Off-Grid Solar System Sizing: Step-by-Step Calculator Guide for Cabins and Homes

Sizing an off-grid solar system is the most important — and most commonly botched — part of going off-grid. Too small and you run out of power every winter. Too large and you've spent thousands more than necessary.

This guide walks you through the exact calculation process step by step, with worked examples for a cabin, a small homestead, and a family home.

Step 1: Calculate Your Daily Load (Wh/day)

List every electrical appliance you plan to run and multiply its wattage by its daily usage hours to get watt-hours per day. Add them all together.

How to find appliance wattage: Check the label on the back or bottom of the appliance, or look in the manual. For appliances that cycle (fridges, pumps), use the average running wattage — typically 30–40% of the peak rated wattage for compressor-based appliances.

Important: add 20% to your total as a safety margin for losses, efficiency variation, and appliances you forgot. So: Final daily load = Sum of (watts × hours) × 1.2

The single most impactful decision you can make before sizing: switch to 12V DC efficient appliances for refrigeration (saves 500–800Wh/day) and eliminate all resistive heating loads (electric heaters, kettles, hair dryers). A homestead that carefully selects its loads can run on 2–3kWh/day instead of 8–10kWh/day, cutting system cost by 60–70%.

Step 2: Find Your Winter Peak Sun Hours

Peak sun hours (PSH) is the number of hours per day equivalent to 1,000W/m² of solar irradiance — the standard measurement for solar panel rated output. It's not the same as daylight hours.

Use December/January figures for your location:

For precise data for your exact location, use the NREL PVWatts calculator → — free and accurate to within a few percent for US locations. European users can use the EU PVGIS tool →

Step 3: Size Your Solar Array (Watts)

Formula: Array size (W) = Daily load (Wh) ÷ (Winter PSH × 0.75)

The 0.75 factor accounts for real-world system losses: panel temperature derating, wiring losses, charge controller efficiency, inverter efficiency, and battery round-trip efficiency. It's a conservative estimate that prevents undersizing.

Always round up to the next standard panel configuration. Add a 20–30% oversizing buffer if you expect to expand your load later.

Step 4: Size Your Battery Bank

Formula: Battery capacity (Wh) = Daily load (Wh) × Days of autonomy ÷ Usable DoD

• Days of autonomy: 3 days minimum, 5 days recommended for year-round homesteads, 7+ days for critical applications
• Usable DoD: LiFePO4 = 0.85, AGM lead-acid = 0.50, flooded lead-acid = 0.50

Convert to amp-hours for purchasing: Ah = Wh ÷ battery voltage (12V, 24V, or 48V system voltage)

Example: 10,000Wh needed ÷ 48V = 208Ah at 48V → buy two 48V 100Ah LiFePO4 batteries (200Ah, ~8,500Wh usable at 85% DoD — slightly under, so three batteries gives good headroom)

Step 5: Choose Your Charge Controller

Always use MPPT (Maximum Power Point Tracking) for any system over 200W. PWM controllers are cheaper but waste 20–30% of panel output — false economy for serious off-grid use.

MPPT controller sizing:

• Controller current (A) = Array wattage ÷ battery bank voltage × 1.25 safety factor
• Example: 1,500W array ÷ 48V × 1.25 = 39A → buy a 40A or 60A MPPT controller

Reputable brands: Victron SmartSolar (best monitoring via Bluetooth app), Epever Tracer (good value), Outback FlexMax (premium, robust).

Step 6: Size Your Inverter

Size to your peak simultaneous load, not average. Identify what appliances could run at the same time at maximum and add those peak wattages together.

Add 25% margin: Inverter size = Peak simultaneous load × 1.25

Always choose pure sine wave. Match voltage to your battery bank. Victron MultiPlus and Quattro series combine inverter and battery charger in one unit — ideal for generator integration.

Worked Examples

Example 1: Weekend Cabin (1kWh/day)

Loads: 8 LED bulbs (40W × 3h = 120Wh), phone/tablet charging (20W × 2h = 40Wh), 12V fridge (30W avg × 24h = 720Wh), water pump (100W × 0.25h = 25Wh). Subtotal = 905Wh × 1.2 safety = 1,086Wh/day

Location: Colorado (3.5 winter PSH). Array = 1,086 ÷ (3.5 × 0.75) = 413W → 2 × 250W panels (500W)

Battery (3 days, LiFePO4): 1,086 × 3 ÷ 0.85 = 3,832Wh → 1 × 48V 100Ah LiFePO4 (4,800Wh nominal, 4,080Wh usable) — adequate.

Charge controller: 500W ÷ 48V × 1.25 = 13A → Victron SmartSolar 20A MPPT

Inverter: peak load ~300W → Victron Phoenix 500VA pure sine

Estimated total component cost: $1,800–$2,500

Example 2: Year-Round Homestead (3kWh/day)

Location: Oregon (2.5 winter PSH). Array = 3,000 ÷ (2.5 × 0.75) = 1,600W → 4 × 400W panels

Battery (5 days, LiFePO4): 3,000 × 5 ÷ 0.85 = 17,647Wh → 2 × 48V 200Ah LiFePO4 batteries (19,200Wh nominal)

Charge controller: 1,600W ÷ 48V × 1.25 = 42A → Victron SmartSolar 60A MPPT

Inverter: peak load ~2,000W → Victron MultiPlus 2,000VA

Estimated total: $5,500–$8,000

Example 3: Off-Grid Family Home (7kWh/day — well-optimised loads)

Location: New Mexico (5 winter PSH). Array = 7,000 ÷ (5 × 0.75) = 1,867W → 6 × 400W panels (2,400W)

Battery (4 days, LiFePO4): 7,000 × 4 ÷ 0.85 = 32,941Wh → 3 × 48V 250Ah LiFePO4 (36,000Wh nominal)

Charge controller: 2,400W ÷ 48V × 1.25 = 62.5A → Victron SmartSolar 100A MPPT

Inverter: peak load ~4,000W → Victron MultiPlus-II 5,000VA

Estimated total: $12,000–$18,000

Common Sizing Mistakes to Avoid

• Using summer or annual average sun hours: The single most common mistake. A system sized on annual averages will run out of power every December.
• Underestimating battery capacity: Too many first-time systems have 1–2 days of autonomy. Extended cloudy periods kill these systems every winter.
• Including resistive heating loads: If you plan to heat water or your home with electricity from the battery bank, the required system size becomes enormous. Use solar thermal and propane/wood for heating.
• Choosing modified sine wave inverters: Damages compressor motors, some electronics, and anything with a transformer. Always pure sine wave.
• Undersizing the charge controller: A cheap undersized controller throttles your panel output. Buy one size up from minimum.
• Not including generator integration: Even in sunny climates, a backup generator for 5–10 days per year prevents misery. The inverter-charger approach (Victron MultiPlus etc.) makes generator integration seamless.

For more detailed off-grid system design help, the DIY solar forums → — particularly r/DIYsolar and DIYSolarForum.com — have detailed build threads for systems of every size. The full off-grid homestead guide → covers component selection and load optimisation in depth.

Frequently Asked Questions

How do I calculate what size solar system I need?

Step 1: add up all your daily electricity consumption (watts × hours per day for each appliance), multiply by 1.2 for losses. Step 2: find your winter peak sun hours. Step 3: divide daily load by (winter PSH × 0.75) to get your panel array size. Step 4: multiply daily load by days of autonomy and divide by battery depth of discharge to get battery capacity needed.

What is the formula for off-grid solar sizing?

Panel array (W) = Daily load (Wh) ÷ (Winter peak sun hours × 0.75). Battery bank (Wh) = Daily load (Wh) × Days of autonomy ÷ Usable depth of discharge (0.85 for LiFePO4, 0.50 for lead-acid). These two formulas are the core of any off-grid sizing calculation.

How many peak sun hours do I have?

Use NREL PVWatts (US) or EU PVGIS (Europe) for precise figures for your exact location. As a rough guide: UK and Pacific Northwest US get 1.5–2.5 peak sun hours in winter, mid-US and central Europe get 3–4, and the desert Southwest US gets 5–6. Always use December or January values, not annual averages.

How big a battery bank do I need for 3 days of autonomy?

Multiply your daily load in Wh by 3, then divide by your battery's usable depth of discharge (0.85 for LiFePO4, 0.50 for lead-acid). For a 2kWh per day load with LiFePO4: 2,000 × 3 ÷ 0.85 = 7,059Wh needed — approximately two 48V 100Ah LiFePO4 batteries.