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Calculators: how much do you need

Water, food, land, seed, solar panels, battery and firewood, calculated for your household.

HouseholdWaterFoodLandSeedSolarBatteryFirewoodAntenna

1 Start here

Your household

Enter it once. Every calculator on this page uses these numbers, and everything you type is remembered on this device.

People–children counted at about 1500 kcal a day
Food per day–kcal, whole household

2 Water

How much water to store and collect

Drinking is only part of it. Cooking and minimal washing roughly double the need.

Daily need and storage

Per day–litres, whole household
Drinking only–litres per day, the absolute minimum
For the plan–litres
Containers–full containers needed
How it is worked out

Adults: drinking × weather factor. Children: 2/3 of an adult. Plus cooking and washing per person. Toilet flushing is not included: use a dry toilet or grey water.

Rain from your roof

Collected per year–litres (80 % captured)
Per day on average–litres
1 mm of rain gives–litres from this roof
Tank to bridge dry spell–litres of storage
Formula

Litres = roof m² × rain mm × 0.8. Europe ranges from about 400 mm/year (dry south-east Spain, eastern plains) to over 1500 mm (western coasts, mountains). Rain falls unevenly: size the tank for the dry season, not the average. Treat roof water before drinking (Water).

Chlorine bleach dose

Bleach–millilitres
Or drops–at 20 drops per ml
Wait–minutes before drinking
Only plain bleach

Unscented, no detergent, not "thick" or "splash-less", not expired (it loses strength, roughly a fifth per year). After the wait the water should smell faintly of chlorine; if not, dose again and wait again. Doses about 6 mg of chlorine per litre, matching the usual "2 drops of 5–6 % bleach per litre".

3 Food

Food: what to store, how long it lasts

Calories keep you alive; the mix below is the classic long-storage base of grain, legumes, fat and sugar. Add salt, vitamins and whatever you like to eat.

Store for the plan

Calories total–kcal
Grain, rice, pasta, flour–kg (60 % of calories)
Dried beans, lentils, peas–kg (15 %)
Oil or fat–litres (15 %)
Sugar or honey–kg (10 %)
Salt–kg (10 g per person per day)
Rule of thumb

One adult for one year at 2500 kcal: about 160 kg grain, 40 kg legumes, 17 L oil, 23 kg sugar, 4 kg salt. Add milk powder, canned fish or meat, and a multivitamin or sprouted seeds for vitamin C.

How long will what you have last?

Calories you have–kcal
Lasts–days for the household
Short of the plan–kcal still to find
Values used, kcal per kg

Grain 3500 · legumes 3400 · oil 8100 per litre · sugar and honey 3500 · nuts and chocolate 5500 · mixed cans and jars 800 · potatoes 770. Eat the fridge and freezer first; those are extra days on top.

4 Land

How much land feeds your household

Calories, not vegetables, decide the area. Potatoes give the most food per square metre in most of Europe.

Calorie crop area–m² growing
Total with rest, veg, paths–m²
That is a square of–metres each side
Harvest–kg per year
CropYield, kg/m² (experienced)kcal/kgkcal per m²
Potatoes2.5770≈ 1900
Maize, dried0.453650≈ 1600
Oats / barley0.353800≈ 1300
Wheat / rye0.353350≈ 1200
Winter squash2.0400≈ 800
Dry beans0.183400≈ 600
Reality check

Grow in the first year as if the harvest will be half of this. Pests, drought and late frost happen. Grain needs threshing and milling, beans need protein-rich soil care, potatoes need blight watch (Farming). Animals need more land again: a goat roughly 0.1–0.2 ha of grazing plus hay.

5 Seed

Seed: how much to plant and keep back

If you eat your seed, there is no next year. Set it aside at harvest before anything else.

Seed to plant–kg
Plants–spacing
Expected harvest–kg (experienced grower)
Keep back for seed–kg
Keep diversity

Save seed from many plants, not the best one: at least 20 for beans and wheat, 50–100 for maize, several fruits for squash. Keep seed dry, cool and dark in labelled jars. More in Farming.

6 Power

Solar: how many panels

List what you want to run, then size the panels for the darkest month you care about.

Daily energy use

DeviceQtyWattsHours/dayWh/day

Energy per day–Wh incl. inverter losses
Everything on at once–W
Panel power–W peak
Panels–panels
Controller at least–A
Formula

Panel W = Wh per day ÷ (peak sun hours × efficiency). A "peak sun hour" is one hour of full 1000 W/m² sun; a grey December day in Germany is under 1. In winter most off-grid homes in the north cut use, add a generator or a wood-fired alternative. Wiring and fuses: Electricity.

7 Storage

Battery: how big for the nights and cloudy days

Size for days without sun. Lead-acid dies early if you empty it past half; lithium iron phosphate takes deep cycles.

Capacity needed–Ah
Energy stored–kWh nominal
Batteries–12 V units
Weight, about–kg
Formula

Ah = Wh per day × days ÷ (volts × usable fraction × round-trip efficiency). Efficiency used: LiFePO4 95 %, lead-acid 85 %. For 24 V, wire two 12 V batteries in series; for 48 V, four. Lead-acid capacity drops by a third in frost; LiFePO4 must not be charged below 0 °C.

Fuse it

A 12 V battery shorted by a spanner delivers thousands of amps: molten metal, fire, burst battery. Every battery needs a fuse right at the positive terminal (Electricity).

8 Heat

Firewood: how much for one winter

Wood must dry one to two summers. What you burn this winter should have been cut last year.

Heat needed–kWh per year
Dry wood–tonnes per year
Stacked split logs–m³
Woodland to cut sustainably–hectares
Values used

Air-dried wood (under 20 % moisture) ≈ 4 kWh per kg. A stacked cubic metre of split hardwood ≈ 470 kg, softwood ≈ 330 kg. A managed European wood grows roughly 4 solid m³ per hectare per year ≈ 5.6 stacked m³. Heating only one room (Shelter) is the quickest way to cut this number.

9 Radio

Antenna length and radio range

Cut a wire antenna for any frequency, and see how far line-of-sight radio reaches. Background in Comms: antennas.

Quarter wave–vertical, or one dipole leg
Dipole total–tip to tip
Full-wave loop–wire length
Radio horizon–km, open country; less among hills and buildings
Formula

λ/4 ≈ 71.3 ÷ MHz metres, dipole ≈ 142.5 ÷ MHz, loop ≈ 306 ÷ MHz. Horizon km ≈ 4.1 × (√h₁ + √h₂). Cut a few percent long and trim.