Interactive lesson

Homestead energy, step by step

Interactive lesson: numbers first, then panels — watts and kilowatt-hours, a snapshot of your use, the big users, winter, power cuts and saving.

Free sample Chapters 1 and 2 (8 steps) are free. The full lesson comes with the How much energy a homestead needs module. Already bought? Unlock

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The whole lesson as text

1. Watts and kilowatt-hours

Three measures are enough for everything: power, energy and current.

Power in watts — Power tells you how much an appliance draws while it runs. A kettle is 2,000 W, an LED bulb 8 W.
Energy in kWh — Energy is power × time, and that is what you pay for. A 2,000 W kettle running for 6 minutes uses 200 Wh, the same as an 8 W bulb lit for 25 hours.
Amps and volts — Current is power divided by voltage. At 230 V (UK, Ireland) a 2,300 W appliance draws 10 A, at 120 V (US, Canada) 19 A, and at 12 V it would draw almost 200 A.
Power or energy? — A 5 kW array doesn't give 5 kWh a day: in southern England it gives roughly 20 kWh a day in July and 3–4 in December. Always ask the supplier: is that power or energy?

Why: Power decides how big the inverter, the generator or the grid connection must be. Energy decides the size of the battery, the number of panels and the bill. Mix them up and you buy the wrong equipment.

W × hours ÷ 1,000 = kWh. Example: 100 W × 5 h = 0.5 kWh.
Ah × voltage = Wh. A 100 Ah battery at 12 V holds 1,200 Wh.

2. Where the energy goes

On a homestead energy doesn't only come from the socket.

Space heating — Heating is roughly 55–70 % of a house's energy. It is usually covered by wood, pellets, oil, gas, electricity or a heat pump.
Hot water — Hot water is roughly 10–20 %. An electric water heater is often the single biggest electricity user.
Cooking and appliances — Cooking is 5–10 %, appliances and lighting 10–20 %. Electricity for appliances is a smaller part than heat.
Machinery on the land — The mower, the two-wheel tractor and the generator burn petrol and diesel. Put them in the total of all the homestead's energy too.

Why: Wood heats the house and the water, gas or oil cooks and heats, petrol and diesel drive the machines. Once you convert it all to kWh, electricity is usually the smaller part and heating the biggest.

A typical UK or Irish household without electric heating uses roughly 2,500–4,500 kWh of electricity a year; a US home with air conditioning often twice that.
If insulating the loft cuts your heating by 20 %, that is more energy than a small solar system gives in a whole year.