Off-Grid Backup Power: Sizing a System for a Month, Not a Weekend

Before buying anything, list what genuinely must run and for how long. A plug-in energy meter costs very little and turns guesswork into watt-hours. Multiply each device's draw by the hours per day you need it.

A typical essentials-only household lands between 1 and 3 kWh per day — an order of magnitude below normal consumption, and completely achievable off-grid.

Batteries, not panels, are what carry you through the night and through bad weather. Size for two to three days of essential load so a cloudy stretch does not end the plan.

LiFePO4 (lithium iron phosphate) is the practical default: 3,000 to 6,000 cycles, usable to about 90% depth of discharge, stable chemistry, and no maintenance. Lead-acid is cheaper up front, tolerates only about 50% discharge, and lasts a few hundred cycles — which is why it usually costs more per usable kilowatt-hour over its life.

Worked example: 2 kWh/day of essentials × 2 days of autonomy = 4 kWh usable. In LiFePO4 at 90% usable depth that is roughly a 4.4 kWh bank — about 350 Ah at 12 V, or 170 Ah at 24 V. Higher voltage means thinner cables and lower losses; choose 24 V or 48 V for anything above about 2 kWh.

Panels are rated at laboratory conditions. Real output is the rating multiplied by your location's peak sun hours multiplied by system losses of roughly 25% for wiring, heat, controller efficiency and dust.

Size for a bad day rather than an average one. If you need 2 kWh/day and expect 3 peak sun hours in winter: 2,000 Wh ÷ 3 h ÷ 0.75 ≈ 900 W of panels. Round up. Panels are now the cheapest component in the stack and extra capacity buys you cloudy-day resilience.

Use an MPPT charge controller rather than PWM — it recovers 20–30% more energy in cold or low light — and keep a spare controller stored disconnected. Mount panels where you can clear snow and adjust the tilt seasonally.

A generator is the wrong tool for continuous background power and the right tool for short, heavy tasks: refilling a battery bank after four grey days, running a well pump, powering tools during repairs.

Inverter generators run quieter, burn far less fuel at partial load, and produce clean power safe for electronics. Dual-fuel models let you burn propane, which stores indefinitely — unlike petrol, which degrades in three to six months even with stabiliser.

Carbon monoxide from portable generators kills roughly 85 people a year in the United States, according to the Consumer Product Safety Commission, and injures thousands more. The gas is odourless and the symptoms — headache, nausea, confusion — are easy to mistake for stress or fatigue.

Run a generator outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents. Never operate one in a garage, shed, porch or crawlspace, even with the door open. Install battery-powered CO alarms on every sleeping level and test them before storm season.

Never backfeed a generator into a wall outlet. It energises the utility line and can kill line workers. Use a transfer switch or interlock kit installed by a licensed electrician, or run appliances on heavy-gauge outdoor extension cords.

Every watt you remove is a watt you never have to generate, store, wire or maintain — and it is always the cheapest part of the system.

Swap every bulb you would use in an outage for LED. Consider a small chest freezer, which draws far less than an upright fridge and holds cold for a day unopened. Keep a battery-powered fan for heat and layered bedding and insulated window coverings for cold; heating and cooling by electricity is the fastest way to drain any realistic bank.

Stage one, under a few hundred: LED lights, headlamps, a 200–300 Wh portable power station, a hand-crank NOAA weather radio, and CO alarms.

Stage two: a 1–2 kWh LiFePO4 station with 200–400 W of folding panels, enough for lights, comms and device charging indefinitely.

Stage three: a 4–5 kWh bank, 900–1,200 W of fixed panels, an MPPT controller, and an inverter generator with propane storage — the point at which the fridge keeps running and week four looks like week one. For the next step, read DIY Off-Grid Power Systems for Beginners: What You Actually Need Before You Build.