How to Size an Off-Grid Solar System: The Complete Guide

An off-grid solar system has exactly four sized components — panels, charge controller, battery bank and inverter. Get any one wrong and the whole system underperforms, dies young, or costs far more than it should. The good news: the math is not hard. This guide walks you through it, with the formulas and a worked example you can reproduce on the sizing calculator.

Step 1 — Know your load (the step everyone skips)

Everything is sized from your daily energy use in watt-hours (Wh). Don't guess it — measure it. List every device, multiply its wattage by hours of use per day, and total it. A plug-in watt meter will catch the surprises (a fridge that cycles more than you think, a "turned-off" TV still drawing phantom load).

Step 2 — Size the panel array

Panels must replace your daily energy use during your peak sun hours (not daylight hours — peak sun hours is the equivalent of full-sun hours per day, typically 3–6 depending on latitude and season). The formula:

Panel watts = daily Wh ÷ (peak sun hours × 0.75) × 1.25

• 0.75 is the real-world derate: panel temperature, tilt, wiring losses and controller inefficiency.
• 1.25 is a safety margin so a cloudy stretch doesn't leave you flat.

Step 3 — Size the battery bank

The battery stores energy for nights and cloudy days. Capacity is quoted in amp-hours (Ah) at a nominal voltage, but what you can actually use is limited by depth of discharge (DoD):

Chemistry	Usable DoD	Notes
Lead-acid (flooded/AGM)	~50%	Cheaper up front; deeper discharge kills it fast
LiFePO₄ (lithium)	~85%	Higher cost, far longer cycle life, less affected by temperature

Battery Ah = daily Wh × autonomy days ÷ (system volts × usable DoD × 0.9)

The 0.9 accounts for inverter and wiring losses. "Autonomy days" is how many sunless days you want covered — 2 is a common minimum, more if your climate has long cloudy stretches.

Step 4 — Size the charge controller and inverter

• Charge controller: panel watts ÷ system volts × 1.25, rounded up to a standard rating (30/40/60/80/100 A). Use MPPT, not PWM, for anything above a few hundred watts — it recovers 10–30% more energy.
• Inverter: sized to your largest simultaneous load × 1.25, plus surge headroom. Motors (fridges, pumps) draw 3–5× their running watts for a moment on start-up, so check the inverter's surge rating.

A worked example

A small cabin uses 1,800 Wh/day, in a location with 4 peak sun hours, on a 24 V system with lithium batteries and 2 days of autonomy:

• Panels: 1,800 ÷ (4 × 0.75) × 1.25 = 750 W (≈ 2 × 400 W panels).
• Battery: 1,800 × 2 ÷ (24 × 0.85 × 0.9) = 196 Ah (≈ 200 Ah at 24 V).
• Controller: 750 ÷ 24 × 1.25 = 39 A → a 40 A MPPT.
• Inverter: for a 1,500 W simultaneous load → ≈ 2,000 W continuous.

System voltage — 12, 24 or 48 V?

Higher voltage means lower current, thinner cables and less loss. A rough guide: 12 V for tiny weekend systems; 24 V for most cabins; 48 V for full-time homes and large arrays. Above roughly a 1,500 W array or 2,000 W inverter, stepping up from 12 V is strongly advised.

Sources & further reading

• NREL PVWatts — location-specific solar resource (pvwatts.nrel.gov).
• Batteries for Off-Grid: chemistry & sizing
• Battery & runtime calculator
• Your national electrical code (e.g. AS/NZS 3000, NEC) for wiring and protection requirements.