Dreaming of going off-grid with a solar-powered cabin or RV, but feeling overwhelmed by the math? No worries! Sizing a solar system is totally doable with the right know-how. In this guide, we’ll walk you through simple formulas, a real-world example, and handy tools to match solar panels and batteries to your needs. Whether you’re a DIY beginner or a seasoned solar enthusiast, you’ll be designing like a pro in no time. Let’s get started!

Solar System Basics: Components and Calculations

A solar system has four main parts: solar panels, batteries, charge controllers, and inverters. Each needs to work together to turn sunlight into power for your devices. The key is figuring out how much energy you need and sizing your gear accordingly. For more on choosing the right storage solutions, check out this solar energy storage guide.

Peak Sun Hours (PSH) are the equivalent hours per day when panels produce at max power, usually 4-6 hours depending on your location and season. Here are the core formulas we’ll use (note: these are simplified for ideal conditions, but we’ll adjust for real-world losses):

  • Charging Time = Battery Capacity (Ah) × Battery Voltage (V) ÷ Panel Power (W) ÷ System Efficiency (0.85)
  • Panel Quantity = Load Power (Wh) ÷ Panel Power (W) ÷ PSH ÷ System Efficiency (0.85)

The 0.85 system efficiency accounts for losses from inverters (around 90% efficient), wiring, and other components (5-10% loss). For batteries, Depth of Discharge (DoD) is crucial—lithium batteries, like LiFePO4 cells, can discharge 80% (DoD = 0.8), while lead-acid ones manage about 50% (DoD = 0.5). These tweaks keep your calculations grounded in reality.

Real-World Example: Powering a 120W 24V Fridge

Let’s see how this works with a practical case: you want to power a 120W, 24V fridge that runs 8 hours a day off-grid.

  1. Calculate Daily Energy Use: 120W × 8h = 960Wh. Convert to amp-hours: 960 ÷ 24V = 40Ah.
  2. Choose a Battery: For a lithium battery (DoD = 0.8), capacity needed = 40Ah ÷ 0.8 = 50Ah. A 24V 50Ah battery is perfect.
  3. Size the Panels: Panel power = 960Wh ÷ 5h (PSH) ÷ 0.85 (efficiency) ≈ 226W. Go with 2 × 150W panels (300W total) for a buffer.

Heads-up: If winter PSH drops to 3 hours in your area, consider adding a third 150W panel to stay powered up. For a full planning checklist, see this off-grid home solar checklist.

Solar Panel Output for 120W Fridge at Different PSH

Seasonal and Regional PSH: Optimizing Your System

Your solar output depends on Peak Sun Hours, which change with seasons and where you live:

  • Summer: 5-6 hours
  • Spring/Fall: 4-5 hours
  • Winter: 3-4 hours
  • By Region: Tropical spots might hit 5-7 hours; high-latitude areas could dip to 2-4 hours.

To keep your system reliable, size it for the lowest PSH (like winter in your region). Tools like PVWatts from NREL give you local PSH data. Pro move? Use adjustable panel mounts to tilt toward the sun in winter for extra juice. A PSH chart for your area can help you plan.

Seasonal and Regional PSH

Handy Tools to Simplify Your Design

No need to break out a calculator for every step. These tools make solar design a snap:

  • PVWatts: Free tool for PSH and energy estimates.
  • Renogy Solar Calculator: Enter your load and battery details for instant setup suggestions.
  • Excel Spreadsheets: Build one with the formulas above—just plug in load, PSH, and efficiency (0.85).
  • SolarGIS: An excellent resource for mapping local solar potential and finding accurate PSH data for your specific site. Check it out at SolarGIS.

These tools save time and make you look like a solar wizard.

FAQ: Common Solar Sizing Questions Answered

Got questions? Here are answers to some of the most common ones we hear from DIY solar folks.

How many 600W solar panels match a 120A charge controller?

For a 48V system, max controller power = 120A × 48V = 5760W. Divide by panel power: 5760 ÷ 600 = 9.6. You’ll need 10 panels.

How long does a 200W panel take to charge a 12V 70Ah battery?

Battery capacity = 70Ah × 12V = 840Wh. Charging time = 840 ÷ 200 ÷ 0.85 (system efficiency) ≈ 4.94 hours, assuming full PSH.

How many 550W panels are needed to produce 1000kWh?

Daily output per panel = 550W × 5h (PSH) × 0.85 = 2337.5Wh = 2.3375kWh. For 1000kWh in one day: 1000 ÷ 2.3375 ≈ 428 panels. For smaller setups, spread this over more days.

How long does a 40W panel take to charge a 12V 20Ah battery?

Battery capacity = 20Ah × 12V = 240Wh. Charging time = 240 ÷ 40 ÷ 0.85 ≈ 7.06 hours.

How many panels charge a 150Ah battery?

Capacity = 150Ah × 12V = 1800Wh. Panel power needed = 1800 ÷ 5h ÷ 0.85 ≈ 424W. Go with 3 × 150W panels (450W total).

How many panels for an 8kW load with a 51.2V 300Ah battery?

Daily load = 8000W × 24h = 192kWh. Panel power = 192000 ÷ 5h ÷ 0.85 ≈ 45176W. That’s about 76 × 600W panels.

How many 350W panels support a 7.5kVA inverter system?

Assuming a 7.5kW load for 8 hours daily: 7500 × 8h = 60kWh. Panel power = 60000 ÷ 5h ÷ 0.85 ≈ 40 panels. The 0.85 covers inverter efficiency (~90%) and wiring losses (~5%).

Wrap-Up: Kick Off Your Solar Journey

Designing a solar system is all about knowing your energy needs, factoring in PSH, and accounting for efficiency losses. With the formulas, tools, and examples in this guide, you’re ready to power your off-grid dreams—whether it’s a tiny fridge or a full cabin.