In This Guide
Why Getting the Size Right Matters
Undersizing your solar system means you run out of power by evening. Oversizing means you spent thousands of dollars on panels and batteries you’ll never use. The sweet spot is a system sized to meet your actual load with a reasonable safety margin — and that requires math, not guesswork.
The good news: the math is straightforward. You need four numbers: your daily energy consumption in watt-hours, your location’s peak sun hours, a panel array size, and a battery bank capacity. This guide walks you through each step.
Before You Begin
This guide assumes you’ve already completed a power audit and know your estimated daily watt-hour consumption. If you haven’t done that yet, start there. The number you get from your power audit is the foundation of everything in this guide.
Step 1: Establish Your Daily Wh Consumption
Your power audit gives you a total daily watt-hour (Wh) figure. For the worked example throughout this guide, we’ll use 1,500 Wh/day — a reasonable load for a small off-grid cabin running LED lighting, a 12V refrigerator, phone and laptop charging, a water pump, and a few small appliances.
Add a 20% efficiency buffer to account for real-world losses (wiring resistance, charge controller efficiency, inverter conversion losses, temperature derating of panels). This is not optional padding — it reflects genuine system losses.
Adjusted daily consumption: 1,500 Wh × 1.20 = 1,800 Wh/day
Step 2: Find Your Peak Sun Hours
Peak sun hours (PSH) are not the same as hours of daylight. A peak sun hour is one hour of sunlight at an irradiance of 1,000 W/m². In practice, this is the daily average solar energy your location receives expressed as equivalent full-power hours.
Typical values across the US:
- Southwest (AZ, NV, NM): 5.5–7 PSH
- Southeast / Mid-Atlantic: 4.5–5.5 PSH
- Pacific Northwest / Northeast: 3.5–4.5 PSH
- Northern Midwest / Mountain (winter): 3.0–4.0 PSH
Always size to your worst-month PSH, not the annual average. If you live in a location that gets 6 PSH in July but only 3.5 PSH in December, size to 3.5 PSH. Otherwise you’ll be short on power for months every year.
For our example, we’ll use 5 PSH (a common mid-latitude figure).
Step 3: Calculate Your Panel Array Size
The formula is simple:
Panel array (W) = Adjusted daily Wh ÷ Peak sun hours
Using our example: 1,800 Wh ÷ 5 PSH = 360W minimum
This is your mathematical minimum. In practice, add another 15–25% safety margin to account for panel degradation over time, partial shading, and non-ideal panel orientation. A practical recommendation is to round up to the next common array size.
Worked Example
- Daily load: 1,500 Wh
- Efficiency buffer (20%): × 1.20 = 1,800 Wh adjusted
- Peak sun hours: 5 PSH
- Minimum array: 1,800 ÷ 5 = 360W
- Recommended with safety margin: 400–600W
- Practical choice: 2× 200W panels or 3× 200W panels
The 400–600W recommended range gives you headroom to add a load later, covers a run of cloudy days better, and keeps your batteries more consistently topped up, which extends battery life.
Step 4: Size Your Battery Bank
Your battery bank stores energy for nighttime use and cloudy days. Most off-grid designers size for 1.5–3 days of autonomy — meaning the system can run your load without any solar input for that many days. Two days is a practical starting point for most setups.
Required bank capacity (Wh) = Daily Wh × Days of autonomy ÷ Depth of discharge (DoD)
DoD depends on battery chemistry:
- LiFePO4: 80–90% DoD usable
- Flooded Lead-Acid (FLA): 50% DoD (to protect cycle life)
- AGM: 50–60% DoD
For our 1,500 Wh/day example with 2 days autonomy and LiFePO4 batteries at 85% DoD:
1,500 × 2 ÷ 0.85 = ~3,530 Wh of battery capacity needed
A 100Ah 48V LiFePO4 battery holds 4,800 Wh — one bank comfortably covers this load. With lead-acid at 50% DoD you’d need 6,000 Wh nominal capacity (3 × 100Ah 12V batteries in series, or equivalent).
Tip: Match Battery Voltage to Your Inverter
Small systems (under ~2,000W continuous) often run 12V or 24V. Larger systems benefit from 48V because higher voltage means lower current, which reduces wire size and heat. Most quality inverter-chargers are available in 12V, 24V, and 48V configurations.
Step 5: Choose Your Inverter Size
Your inverter must handle your peak simultaneous AC load, not just your daily average. Add up the wattage of every AC appliance that might run at the same time. Don’t forget startup surge — motor-driven loads (pumps, refrigerators, power tools) can draw 2–7× their running watts at startup for a fraction of a second. Your inverter must handle this surge without shutting down.
As a rule of thumb: size your inverter to 1.25–1.5× your continuous AC peak load. If your peak simultaneous load is 1,000W running, choose a 1,500W or 2,000W inverter.
For the worked example above (1,500 Wh/day, mostly 12V loads with moderate AC use), a 1,500–2,000W pure sine wave inverter is appropriate.
System Size Reference Table
Here is a quick reference for three common off-grid build scales:
| System Scale | Daily Load | Panel Array | Battery Bank | Inverter | Typical Use Case |
|---|---|---|---|---|---|
| Small | 300–800 Wh | 200–400W | 100–200Ah @ 12V | 500–1,000W | Van, tiny cabin, shed |
| Medium | 800–2,500 Wh | 400–1,200W | 200–400Ah @ 24V | 1,500–3,000W | Small cabin, couple off-grid |
| Large | 2,500–6,000+ Wh | 2,000–6,000W | 400–800Ah @ 48V | 3,000–6,000W | Family home, small farm |
Don’t Skip the Power Audit
These reference figures are guidelines only. Your actual system size must be driven by your real daily watt-hour number. Using a reference table as a substitute for a proper audit is one of the most common causes of an undersized — and deeply frustrating — off-grid solar system.
Where to Go Next
With your system sized, the next decisions are battery chemistry and specific equipment. These guides will help:
- How to Do a Power Audit — Get your daily Wh number right
- LiFePO4 vs Lead-Acid — Choose the right battery for your budget
- Solar Gear Reviews — Specific equipment recommendations
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