Going Off-Grid at a Rural Home: Sizing Solar, Batteries, and the Generator That Backs Them Up

Going off-grid means your home makes and stores all of its own electricity, with no utility connection at all. No meter, no monthly bill, and no grid to fall back on when the sun does not shine. That last part is the whole job: an off-grid system has to cover your worst realistic week, not your average day. On a rural property, the hardest loads are the ones a suburban design never thinks about — the well pump, the septic aerator, and a long driveway in February.

This guide sizes the system in order — daily load, battery bank, solar array, generator — then honest costs, what off-grid does not fix, and when to stay grid-tied.

What "off-grid" actually means

Three setups get confused with each other, and the confusion costs people money. Only one of them is off-grid.

A grid-tied home with a battery is a resilient grid-tied home, not an off-grid one. Undersize a grid-tied battery and the grid quietly covers the shortfall; undersize an off-grid system and the lights go out — which is why every step below sizes to the worst month, not the average. There is also a middle path, sometimes called a hybrid: grid-tied solar plus a battery plus a generator. You keep the utility connection but can run for days without it — for most rural homes, the practical version of independence, and usually cheaper than cutting the line.

Step 1: Build your load inventory

Everything starts with a list: each load's running watts, hours a day it runs, and whether it is a motor with a starting surge. The U.S. Department of Energy's formula is simple: (watts × hours used per day) ÷ 1,000 = daily kilowatt-hours (kWh). Daily kWh sizes the battery and the array; peak kW sizes the inverter and the generator.

For scale, the average American home uses roughly 29 kWh a day (about 10,500–10,800 kWh a year, per the Energy Information Administration). An off-grid home usually uses less — but measure your own loads. A nameplate gives maximum draw, not average: a fridge cycles its compressor, so the Department of Energy suggests counting about one-third of its plugged-in time as running time.

Here is a worked example — a conservation-minded rural household on propane or wood heat, well and aerobic septic:

That 12.5 kWh a day is the number the rest of this guide sizes from. Your number will differ — a heat pump, an electric water heater, or a shop full of tools changes it fast. An electric water heater alone draws about 4,500 W while it runs. So the first off-grid decision is usually about loads, not panels: cut the big electric loads first, then size the system for what is left. Use your worst season, not your average — sizing to July is how off-grid systems fail in January.

Step 2: Size the battery bank

The battery gets you through the night and the cloudy stretch. The standard design convention is 2 to 3 days of autonomy — days the house runs with no meaningful solar input. Two days is the common starting point for a full-time home with a generator behind it; three days is the choice when the generator is small, fuel delivery is slow, or you want more margin.

The math: daily kWh × days of autonomy = usable kWh needed. For the worked example: 12.5 kWh a day × 2 days = 25 kWh of usable storage — roughly two 13.5 kWh battery units (27 kWh combined). One 13.5 kWh unit alone is about a single day of this household's use: a bridge, not independence.

Usable capacity is not nameplate capacity. Lithium batteries (lithium iron phosphate is the common residential chemistry) give up roughly 80–100% of nameplate energy; lead-acid gives up about 50%. A lead-acid bank for the same 25 kWh of usable storage needs roughly twice the nameplate size. Most new residential systems are lithium, and the rest of this guide assumes it.

Round-trip efficiency is about 90% on a typical AC-coupled battery — for every 10 kWh you put in, roughly 9 kWh come back out. Batteries are also a replaceable component, not a lifetime one: a common warranty is 10 years at 70% capacity retention, and some units carry 15-year warranties. Panels, by contrast, are warrantied for 25 years and degrade slowly — about 0.5% of output per year at the median, per National Renewable Energy Laboratory field data.

Step 3: Size the solar array for the worst month

Solar sizing starts from the same daily number, divided by your local peak sun hours — the equivalent hours of full-strength sun (1,000 watts per square meter) your site averages per day. Peak sun hours are not daylight hours: a December day can have nine hours of daylight and under two peak sun hours.

The formula, with a loss allowance for wiring, dust, heat, and battery round-trip losses: array watts = daily watt-hours ÷ (peak sun hours × 0.75). Use the winter peak sun hours, not the annual average — the same array makes roughly twice as much power in the desert Southwest as in the Pacific Northwest in December.

For the worked example in the Northeast: 12,500 Wh ÷ (1.75 × 0.75) ≈ 9,500 W of panels — about a 9.5 kW array. Run the same math for June (5.25 peak sun hours) and the answer is about 3.2 kW. The winter array is roughly three times the summer array for the same loads — the single biggest surprise in off-grid sizing, and why quotes built on annual-average production are worthless for an off-grid home.

Look your own numbers up in the National Renewable Energy Laboratory's free PVWatts calculator before buying anything. If a quote does not show December production separately, ask for it.

Step 4: Size the inverter for the surge, not the average

The inverter turns battery DC into household AC, and its rating has two numbers: continuous power (what it sustains) and surge power (what it delivers for a few seconds). Motors spike hard at startup, and the spike is what trips an undersized inverter.

The well pump decides this. A typical 1/2 HP submersible draws roughly 1,000 W running but needs about 7,700 W for the second or two it takes to start — its locked-rotor amps (32.2 A on a common Franklin Electric 230 V motor) times 240 volts. Locked-rotor amps vary by motor, so check your pump's nameplate before sizing anything. The rule is the same one our generator sizing guide uses: size for the surge, then check the running watts.

A Powerwall 3's 185-amp motor-start rating clears a 1/2 HP pump's ~32 amps with room to spare. A single Enphase 5P's 32-amp peak lines up almost exactly with the pump's 32.2-amp draw — it can start the pump on paper, with nothing left over for anything else starting at the same moment. Two 5P units give real margin. A soft-start on the pump can cut that starting current to under half of the locked-rotor figure — ask your well contractor about it. Our well pump guide and troubleshooting guide cover finding your pump's horsepower and nameplate figures.

The rural loads everyone forgets: well pump and septic

On a grid-tied suburban home, the well pump and the septic system are background details. Off-grid, they are the design drivers — and the septic loads are the easiest to forget, because they sit outside and run without anyone watching.

The well pump is the hardest load in the house. It is the largest motor, it surges hardest at startup, and without it there is no water at all. It also runs on 240 volts on most rural properties, so the inverter and the generator both need a 240-volt output; a 120-volt-only unit cannot run it no matter how many watts it advertises. Size everything to the pump first. And the pump is only half the water story: the well itself has to yield enough water. No amount of solar makes a dry well produce.

Septic loads are small but constant. A conventional gravity septic system needs no electricity at all. An aerobic treatment unit is different: its air pump runs continuously, and a common aerator for a 500–600-gallon-per-day unit draws about 71 watts around the clock, or roughly 1.7 kWh a day. If the aerator stops, treatment stops with it, so it belongs on the backed-up circuits. If your system has an effluent or lift pump, count that too: a typical 1/2 HP effluent pump (a Zoeller M98, for example) draws about 1,080 watts running and roughly 3,360 watts for a moment at startup. Read the nameplate on your actual pump; the range across models is wide.

Winter production reality

Winter is when off-grid systems earn their keep or fail: short days, low sun, snow-covered panels, and peak loads — heat running hardest, lights on longer, everyone indoors.

A Northeast site that sees 5-plus peak sun hours in June gets under 2 in December. Snow-covered panels produce essentially nothing until they shed: a multi-site study found annual snow losses from 0% to about 20% of snow-free yield depending on location and tilt, with steeper tilts shedding faster. Ground-mounted arrays have a real advantage — they can be tilted steeply, and you can reach them with a long-handled brush instead of climbing onto an icy roof. Never climb onto a snow-covered roof to clear panels.

Wooded lots and solar are natural enemies. Even partial shade cuts output disproportionately on a string-inverter system, which is one place microinverters earn their keep. A proper shade analysis — not a salesman's glance at the yard — is non-negotiable before you sign. Cold is not the problem: panels run slightly more efficiently in cold air. The problem in winter is the low sun, the short days, and the snow.

The generator: backup to the backup

Every honest off-grid design has a generator. Sizing solar and batteries for the absolute worst week of the year is enormously expensive; sizing for the typical winter and covering the dark stretch with a generator is the rational trade. In an off-grid home the generator does not just run the house — it recharges the batteries while it runs the house, so the inverter can take over again when the generator stops.

That changes the sizing: the generator has to cover your house loads and the battery charging rate at the same time. The rule of thumb is roughly twice the inverter's continuous output — a 4 kW inverter pairs with about an 8 kW generator, because charging alone can draw close to 3 kW on top of a 4 kW house load. Worked example: the house pulls 3 kW while you want 6 kW going into the batteries, so the generator sees about 9 kW plus conversion losses — call it 10 kW, which lands you in the 12 kW generator class. A smaller generator still works if you turn the charge rate down; it just takes longer to refill the batteries.

Two features matter more than brand: 2-wire auto-start, so the inverter can start the generator itself when batteries run low (not every generator accepts that signal), and the off-grid-use warranty — some standby units limit off-grid hours, one common model to 18 months / 1,000 hours. An off-grid generator runs far more hours than a standby unit ever will, so read that line before you sign.

Propane vs. diesel

Runtime math, honestly. A 22 kW standby burns about 2.5 gallons of propane per hour at half load. A 500-gallon propane tank holds 400 usable gallons (tanks fill to 80%), so that is roughly 150-plus hours without a delivery. But an off-grid generator runs hard for a few hours to recharge batteries, then stops — work the fuel math from your actual charge cycles, and size the tank for the longest plausible dark stretch plus margin. The delivery truck cannot reach you if the driveway is not plowed. Gasoline goes stale in months, so it is the wrong fuel here. For transfer switches, the backfeeding rules, and the sizing worksheet, see our home backup power guide, what size generator you need, and the generator size calculator. Our solar-vs-generator comparison covers the same equipment from the grid-tied side.

What it costs

There is no published benchmark for a complete off-grid package, because every one is custom. What exists are solid benchmarks for the parts, and the honest way to price the job is to add them up. The figures below are built that way, from the National Renewable Energy Laboratory's 2025Q1 residential benchmarks and the standby-generator ranges in our own generator guides. Every line is before any incentive.

Worked example — the 9.5 kW array, 27 kWh battery home from the sizing sections. Solar: 9,500 W × $2.78–$2.95 ≈ $26,400–$28,000. Batteries: 27 kWh at ~$900/kWh ≈ $24,300. Generator: $8,000–$23,000 installed. Equipment total: roughly $59,000–$75,000 before site work — racking and trenching, a propane tank if you need one, and any panel upgrade are on top. That is our arithmetic from sourced parts, not a quote. Get at least two itemized quotes, and make sure both show December production.

The incentive fine print. The old federal Residential Clean Energy Credit (Section 25D) was a 30% income-tax credit covering solar panels and battery storage, terminated by the One Big Beautiful Bill Act (P.L. 119-21) for expenditures made after December 31, 2025 — installations completed in 2026 or later, per the IRS. If a quote still prices in a 30% federal credit for a new installation, that quote is wrong. Verify current law with a tax professional before counting any incentive dollar.

There is no payback math here. A grid-tied system pays you back in lower electric bills. An off-grid system has no bill to lower — it replaces the bill with equipment, fuel, and maintenance you own outright. What you are buying is independence, not a return on investment.

When off-grid won't help

Off-grid solves one problem: electricity supply. It does not solve the other things that make rural living hard.

• Water quantity. Solar and batteries run the pump; they do not make the well yield more water. A well going dry is a water-system problem. Fix the water system first.
• Septic capacity. Off-grid power keeps an aerobic unit's aerator running, but it does not add a gallon of treatment capacity. A failing drainfield will still fail — powered, but failing.
• Winter road access. The generator needs fuel deliveries and the installer needs to reach you. A long driveway that drifts shut cuts off the fuel truck exactly when the generator is running hardest.
• Heat. Most off-grid homes heat with propane, wood, or a heat pump — electric resistance heat would swallow the battery bank in hours. Going off-grid does not fix a heating system that was already the wrong fit; it exposes it.

When to stay grid-tied instead

Stay grid-tied — ideally with solar plus a battery and a generator behind it — when any of these is true:

• The grid reaches you cheaply. If the utility line is already at the road, cutting it means giving up the cheapest backup power ever built and paying tens of thousands to replace it.
• Your loads are large and electric. All-electric heat, an electric water heater, a shop with welders, a hot tub — each roughly doubles the system. Change the loads before cutting the line.
• Your site is shaded or north-facing. Heavy tree cover or a steep north slope will starve an off-grid array in exactly the season it matters.
• You want savings, not independence. Grid-tied solar without a battery is the cheapest solar there is, and it pays you back every day. Do not pay the off-grid premium for a job the grid does cheaper.
• You have not done the load inventory. Both a battery system and a generator get sized from the same list — well pump horsepower, heating type, fridge, septic loads. Buying either one before you know your loads is how people end up with a battery that cannot start the pump. Our generator size calculator walks through the same arithmetic.