How to size an off-grid solar system, step by step

Step 1: the load list (watt-hours per day)

List every appliance with its running watts and the hours it runs each day. Multiply and add. AC appliances on an inverter lose about 10% more — Victron puts modern inverters above 90% efficient — so divide their total by 0.9.

Wh per day = Σ (watts × hours) for DC loads + Σ (watts × hours) ÷ 0.9 for AC loads

Be honest about fridges and anything on a thermostat: use running hours, not hours plugged in. The extension-service rule is a third of the day for a fridge; Victron’s example 12 V fridge runs 25–50% of the time. Starlink’s spec sheets give 75–100 W average for the Standard kit and 25–40 W for the Mini — see how much power the Starlink Mini uses and what size power bank for a Starlink Mini. The solar load calculator adds it all up.

Step 2: the battery bank

bank Ah = Wh per day × days of autonomy ÷ usable depth of discharge ÷ battery voltage

Days of autonomy is how long you want to go with no charging. Usable depth of discharge is about 80% for LiFePO4 (Victron rates cycle life there) and 50% for lead-acid (Trojan recommends 20–50%). For our van: 2,000 Wh × 1.5 days ÷ 0.8 ÷ 12.8 V ≈ 293 Ah — three 100 Ah LiFePO4 batteries. The battery bank calculator does this; what size battery bank for a 2000W inverter shows why a big inverter can need more batteries for current alone.

Step 3: the solar array

array W = Wh per day ÷ (peak sun hours × (1 − system losses) × battery round-trip efficiency)

Peak sun hours come from NREL’s PVWatts for your location and season — for year-round use, the worst month you need. System losses default to PVWatts’ 14%. The battery round trip is about 92% for LiFePO4 (Victron). For the van at 4.5 peak sun hours: 2,000 ÷ (4.5 × 0.86 × 0.92) ≈ 562 W — three 200 W panels, if the roof has room. See peak sun hours by city, the best angle for solar panels in winter and the solar panel calculator.

Step 4: the charge controller

MPPT amps = array W ÷ battery V × (1 + headroom)
max PV voltage > array open-circuit voltage at your coldest temperature

Three 200 W panels into 12 V is 600 ÷ 12 = 50 A, or about 63 A with 25% headroom — a 70 A controller, or move to 24 V and halve it. Then check the voltage: panels in series must stay under the controller’s maximum PV voltage on the coldest morning, when their voltage is highest. What size charge controller for a 400 watt solar panel, what size charge controller for 1000 watts of solar and four solar panels in series vs parallel walk through both checks.

Step 5: the inverter

continuous W ≥ everything that runs together × (1 + headroom)
surge W ≥ that + the largest motor’s start-up extra

If the van’s biggest AC moment is a 1,200 W microwave plus a 200 W fridge, the inverter needs about 1,750 W continuous with headroom — a 2,000 W pure sine inverter — and a surge rating above the fridge’s start. At 12 V that inverter pulls up to about 185 A at full load. See what size inverter to run a microwave.

The worked example in one table

Common mistakes

• Sizing panels on summer sun and wondering why winter runs flat.
• Counting lead-acid at 100%. Plan on about half.
• Forgetting current. A big inverter on 12 V can exceed what one battery is designed to deliver.
• Ignoring cold Voc. Too many panels in series can exceed the controller’s voltage limit on a frosty morning.
• Skipping fuses and proper cable. This site deliberately doesn’t size them — wire and fuse sizes come from NEC/ABYC ampacity tables and the equipment manual; have a qualified installer check them.

Bigger examples: how much solar for an RV, how many solar panels for a small cabin and an off-grid solar system for a house.