: ebay link)
Store

    • 20A MPPT Solar battery charger (eg: ebay link)
    • 2 x Car battery (150-200Ah total) plus short jump leads to connect them together
Use

    • Lighting – LED strips with tails (eg: ebay link – this is a 5m length; if you need shorter lengths you’ll need very basic soldering skills. Oh, and a soldering iron)
    • Car cigarette lighter socket (eg: ebay link)
    • Car USB charger for phones, laptops etc (eg: ebay link)
Connect

    • Fuse holder plus fuse (eg: ebay link)
    • Speaker cable (or mains flex – anything chunky will do)
    • Terminal blocks (for joining wires up and splitting power between multiple things – buy from hardware store or this sort of thing)
    • Crocodile clips for connecting to the car battery (eg: ebay link)
Essential tools and bits

    • Voltmeter
    • Screwdriver
    • Blade for stripping wire

[/EXPAND]

And lastly, here’s what I power my house with:

h’s current solar setup – £900-£1000ish

[EXPAND Click to expand:]

Gather

    • 5 x 80 Watt, 12 V solar panels (eg: ebay link)
Store

    • 30A MPPT Solar battery charger (eg: ebay link)
    • 4 x Car battery (360Ah total) plus short (cut up) jump leads to connect them together
Use

    • Lighting – multiple LED strips with tails (eg: ebay link – this is a 5m length; if you need shorter lengths you’ll need very basic soldering skills. Oh, and a soldering iron)
    • Multiple car cigarette lighter sockets (eg: ebay link)
    • Car USB chargers everywhere for phones, laptops etc (eg: ebay link)

Connect

    • Lots of fuse holders plus fuses (eg: ebay link)
    • Miles of speaker cable
    • Terminal blocks (for joining wires up and splitting power between multiple things – buy from hardware store or this sort of thing)
    • Crimp connectors and crimp tool (for quicker and easier connections)
    • Crocodile clips for connecting to the car battery (eg: ebay link)
Essential tools and bits

    • Voltmeter
    • Battery monitor (gives much more accurate reading of current battery capacity, load currents etc – I use one of these, they’re designed for use on boats but they suit our purposes perfectly)
    • Screwdrivers
    • Blades for stripping wire

[/EXPAND]

 

 

Connecting it up

I’ll expand this section when I have a moment, but basically all you need to do is follow the connection instructions that come with (or printed on) the charge controller.

Assuming you’ve bought a panel, charge controller, fuse holder, and a car lighter socket to connect phone chargers and things to, the process is normally:

  1. Pop the fuse out of the fuse holder you’ve bought.
  2. Use chunky speaker wire or mains cable to connect the battery – (negative) connection on the charge controller to one side of the fuse holder, then connect the other end of the fuse holder to the (negative) – terminal on the battery.
  3. Use another wire to connect the + terminal on the car battery to the battery + connection on the charge controller.
  4. Connect some long-ish wires to the load + and load – connectors on the charge controller and connect them to the back of the car lighter socket. If the lighter socket doesn’t have + and – connections marked, whichever pin or wire connects to the metal body of the socket is the negative (-) one. The positive connection goes to the little nub in the centre at the end of the socket.
  5. Pop the fuse back into the fuse holder. The fuse rating isn’t too critical as long as it’s smaller than 15 Amps – it’s really just to protect against fires in case something damages your wiring and creates a short-circuit. Have some spares handy nearby.
  6. If there’s any charge in the battery, you should be able to plug stuff into the lighter socket and it should work straight away.
  7. Finally, connect the solar panel to the charge controller. The wires on the back of the panel will be marked positive and negative. It’ll probably have some chunky looking black plugs on the wires; you can buy matching plugs on eBay if you like (search for ‘MC4 connectors’), or if you’re on a budget just cut them off. Connect the wires to the controller, positive to positive, negative to negative.
  8. Point the solar panel at the sun!

 

Tips:

  1. If you have more than one panel, connect them in parallel. That is, connect all the positive wires together, all the negative wires together, and then connect that to the charge controller as if it were just one panel. (NB there are slightly more efficient ways of connecting multiple panels together, particularly if the panels are some way away from the charge controller, but that’s beyond the scope of this article for now)
  2. If you want to put the panel somewhere awkward (like on a roof) you can always use long bits of wire to connect the panels to the charge controller. The longer the wire or the thinner the wire, the more power you lose, so try and use the thickest cable you can, and try and keep cables reasonably short where possible.
  3. The best direction to face panels in the UK is usually south, and tilted up by 30-35° so the panel will get as much sun as possible.
  4. The whole solar panel only works as well as the least lit part of it. That means that if something casts a shadow across part of the panel, it affects the whole panel’s output. So – keep the panel clean. A few leaves lying across the corner of the panel could cost you 70-80% of its power.

 

 

To come:

  • A breakdown of how to connect it all up
  • More FAQs

For now, though, you know what to stick in your garage. I’ll be selling full kits shortly – everything you need apart from the car battery. Watch this space – and drop me a line at h@howiem.net if you want to know more…

h at howiem dot com — animation, music, electronics, ramblings