Energy and light

The page above specifies the kit well. This is about the three seasonal facts that decide whether that kit delivers in the week it is actually wanted, which in Britain is a dark one.

The sizing rule is a summer rule

The parent page gives the sizing arithmetic as daily watt-hours divided by panel watts times four real sun hours, then doubled for cloud. That is a sound method and the four is a summer figure.

Southern Britain averages roughly four to five peak sun hours a day in June and under one in December. That is a swing of six to eight times, and it falls in exactly the season with the longest nights, the highest lighting load and the greatest need for heat.

So a rig sized on the summer figure runs a deficit all winter, drawing the battery down a little further each day until it is flat, at which point it looks like a fault rather than what it is.

What to do about it, in order of cost:

1. Size the winter case, not the average. If the system has to work in January, do the arithmetic with one sun hour and accept that this means more panel than feels reasonable in July.
2. Cut the winter load instead, which is usually cheaper than more panel. A Pi running 24 hours is 120 Wh a day; running it only when needed removes most of the winter problem on its own.
3. Angle the panels steeply. A near-vertical panel catches a low winter sun far better than a summer-optimised angle, and sheds snow.
4. Keep them clean and unshaded. Winter sun is low, so shadows are long, and a shaded corner of a panel costs far more than its share.
5. Have a non-solar route to a charged phone. The hand-crank radio with USB out on the parent page is the honest answer for a dark fortnight.

What to shed, in what order

A battery system is a budget, and the household that decides the order in advance does not spend the last of it on the wrong thing.

Turn the inverter off when nothing is plugged in. It draws standby power continuously and will quietly empty a battery overnight doing nothing at all.

Run 12 V directly wherever possible, as the parent page says. Every conversion costs, and USB and 12 V lighting skips the inverter entirely.

Butane stops working when you need it

Butane boils at about minus 0.5 degrees C. Below that it stops producing gas pressure, and it starts losing performance well before it, which is why a camping stove that worked in the garden in August produces a feeble flame in an unheated kitchen in February.

Propane boils at about minus 42 degrees C and keeps working through any British winter.

The battery and the cold

Two things about batteries that the specification sheet does not put on the front.

Nameplate capacity is not usable capacity, and the two chemistries differ. A 100 Ah lead-acid battery gives roughly half of that before you are damaging it. A 100 Ah lithium iron phosphate cell gives most of it. So the parent page's preference for LiFePO4 is right, and the comparison is not one to one: the lithium unit is worth close to twice the lead-acid one of the same number.

Light is the cheap win

Light is the part of this chapter that costs almost nothing and does the most for how a house feels.

• A headlamp per person is the single best purchase here, exactly as the parent page says, and the reason is hands. Everything else in this app needs both of them.
• Solar garden stakes are the sleeper item. A pound or two each, charge outside all day, and give a night's soft light indoors with no wiring, no battery management and nothing to go wrong.
• Rechargeable AA cells and a solar charger outlast any quantity of disposables and stop the torch drawer being a lottery.
• Candles are for atmosphere, not for light or heat. They are a fire risk in a house full of blankets and improvised arrangements, they produce carbon monoxide, and one candle heats a room by an amount you cannot feel. Use them knowing that.
• Mark the route. A strip of glow tape on the stairs, the door frames and the meeting point costs almost nothing and prevents the falls that are one of the real causes of harm in a dark house.