How do I work out how much power my solar panels will produce?
Multiply the number of panels by their rated watts, then by your location's peak sun hours, then by a system efficiency factor. Eight 400 W panels at 4.5 peak sun hours with 20% losses give 8 × 400 × 4.5 × 0.8 = 11,520 Wh, or about 11.5 kWh a day. The rating on the panel is measured under laboratory conditions that almost never occur outdoors, which is why the losses factor is not optional — leaving it out overstates output by a quarter.
What are peak sun hours, and why are they not the same as daylight hours?
A peak sun hour is one hour of sunlight at 1,000 watts per square metre, the intensity panels are rated against. A location with fourteen hours of summer daylight might deliver only five peak sun hours, because early morning and late evening sun arrives at a shallow angle and carries far less energy. Sizing an array on daylight hours rather than peak sun hours is the single most common way people end up with a system that underperforms.
What system losses should I assume?
Between 15% and 25% for a typical installation, with 20% a reasonable default. The losses come from several places at once: panels lose roughly 0.4% of output per degree above 25 °C, inverters are 94–97% efficient, wiring and connectors take 2–3%, soiling and dust take another 2–5%, and panels degrade about 0.5% a year. Shading is separate and much more punishing — one shaded panel can drag down a whole string.
How many panels do I need to run my house?
Add up each appliance's watts multiplied by the hours it runs daily to get your consumption in kWh, then divide by what a single panel produces in a day. If you use 12 kWh a day and each 400 W panel gives 1.44 kWh, you need nine panels to break even on an average day. Real installations are sized above that, because averages hide bad weather and because loads grow. Cutting your largest load is almost always cheaper than adding panels to cover it.
Does this calculator size the battery too?
No, and that is a genuine limitation for anyone going off-grid. This balances daily production against daily consumption, which tells you whether the array is big enough on an average day. A battery bank is sized on different questions: how many days of autonomy you need through cloudy weather, how deeply you are willing to discharge the chemistry you have chosen, and what your peak simultaneous load is. An array that balances on paper can still leave you dark for three overcast days.
Can I cook on solar power?
You can, but cooking is the load that most often forces a much bigger system than people expect. An electric hob ring pulls around 1,800 watts and an oven about 2,200 — more than a fridge, a television, a laptop and every light in the house running together. The daily energy looks modest because they run briefly, so an oven used for an hour is only 2.2 kWh, but the instantaneous draw is what your inverter and battery have to survive. This is why most off-grid installations cook with gas and leave the panels to lighting, refrigeration and electronics.
Which cooking appliances are kindest to a solar system?
The ones that use less power for longer rather than a lot for a short burst. A slow cooker at around 220 watts for seven hours uses about 1.5 kWh with a peak your inverter will barely notice; an electric hob reaching the same energy does it at eight times the draw. Pressure cookers and rice cookers sit in between and are efficient because they cook faster. Induction is more efficient than a radiant hob — it puts more of the energy into the pan rather than the air — but its peak draw is just as high, so it helps the array without helping the inverter.
Why does my system produce so much less in winter?
Because peak sun hours fall with the sun's angle and the shorter day. In temperate latitudes December output can be a third of June output, or worse — the UK averages around 1 peak sun hour in midwinter against 5 in midsummer. An array sized on the annual average will be comfortable in summer and short every winter. If you need year-round coverage without a grid connection, size for the worst month you must cover and accept the summer surplus, or plan for a generator.