Off-Grid Water and Power Basics

How to plan water and power for a rural property from demand upward, with the sizing logic and worked examples behind each decision.

Plan from demand, not from equipment

Every off-grid system starts with the same question: how much do you actually need? A well, a cistern, a generator, a solar array and a septic system are all sized to demand. Start by listing how much water your household, animals and garden use, and how much electricity your loads draw and when. Equipment sized without that list ends up too small, which causes shortages, or too large, which wastes money and can shorten the life of the equipment.

These systems also fall under local codes. Wells, septic systems, electrical work and fuel storage are regulated almost everywhere. This guide explains the sizing logic so you can plan and ask good questions, but a licensed professional and your local authority make the final decisions.

Water: estimate demand

Indoor household water use averages about 80 gallons per person per day in the United States, according to the Environmental Protection Agency, though careful households use much less. A common planning figure for a rural home is 50 to 100 gallons per person per day, plus allowances for animals, irrigation and any special uses. Livestock needs vary with species, size and weather, and drinking water for a horse or a cow can run to a dozen or more gallons per animal per day in hot weather. Add your own numbers for the garden. Then plan for peak demand, not just the daily average, because several fixtures running together draw much more than the total would suggest.

Water: wells and pumps

A pump has to deliver enough flow, in gallons per minute, against the total head it must overcome: the vertical lift from the water level, the pressure you want at the house (commonly 40 to 60 psi, where 1 psi equals about 2.31 feet of head), and friction loss in the pipe. The well pump sizing calculator combines these. The pressure converter turns psi into feet of head and kilopascals.

Bigger is not better. A pump much larger than the well or the demand can pump the well dry and will start and stop frequently, called short cycling, which wears out the motor and the pressure switch. A pressure tank gives the pump a buffer and reduces cycling. Your well driller can tell you the well's tested yield, which limits how much water you can sustainably draw.

Water: collecting rain

Rainwater harvesting follows simple arithmetic. One inch of rain on 1 square foot of roof yields about 0.623 gallons, and real systems capture less because of splash, first flush and overflow, often 75 to 90 percent. A 1,000-square-foot roof catching 1 inch of rain collects about 620 gallons at best, or roughly 500 gallons after losses. The rain barrel and cistern calculator estimates collection from your roof area and local rainfall and shows what size tank you need to bridge dry spells.

Rainwater from a roof is not safe to drink without treatment and testing, and rules vary by state. It is excellent for gardens and animals when collected in clean, covered containers that keep out mosquitoes and light.

Water in winter

Frozen troughs are a daily chore in cold climates. The water trough de-icer calculator sizes a heater to the trough volume and the lowest expected temperature, so you buy one wattage that works, and not an undersized heater that fails on the coldest night. Use only heaters rated for livestock and plug them into properly grounded, protected outlets.

Power: start with a load list

Electricity is measured in watts (rate of use) and watt-hours (energy used over time). A 100-watt device that runs for 5 hours uses 500 watt-hours. List every device, its running watts and the hours it runs per day, and add them to get daily watt-hours. Note which devices contain motors, such as pumps, freezers and power tools, because they draw several times their running power for a moment at startup. This surge, or starting load, often decides the size of a generator or inverter.

Power: generators

A generator must supply the running watts of everything on at once plus the largest starting surge. A common practice is to size the generator so that the load is no more than about 80 percent of its rated output, which helps with heat, fuel use and engine life. The generator size calculator adds running and starting loads for you.

Generators carry real hazards. Run them outdoors, well away from windows, doors and vents, because carbon monoxide kills quickly and without warning. Never connect a generator to a house circuit with a plug that backfeeds, which can electrocute utility workers. A properly installed transfer switch by a licensed electrician is the safe way to power household circuits. Store fuel in approved containers away from living spaces.

Power: solar and batteries

Solar sizing follows a short chain of steps. Take your daily watt-hours and divide by the peak sun hours for your location and season, which is the equivalent number of hours of full sun, often 3 to 6 depending on place and time of year. Then divide by about 0.75 to allow for losses in wiring, the inverter, temperature and dust. The answer is the array size in watts.

For example, a home using 3,000 watt-hours a day in a place with 4 peak sun hours needs about 3,000 ÷ 4 ÷ 0.75 = 1,000 watts of panels. Batteries then store energy for nights and cloudy days. Take the daily use, multiply by the days of autonomy you want, and divide by the usable fraction of the battery. Lead-acid batteries are often limited to about 50 percent usable depth of discharge to protect their life, while lithium iron phosphate batteries commonly allow 80 to 90 percent. Two days of autonomy for that 3,000 watt-hour load on lead-acid is 3,000 × 2 ÷ 0.5 = 12,000 watt-hours, or 12 kilowatt-hours of nominal battery capacity. The solar and battery bank calculator runs this sequence for your numbers.

Batteries store a lot of energy and can cause fires and burns if wired or fused incorrectly. Use properly rated wire, fuses and disconnects, and have the system checked by a qualified installer or inspector.

How generator and solar work together

Many off-grid homes use solar and batteries for daily needs and a generator as a backup or to run heavy loads such as welders or well pumps during long cloudy spells. Running the two tools together answers whether a generator is a backup or the main source: if you must run the generator every day, your array or battery is probably too small, and if you almost never run it, it is a safety net.

Waste: septic systems

If you are not on a sewer, a septic system treats household wastewater. Tank size and drain field area depend on the number of bedrooms or daily flow and on the soil's ability to absorb water, which a percolation or soil test measures. Rules vary by state and county. The septic tank and leach field calculator gives a planning estimate for scale and cost. A licensed designer or installer and the local health department must design and approve the actual system. Maintenance, mainly pumping the tank every few years and keeping heavy vehicles, trees and excess water off the field, protects an expensive asset.

A sensible order of work

Write down demand for water and power, including peak loads.
Check local codes, permits and the rules for wells, septic and electrical work.
Design for efficiency first: fix leaks, choose efficient fixtures and appliances, and reduce loads before you size equipment.
Size the water system, then power, then waste.
Hire licensed professionals for the work that requires them, and test water before you drink it.

Common mistakes to avoid

Sizing from the equipment you can afford instead of the demand you have.
Forgetting starting surges, which crash undersized generators and inverters.
Oversizing a well pump.
Treating rainwater as safe to drink without testing and treatment.
Running a generator in a garage or near a window.

See our disclaimer and safety notes before relying on any number here. For heating, read the wood heat guide.

Frequently asked questions

Do I need a permit for a well, septic system or solar array?

Almost always for wells, septic systems and electrical work, and often for generators wired into a house and for rainwater systems. Check with your county or state before you buy equipment.

Should I choose lithium or lead-acid batteries?

Lithium iron phosphate batteries allow more usable capacity and last longer, but cost more up front. Lead-acid batteries cost less per kilowatt-hour of nominal capacity but are usually limited to about half of their capacity for good life.

How often should I test well water?

Many health departments recommend testing at least once a year for coliform bacteria and nitrates, and testing for other contaminants if there is a local concern. Use a state-certified laboratory.

Can a generator power my whole house?

Only if it is sized for the combined running loads plus the largest starting surge, and connected through a properly installed transfer switch. Ask a licensed electrician before wiring anything into the house.