What to sort out before solar

Solar needs less preparation than a heat pump. Nothing about the inside of the house has to change, there is no design to get wrong in the way a heating system can be got wrong, and the panels do not care how draughty your hall is.

What preparation there is falls into two halves: knowing what your household uses and when, and knowing what is up on your roof. Both take an evening, and both change what a sensible system looks like.

Find out what you use, and when

One figure decides how many panels are worth fitting.

Electricity used last year, in units. It is on the annual statement your supplier has to send you, in the app under something like "your usage", or in twelve months of bills added up. An average household is around 2,700 units a year; over 4,500 usually means electric heating, a hot tub or a car being charged at home.

The second thing has no single number, and it matters nearly as much: when the house is using electricity. Panels generate in the middle of the day. A unit you use as it is made is worth what you would have paid for it; a unit you export is worth a good deal less. So the same array on the same roof is worth noticeably more to a household with somebody at home during the day than to one that is empty from eight until six.

If you have a smart meter, the supplier's app usually holds half-hourly data going back months, and that shows you the shape directly — the morning peak, the evening peak, the quiet middle of the day. Failing that, an honest guess about who is in and when is enough to start with. There is more on where the figures live in reading your electricity bill.

Go and look at your roof

Five minutes outside will tell you most of what a quotation is going to assert later.

Which way does the main slope face? A compass app will tell you. South is best, east and west give up about a fifth, and north-facing slopes are generally left alone.

What is in the way? A chimney, a tall tree, a neighbour's roof standing higher than yours. Look in the morning and again in the afternoon, because shadows move, and remember that the winter sun is low and throws shadows several times longer than the summer one. Shading costs more output than its share of the roof suggests, so it is worth being thorough about.

How much clear roof is there? Vents, aerials, dormers and hips all break up the area, and what matters is the largest unbroken rectangle, not the total size of the slope.

Sort the roof itself out first, if it needs it

This is the solar equivalent of insulating before you buy a heat pump, and it is the one that costs real money to get wrong.

Panels last twenty-five to thirty years. If the tiles or the felt underneath are near the end of their life, the roof will need work well inside the life of the array, and taking an array off and putting it back costs a substantial fraction of what it cost to fit. Doing the roof first, with the scaffolding already up, is much cheaper than doing it twice.

So look at the age and the state of the covering, and if you are unsure, get a roofer to look before you get an installer to quote. A slipped tile or two is nothing. A roof that a builder describes as having a few years left in it is a reason to wait and do both together.

What you do not need to sort out

Insulation makes no difference to solar. Panels generate the same electricity above a draughty house as above a warm one, because nothing about the fabric affects what arrives on the roof.

This matters because the two technologies get bundled in conversation, and the preparation for one gets attached to the other. Insulating your loft is excellent for a heat pump and worth doing on its own terms, but nobody should be telling you to do it before you can have panels.

Think about where the electricity will go

Without a battery, a household typically uses something like a quarter to a third of what its panels make and exports the rest. Shifting a little of the use into the middle of the day lifts that, and it costs nothing: the washing machine at lunchtime, the dishwasher set to start at noon, the immersion heater in the afternoon.

Worth settling before anyone quotes: whether you want a battery now, later, or not at all. A battery stores the midday surplus for the evening peak and is the single biggest lever on how much of your own electricity you keep. It is also the most expensive part of the system, and a quotation that includes one should say what it adds to the saving. The self-consumption calculator will show you the difference for your own usage.

You will also want a smart meter to be paid for what you export, which is worth arranging early since it is a separate conversation with your supplier.

The practical bits people forget

• Scaffolding access. A narrow side return, a locked shared alley or a neighbour's garden in the way is worth flagging before somebody quotes on the assumption of an easy setup.
• Somewhere for the inverter. It is about the size of a small suitcase and wants somewhere ventilated and reachable, usually a loft or a garage.
• Who owns the roof. On a flat, or a leasehold house, the roof may not be yours to fit anything to, and that is a conversation with a timescale of its own.
• Listed buildings and conservation areas. Most domestic solar needs no planning permission, but these are the exceptions where it can.

Get your expectations straight

Generating as much as you use in a year does not mean you run on solar. It generates at midday and you use it at breakfast and teatime. Without a battery you will use a fraction of what you make and sell the rest cheaply. Both of those are fine, as long as nobody has led you to expect otherwise.

The standing charge does not go away. Panels reduce the units you buy. They do nothing about the daily amount you pay for being connected, so any illustration showing a bill going to zero has left something out.

It is not a money-making scheme. Solar can reduce a bill substantially and can pay for itself over a decade or so. Anybody presenting it as an investment with a two-year payback is selling something.

Then get quotes

With last year's units, a sense of when the house is occupied, and an honest look at the roof, you know more about your own situation than most people who ring three installers on a Tuesday.

Put your figures and your postcode through the solar system size calculator first. You are not trying to design anything — you are trying to know roughly what the answer should be, so that a quotation which is nothing like it prompts a question and not a signature.

Then read what a good quote looks like.