Off-Grid Solar Power: How to Size a System

An off-grid solar system has to carry you through the shortest, cloudiest days of the year without a utility to fall back on. Sizing it correctly is a sequence of simple calculations: measure what you use, find how much sun your site gets in winter, then size the array, charge controller, inverter, wiring and protection to match. This guide walks through each step for cabins, homesteads and rural properties in Canada and the northern United States, and finishes with a complete worked example.

Start with a load audit

Every sizing decision depends on how many watt-hours (Wh) you use per day.

For each appliance, multiply its power in watts by the hours it runs per day. Measure with a plug-in energy meter over 24 hours where you can; nameplate figures for refrigerators and anything with a thermostat are maximums, not averages.

Audit winter separately: lights run longer and furnace blowers and heat tape come on. The winter audit sizes the system.

Two lines are commonly missed. A microwave rated at 1,000 W of cooking power draws about 1,500 W from the outlet. And the inverter consumes power whenever it is on: 15 W over 24 hours is 360 Wh. Larger inverter-chargers commonly idle at 20–40 W; many have a search or power-saving mode that cuts this sharply when no load is present, and the 15 W figure assumes that mode is used.

Peak sun hours and the winter design case

Solar resource is expressed in peak sun hours (PSH): the number of hours per day the sun would need to shine at 1,000 W/m² to deliver the day's total energy. One peak sun hour equals 1 kWh/m². A site receiving 1.8 kWh/m² per day on the array surface has 1.8 peak sun hours, regardless of how many hours the sun is above the horizon.

At Canadian latitudes, December delivers a fraction of June's energy. The sun is low, days are short, and cloud is often heaviest from November to January. Size for the worst month you occupy the property, usually December.

Values are approximate long-term daily averages in kWh/m² (equal to peak sun hours) on a south-facing array tilted near the site's latitude. Terrain, valley shading and fog can move a site outside these ranges; check Natural Resources Canada's photovoltaic potential data for your location and observe winter shading on site.

Tilt and snow

For a winter design, tilt steeply: latitude plus 15°, or 60° and steeper in heavy-snow regions, so snow slides off and panels face the low sun. Keep the lower edge of a ground mount at least 1 m (3.3 ft) above typical snow depth. Snow on the ground reflects extra light onto the panels on clear days.

Sizing the array

Energy is lost between the panels and the appliances. Typical efficiencies are 90–94% for the inverter, about 95% round trip for a lithium iron phosphate (LiFePO4) battery or 80–85% for lead-acid, and about 97% for wiring. Multiplying 0.92 × 0.95 × 0.97 gives 0.848, so a combined factor of 0.85 is reasonable for a lithium system. Use about 0.75 for lead-acid.

Panels also produce less than their nameplate rating in practice because of soiling, partial snow, mismatch and controller losses. A derate factor of 0.85 covers these in winter, when cold temperatures partly offset other losses.

The array formula is:

Array watts = daily load (Wh) ÷ system efficiency ÷ (design-month PSH × panel derate)

This break-even array only replaces each day's use on an average day. To recover batteries after a cloudy spell, add 25–50%.

Charge controllers

PWM

A pulse-width modulation (PWM) controller connects the panels directly to the battery and switches them on and off to regulate voltage. A panel with a maximum power voltage (Vmp) of 18 V and current of 5.56 A produces 100 W, but on a 12 V battery sitting at 13.5 V it delivers about 5.56 × 13.5 = 75 W. PWM suits only small systems.

MPPT

A maximum power point tracking (MPPT) controller converts higher array voltage down to battery voltage at 95–98% efficiency. It allows long series strings, thinner array wiring and standard residential panels, and its advantage grows in cold weather. Use MPPT for anything over a few hundred watts.

Voltage limits and cold-temperature correction

Every MPPT controller has a maximum PV input voltage, commonly 100 V, 150 V or 250 V. Exceeding it, even briefly, can destroy the controller.

Panel open-circuit voltage (Voc) rises as temperature falls. Panels are rated at 25 °C (77 °F), and on a clear, cold morning cells sit near air temperature. Correct Voc for your site's record low:

Corrected Voc = rated Voc × [1 + (temperature coefficient × (25 − record low °C))]

The temperature coefficient of Voc is printed on the panel datasheet, typically −0.25 to −0.30% per °C for current monocrystalline panels. Use it as a positive number here. Both the Canadian Electrical Code (CEC) and the US National Electrical Code (NEC) require this correction and provide default correction factors for use when the manufacturer's coefficient is not available; those defaults are more conservative.

Corrected Voc × panels in series must stay below the controller's maximum input voltage.

Controller current rating

Output current is array watts ÷ battery voltage: a 2,400 W array on 24 V needs 2,400 ÷ 24 = 100 A. Check the manufacturer's maximum array size before oversizing.

Choosing 12, 24 or 48 V

Higher voltage means lower current, smaller cables and lower losses. For a 2,000 W load, the battery current is about 167 A at 12 V, 83 A at 24 V and 42 A at 48 V. Cable loss rises with the square of current, so the same cable at 12 V loses four times as much as at 24 V.

If the system may grow, start at 24 V or 48 V. Changing later means replacing the inverter and usually the battery.

Inverters

• Waveform. Use a pure sine wave inverter. Modified sine units make motors run hot and can damage chargers and electronics.
• Continuous rating. Must exceed everything that could run at once.
• Surge rating. Motors draw three to seven times their running current when starting. A 600 W pump may draw 1,800 W or more at start-up. The inverter's surge rating must cover the largest motor starting while other loads run.
• Inverter-chargers. These combine the inverter, a battery charger and an automatic transfer switch. With a generator connected, the unit powers the loads and charges the battery at the same time.
• 120/240 V. Most cabins need only 120 V. A 240 V deep-well pump requires a split-phase inverter or a transformer.

Wire sizing and voltage drop

Check wire for both ampacity (heating) and voltage drop (wasted power), and use the larger result.

Ampacity

Both codes treat PV current as continuous. Under the NEC, maximum PV circuit current is the panels' short-circuit current (Isc) × 1.25, and conductors and fuses are then sized at 125% of that, which works out to 1.56 × Isc. The CEC applies a comparable approach. Battery and inverter cables are sized to the fuse protecting them, and the conductor's ampacity at its terminal temperature rating must equal or exceed the fuse rating.

Voltage drop

Voltage drop = 2 × one-way length (m) × current (A) × resistance (Ω per m)

The factor of 2 accounts for the out-and-back path. Common targets are 2–3% for array and controller circuits and 1–2% for battery-to-inverter cables.

Use code-approved PV wire for array circuits and fine-stranded battery cable with properly crimped lugs for battery connections; a poor crimp is a fire hazard.

Fusing and disconnects

Every battery conductor needs overcurrent protection as close to the battery as possible. A LiFePO4 bank can deliver more than 10,000 A into a short circuit, so fuses need an adequate DC interrupting rating. Class T fuses, rated to interrupt 20,000 A at DC, are the usual choice for the main battery fuse.

• A main fuse at the battery positive terminal, sized to the battery cable.
• Individual fuses or DC breakers on each branch: inverter, charge controller, DC load panel.
• A battery disconnect switch.
• A PV disconnect between the array and the controller. Panels cannot be switched off at the source.
• String fuses when three or more strings are connected in parallel, rated no higher than the panel's maximum series fuse rating.

Recent editions of both codes include rapid-shutdown requirements for PV arrays mounted on buildings. Ground mounts may be treated differently.

Grounding and bonding

Panel frames, racking, enclosures, the inverter chassis and the generator frame are bonded with an equipment grounding conductor to a grounding electrode, typically ground rods. Where the DC system is grounded, the negative conductor is bonded to ground at one point only. Follow the inverter and controller manufacturer's grounding instructions.

On the AC side, the neutral-to-ground bond must exist at exactly one point. Inverter-chargers typically bond neutral to ground when inverting and lift the bond when passing generator power, because many generators have their own. A doubled or missing bond is a shock hazard; confirm how your equipment handles it.

Arrays on exposed ridges or long wire runs benefit from surge protection devices on the PV input and the AC output.

Generator backup

A generator lets you size the array and battery for typical conditions rather than the worst week of the year.

Size the generator for the inverter-charger's charging power plus any loads that run while charging, with margin so the generator runs at 60–80% of its rating. A charger delivering 70 A into a 24 V battery at 28.4 V supplies 70 × 28.4 = 1,988 W to the battery. At 90% charger efficiency, it draws about 2,209 W from the generator, so a 3,000 W generator is a sensible match.

LiFePO4 batteries accept full charging current until nearly full, which makes generator runs short and efficient. Lead-acid charging slows sharply above about 80% state of charge, so charge to about 85% with the generator and let solar finish.

Codes and permits

In Canada, electrical installations are governed by the Canadian Electrical Code as adopted, with amendments, by each province and territory. Section 64 covers renewable energy systems, and Section 26 covers batteries. Electrical permits and inspections are handled by the provincial or territorial authority. Some provinces allow homeowners to obtain a permit for work on their own property; others require a licensed electrician. In the United States, the NEC (Articles 690 for PV and 706 and 480 for energy storage and batteries) is adopted by states and local jurisdictions, which issue permits.

Equipment generally must be certified for use in your country: in Canada, marked by an accredited certification body such as CSA or cUL; in the US, listed by a recognized testing laboratory. Uncertified equipment can fail inspection. Talk to your inspection authority and insurer before buying. This guide describes common practice and is not legal advice.

Key points

• Measure your loads, including inverter idle draw, and audit winter use separately.
• Size the array for the worst month you occupy the property, and add 25–50% for recovery after cloudy days.
• Correct panel Voc for your record low temperature and keep strings below the controller's input limit.
• Use MPPT, pure sine inverters, and 24 V or 48 V for anything beyond a small cabin.
• Check wire for both ampacity and voltage drop, and fuse every battery conductor close to the battery.
• Plan for a generator, keep it outdoors, and get permits and inspections from your local authority.

This guide is general information. Codes, permits and safety requirements vary by province, state and municipality. Check local rules and use a qualified professional for electrical, gas, chimney and septic work where required.

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