Sizing an off-grid solar system properly means working through loads, batteries, array, inverter, wiring, and safety in order — each step depends on the one before it. Skip a step, or guess instead of calculate, and the usual result is a system that runs out of power on cloudy days or trips its inverter the moment a compressor starts. This guide walks through all seven steps with a worked example for a small cabin and a full off-grid home, so you can see the method applied at two very different scales. Treat the numbers here as a planning starting point — an installer or electrician should review the final design, especially the wiring and fusing.
Step 1: Do a load audit
List every appliance you'll run, its wattage, and how many hours a day it typically runs.
| Appliance | Typical watts | Hours/day | kWh/day |
|---|---|---|---|
| LED lighting (whole home) | 40 | 5 | 0.2 |
| Refrigerator | 150 | 8 (cycling) | 1.2 |
| Laptop | 60 | 4 | 0.24 |
| Phone charging | 10 | 3 | 0.03 |
| TV | 100 | 3 | 0.3 |
| Water pump | 500 | 1 | 0.5 |
| Microwave | 1,000 | 0.2 | 0.2 |
| Washing machine | 500 | 1 | 0.5 |
Our appliance wattage list has typical figures for dozens more appliances if yours isn't listed here — use it rather than guessing, since wattage varies a lot between models.
Step 2: Add up daily kWh
Sum the kWh/day column for every appliance you'll actually run, on a typical day. Add a margin of 10–20% for anything you might have missed — off-grid load audits are almost always underestimated on the first pass.
Step 3: Set days of autonomy and size the battery bank
Days of autonomy is how many consecutive days the battery bank should cover with no solar input, sized around your local cloudy-weather pattern. Two to three days is typical for most climates; locations with long overcast stretches may want more.
Battery capacity (kWh) = Daily kWh use × Days of autonomy ÷ Depth of discharge
Depth of discharge depends on chemistry — commonly 80% for lithium (LiFePO4) or 50% for lead-acid, as covered in our lithium vs lead-acid batteries comparison. Divide the resulting kWh by your system voltage, commonly 12V, 24V, or 48V, to get amp-hours. Our solar battery calculator runs this conversion automatically.
Step 4: Size the array using peak sun hours
Array size (kW) = Daily kWh use ÷ (Peak sun hours × 0.8)
Use the peak sun hours for your worst realistic month, not the annual average — off-grid systems are sized around the low point, since there's no grid to fall back on when a cloudy week runs the battery down. See what are peak sun hours for typical values and how to find yours.
Step 5: Size the inverter
The inverter needs to handle two different numbers:
- Continuous load — the sum of everything that might run at once, with margin.
- Surge (starting) load — motors and compressors, including well pumps, refrigerators, and washing machines, can draw 2–3 times their running wattage for a fraction of a second at startup.
Size the inverter's continuous rating above your expected simultaneous load, and check its surge rating covers the largest motor start while other loads are also running. Our solar inverter size calculator walks through both numbers together.
Step 6: Charge controller and wiring
Choose an MPPT charge controller over PWM in almost all cases — MPPT harvests more energy from the same array, especially in cooler weather or with higher-voltage panel strings. Size the controller's amperage above your array's maximum output current with margin.
Wire gauge matters more than it looks — undersized cable between the array, battery, and inverter wastes energy as heat and can be a fire risk. Use the solar wire size calculator to match cable gauge to your current and run length, keeping voltage drop low.
Step 7: Safety and fusing
Every circuit — array to charge controller, charge controller to battery, battery to inverter — needs an appropriately rated fuse or breaker and a disconnect switch. Label everything clearly, keep batteries ventilated per the manufacturer's instructions, and have the final design checked by a qualified electrician before it's energized, particularly for anything above a small, low-voltage system.
Worked example: small cabin (about 3 kWh/day)
A weekend cabin with LED lighting, a small fridge, laptop charging, and a water pump.
- Daily use: about 3 kWh
- Battery bank (2 days autonomy, lithium at 80% DoD): 3 × 2 ÷ 0.8 = 7.5 kWh, roughly a single mid-size lithium battery
- Array (4.5 peak sun hours): 3 ÷ (4.5 × 0.8) = 0.83 kW, around two to three panels
- Inverter: a small pure sine wave inverter sized for the fridge's surge plus lighting and laptop load, typically well under 2,000W continuous
Worked example: full off-grid home (about 15 kWh/day)
A full-time home with a fridge-freezer, washing machine, well pump, lighting, and entertainment electronics.
- Daily use: about 15 kWh
- Battery bank (3 days autonomy, lithium at 80% DoD): 15 × 3 ÷ 0.8 = 56.25 kWh
- Array (4.5 peak sun hours): 15 ÷ (4.5 × 0.8) = 4.17 kW, around 10–12 panels at 400W
- Inverter: sized well above the combined continuous load of the home, with surge headroom for the well pump and washing machine motors starting together with other loads running
Common mistakes
- Sizing around the annual average PSH instead of the worst month — leaves the system short exactly when it's needed most.
- Undersizing the inverter for surge loads — a well pump or compressor starting can trip an inverter sized only for continuous wattage.
- Ignoring voltage drop on long wire runs — especially between the array and the charge controller, where distances are often greater than expected.
- Skipping fusing and disconnects — a working system without proper protection is a fire and safety risk, not just a code issue.
- Forgetting the 10–20% audit margin — almost every first-pass load audit misses something.
Work through each step in order, use the calculators linked above for the battery bank, inverter, and wiring, and have the finished design reviewed locally before installation.