Off-Grid Electrical Planning Checklist

The load audit is non-negotiable

Every other decision on this page flows from one number: the worst-day total in watt-hours. Get that wrong and the array is over-sized, the bank is under-sized, or the inverter trips every evening.

The honest way to get it is a meter. Not a guess from a nameplate, not an internet table. A plug-in meter on each appliance for a week, recording real run-time and real draw. Motors cycle. Compressors idle. Heaters run longer than anyone expects.

Once the table is full, add the simultaneous worst case. Every light on, the pump running, the fridge cycling, the microwave mid-pop. That total is the inverter's continuous rating floor. The daily sum is the bank's job.

A household that skips this stage spends more and gets less. A household that measures once gets a system that matches its actual life rather than its imagined one.

The 30 percent rule: After you calculate the array, add 30 percent. Dust, wiring loss, controller inefficiency and a cloudy morning all take their share. The margin is not waste.

Measure the loads for a week before ordering a single component. The watt-hour total drives every subsequent choice.

Array sizing starts in December

Peak-sun-hours tables give the worst month's daily average for your latitude and tilt. That number is what the array must deliver watt-hours against. Summer data flatters; winter punishes.

The arithmetic is simple: worst-day watt-hours divided by worst-month sun-hours divided by a derating factor of roughly 0.75. The result is the array's nameplate rating. Then check that the battery and controller can absorb what the array produces.

String voltage matters for the controller. Panels in series add voltage, in parallel add current. The cold-weather open-circuit voltage must stay inside the controller's maximum input, which is a number that catches people who sized only for wattage.

Why not just oversize?: A very large array with a small bank means over-voltage on sunny days when the battery is already full. The controller sheds the excess. Oversizing wastes money unless you also expand the bank.

Size the array against the worst month's sun-hours, check cold-weather voltage against the controller input, and derate by 25 percent for real-world losses. Summer numbers teach nothing useful.

The bank stores what the array is not producing

Two overcast days is the minimum autonomy target for a household. Multiply worst-day watt-hours by the number of days, then divide by usable depth-of-discharge. That gives the nameplate capacity to shop for.

Chemistry sets the depth. Lithium iron phosphate tolerates eighty percent routinely. Lead-acid degrades quickly past fifty. The same physical size stores very different usable energy depending on which is inside the box.

Temperature is the silent variable. A battery rated at twenty-five degrees delivers less at five, and far less at minus ten. The bank should live indoors or in an insulated box, never on a frozen slab.

Wiring between cells matters as much as the cells themselves. A bank of four in series at 48 volts carries the same current as one large unit, but each inter-node cable must handle full load current. Undersized links heat, voltage-drops and fail before the cells ever reach their limit.

The C-rate trap: The inverter can pull hard and fast. A bank sized only for capacity may not deliver the required current without voltage sag. Check the continuous discharge rating against the inverter's peak draw.

Bank capacity equals worst-day watt-hours times autonomy days divided by usable depth. Chemistry and temperature decide what usable actually means.

The controller is the traffic cop

A charge controller regulates array voltage down to the bank's charge voltage. MPPT types convert excess voltage to extra current, harvesting ten to thirty percent more energy than the older PWM design.

Sizing the controller means checking two numbers. Maximum input voltage against the array's cold-weather Voc, and output current against the array's short-circuit current times the conversion ratio. The first prevents damage. The second prevents clipping.

The absorption and float stages are chemistry-specific. A controller configured for the wrong chemistry will either under-charge or gas the bank. A temperature sensor on the battery lets the controller adjust the voltage target through seasons.

Read the cold-weather number: Panel Voc rises in cold temperatures. A string that fits the controller window at twenty-five degrees may exceed it at minus ten. The datasheet tells you. Check before wiring, not after.

MPPT for any real system. Voltage window sized for cold-weather Voc. Amperage sized for the array. Charge profile matched to the chemistry.

The inverter defines the household ceiling

The inverter converts stored DC into AC. Its continuous rating is the maximum the house can draw at any instant. Its surge rating must exceed the largest motor-start spike, usually a pump or a compressor.

A distribution panel downstream of the inverter lets circuits trip individually. Without one, a single fault kills the whole house. A small six-way panel is inexpensive insurance against a kitchen appliance failure.

Idle draw is the hidden tax. A large inverter sips power all night even with nothing running. Switching it off between jobs or choosing a low-idle model saves a real percentage of the bank over a winter.

The 120-volt vs 240-volt decision: A 240-volt split-phase inverter halves the current on long wire runs, reducing loss and voltage drop. For a whole house it is worth the extra cost over 120. For a single cabin with short runs, 120 is adequate.

Continuous rating above worst simultaneous load. Surge above the largest motor start. A sub-panel for circuit-level protection. Idle drain planned for.

An off-grid house is its own electrical service

Nothing upstream will trip for you. The utility grid's transformer is not there to clear a fault. The house must provide its own grounding electrode, its own surge protection, and its own over-current devices at every cable transition.

A ground rod bonded to the system bus gives fault current a path and stabilises voltage reference. Surge protective devices at the array, the bank and the inverter handle lightning-induced spikes that would otherwise destroy electronics.

Every point where cable gauge changes needs a fuse or breaker sized for the smaller wire. Without one, a fault downstream can drive hundreds of amps through a thin branch cable and heat it past insulation.

Label everything: A fire service or a locum electrician needs to isolate sections fast. Every disconnect, breaker and bus should carry a label naming what it feeds and what voltage to expect.

A ground rod, surge protection at three points, a fuse or breaker at every cable transition, and labels on every disconnect. Nothing upstream will protect you.

If you cannot measure it, you will be surprised

A bank at forty percent looks identical to a bank at twenty percent if the only gauge is voltage under load. A shunt-based battery monitor shows true state of charge, current in and out, and time remaining.

Production logging during the first year calibrates the design numbers to the site's actual sky. If December delivered half the table predicted, you learn that in January rather than on the darkest evening of year two.

Expandability is a monitoring concern too. Spare conduit runs, bus-bar lugs and controller headroom mean a later addition does not require a complete rewire. Plan for growth before the conduit is buried.

The one-page system diagram is essential: Wire gauges, voltages at each node, disconnect locations, fuse ratings. Tape it inside the inverter cover. The next person who works on the system will thank you.

A shunt monitor for state of charge. A year of production logs to calibrate the design. Spare capacity in conduit and bus for the inevitable expansion.