How do I design a solar power system?

Well, you start at the beginning. Actually that’s not strictly true, really you start at the

end, by optimising your loads. Then you size the battery and solar array, then the

controller and inverter. Finally you decide where all the things are going and size the

cable.

The following process will give you an approximation for the United Kingdom, and

other countries with a similar climate. For detailed worldwide design calculations see

the Solar Power Design Manual.

Optimise my loads?

Yes; before you start designing you need to know that you’re designing the right

thing. That’s what optimising your loads is all about.

So I should start weighing things?

No, not those sort of loads. The load on the system is the amount of power it will have to supply, averaged over time. So optimising your loads is about reducing the number of appliances you have, reducing the time you use them for and picking the appliances with the lowest power consumption.

Why would I want less appliances?

It’s like this; every extra Watt of electricity or every extra hour that something’s on

for is an extra bit of solar panel, an extra bit of battery, an extra bit of controller and

maybe an extra bit of inverter. These things aren’t cheap; the way to make an

affordable solar power system is to do a good job of optimising your loads.

Ok, how do I do it then?

I’ve done it for you, sort of. For a rough idea, try the pop-up calculator on my design

page. Keep trying until you get the smallest possible answer.

Done that, what next?

You need to size the array.

Size the array? What’s that?

The array. That’s the term used for a number of solar panels connected

together. There is a maximum practical size for a solar panel so it’s normal to connect more than one together for big systems.

Is there another calculator?

Yup, on the same page. Beware though, it’s an approximation for the

UK only. If I could predict the weather like that I’d be a millionaire.

I need an array the size of Birmingham. What did I do wrong?

That’s what I mean about optimising your loads. Reduce your expectations and do it

again. Keep doing this until either you realise you can’t solar power a blast furnace or

you get a sensible answer.

Now then, what size does the battery need to be?

Well that all depends on how many days holdover you want.

Speak English man. What’s holdover?

Simple, it’s the amount of time that a fully charged battery would be able to power

the system without the solar panels. That’s your decision.

Great. How do I decide?

It’s 3. Well, not necessarily; for critical medical applications it’s at least 7, but for you

3 will do. Don’t ask why.

And there’s a calculator?

Just like before, at Solar power answers there’s a battery sizing calculator.

It says I need 200 Amp hours of battery, so my 500 Amp car battery will do

won’t it?

A common mistake that, mostly made by the sellers of car batteries. The output of the

calculator is in Amp hours. A 200 Amp hour battery can provide 200 Amps for 1

hour, 1 Amp for 200 Hours or anything that adds up to the same. The number on a car

battery is cold-cranking amps. A 500 Amp battery can provide 500 Amps for 30

seconds on a cold day. Different altogether. Anyway, what you need is a deep-cycle

battery or at the least a leisure battery.

Not a car battery? They’re cheap you know.

I didn’t believe this myself so I tried it. Take it from me, you’ll be lucky if a car

battery lasts a month. In a car it’s always on charge so it will last for years. In a solar

power system it gets discharged daily and will break. Nope, a deep-cycle battery’s

what you want.

I’m getting the hang of this, can I do the controller and inverter?

That’s the next thing. Lucky it’s easy, because there’s no calculators.

No calculators?

No, sorry, but I’m sure you’ll manage. Let’s do the controller first.

Do I really need one?

Yes. Well, almost certainly. The only time you don’t need one is if you’ve got a really

big battery relative to the size of the solar panel or array.

How do I work it out?

On the back of the panels there is a rating for the short circuit current. Multiply this

by the number of panels in parallel and that’s the rating of your controller in Amps.

I haven’t got the panels yet have I?

Oh no so you haven’t. You’ll have to approximate then. Divide the rated output (in

Watts) by 16. That will give you about the right answer. If the total is less than 10%

of the battery capacity you may be alright without a controller if it’s not going to be

left unattended for long periods and you’re not using sealed batteries. I wouldn’t

recommend it though.

What about the inverter?

Even easier. Add up the Wattage of all the mains appliances that will be on at once.

That’s your answer.

How do I attach it all together?

Get an electrician. Seriously, you need to have a certain amount of electrical

knowledge before you start. If you have, then read the instructions before you start

and I’ll take no responsibility if you set your house on fire. First you have to decide

where to put things.

What goes where?

The solar panels need to face the south, or the north if you’re south of the equator.

Can I mount them flat?

No, because you’ll have to keep cleaning them. Tilt them at about your angle of

latitude, or at least 10 degrees.

And the batteries?

Protected from the elements, but well ventilated. They produce hydrogen gas when

charging and it’s highly flammable.

And the electronics?

Nice and close, preferably indoors. You need to keep the cables as short as possible.

Why do the cables need to be short?

Because they’re low voltage cables remember. If you half the voltage then you

double the current, so our current is about 20 times what it would be if it was mains.

Why do I care about the current?

Because it determines the thickness of the cables and that determines the cost. Once

again there’s a calculator which will help.

How is it wired up?

Pretty much the same as the mains really. The 12 Volt stuff will need thicker cable

than you would imagine; use the calculator to get an idea. Earth the battery negative

and make sure that the 12 Volt and 230 Volt wiring is kept completely separate.

Why is that so important?

To make sure that you can never have 230 Volts on the 12 Volt circuit, that’s why.

You could get a nasty shock.

What fittings do I use?

Some mains fittings, some special ones. Look at the wiring page for more

information.

Can you give me an idea?

Of course, look at the sample wiring diagrams on the next page.