Should I get solar panels?

Solar is the simpler of the two decisions. There is nothing to design around, nothing about your radiators to worry about, and once it is up there it does its job without you thinking about it.

The complication is in what the electricity is worth, and that is not the same for every household even on identical roofs.

How much will a roof make?

More than most people expect, and it is genuinely predictable. Britain gets less sun than Spain but a great deal more than the national mood suggests.

A typical array on a house — say ten or twelve panels — will make somewhere around 3,500 to 4,000 units of electricity a year in the south of England, and around 3,000 in Scotland. For most households that is roughly what they use in a year.

Two things move that figure: which way the roof faces, and what is in the way.

Which way does your roof face?

South is best. But the penalty for anything else is gentler than people fear, and this is where a lot of perfectly good roofs get talked out of it.

An east or west roof makes about four fifths of what a south roof makes. That is a real difference but it is not a reason to abandon the idea, and there is a compensation: an east-west roof spreads its generation across the morning and the evening instead of piling it into the middle of the day. As the next section explains, that shape is often worth more than the missing fifth.

The pitch of the roof barely matters. Anything from about 20 to 50 degrees is within a few per cent of the best, and almost every British pitched roof is in that band, so the pitch can be left out of the decision.

What is in the way

Shading matters far more than direction, and it matters more than its share of the roof suggests.

Panels are wired in a chain. A shadow across one panel drags down the ones connected to it, in the way one dim bulb used to take out a whole string of Christmas lights. Modern panels limit the damage, but a chimney shadow crossing the bottom row can cost fifteen per cent of the output while it lies there, not the five per cent of the surface it covers.

So: a chimney, a tall tree, a neighbouring roof to the south. Go and look, at different times of day if you can. Remember that the winter sun is low and casts shadows several times longer than the summer one, so a survey done in June can miss a problem that appears in December.

If your roof is genuinely shaded across its southern side, that is the one honest reason to say no.

When the electricity arrives

Panels and households keep different hours, and this is where most of the disappointment with solar comes from.

Panels make most of their electricity in the middle of the day, while a household uses most of its electricity before work and after it, so the two shapes do not line up.

Only the overlap is electricity you use yourself, and that is the valuable part, because it is electricity you did not have to buy. The rest goes back into the grid, and you are paid for it at a good deal less than you pay to buy electricity in.

How big is that overlap? Without a battery, typically somewhere between a sixth and a third of what the panels make. So most of what your roof produces gets sold cheaply instead of saving you the full price.

That is not an argument against solar. It is an argument for knowing which of the two numbers a salesperson is quoting you.

Which is why your habits matter

Two identical roofs on two identical houses can be worth quite different amounts, depending on who is in the house during the day.

Somebody home all day, running a washing machine at lunchtime, uses a lot of what the roof makes. A house that is empty from eight until six uses very little of it directly.

Things that shift the balance without much effort: running the washing machine, dishwasher and tumble dryer in the middle of the day instead of the evening; heating the hot water cylinder at lunchtime; charging a car at home during the day if you are there.

And this is where an east-west roof earns its keep. It generates when you are more likely to be using electricity, so more of what it makes stays in the house.

Do you need a battery?

A battery catches the midday surplus and gives it back at teatime. It roughly doubles how much of your own generation you use, which is a large improvement.

What it earns you is the gap between the two prices: what you would have paid to buy that unit, minus what you would have been paid to sell it. On typical tariffs that is something like twelve pence a unit. Multiply by the units it shifts and you have its annual worth, which is usually a few hundred pounds against a cost of several thousand.

So on the arithmetic alone, batteries pay back slowly, sometimes more slowly than their warranty lasts. They make much better sense if any of these apply:

• You are on a tariff with a cheap overnight rate, because then the battery can also buy cheap electricity at night and use it by day. That works every day of the year, sunshine or not, and is often worth more than the solar shifting.
• You want the lights to stay on in a power cut — but only if the system is specifically built for it, because most solar shuts down in an outage by design.
• You simply prefer using your own electricity to selling it cheaply and buying it back dear. That will not show up in the payback sum, which does not make it a bad reason to buy one.

The practical questions

Will the roof take it? An array adds roughly the weight of a person standing on every couple of square metres. Most roofs manage easily. Older roofs with thin timbers, or ones already carrying a heavy covering, need somebody to look properly.

How long does it last? Panels are warranted for twenty-five or thirty years and lose about half a per cent of their output a year, so after two decades they are still doing around ninety per cent. The inverter — the box that converts the electricity — is the part that does not last that long. Budget for replacing it once, somewhere around year twelve.

Does it need maintenance? Almost none. Rain does the cleaning, provided the roof has enough pitch for water to run off. Worth a look once a year for moss creeping up the bottom edge or bird droppings, which are opaque and can affect a whole string.

Do I need permission? Usually not for a normal roof array. Listed buildings, conservation areas and flats are all different, and the council can tell you which rules apply.

What to do next

Find last year's electricity usage in units — it is on an annual statement — and put it, your postcode and the direction your roof faces into the solar system size calculator. It will tell you roughly how many panels you need, what they would generate, and how much of that you would use yourself and how much you would sell.

That last figure is the one to hold on to when you start reading quotes, because it is the one most likely to be quietly replaced with a rosier one.