Last Updated: April 13, 2026
How to Size a Solar System: The Complete Worksheet for Off-Grid Homes With Real Examples
TL;DR — The Math of Independence
Sizing a solar system is a four-step sequence: 1) Load Audit (Watts × Hours), 2) Battery Sizing (Load × Autonomy ÷ DoD), 3) Array Sizing (Load ÷ Sun Hours), and 4) Component Matching. If you skip step one, every subsequent calculation will be wrong. Real off-grid stability comes from designing for your peak winter load, not your summer average.
Are you buying hardware before you've done the math?
Most beginners start with a "package deal" and try to fit their life into it. This is backward. Your life dictates the math, and the math dictates the gear. This guide gives you the exact worksheet Wattson uses to size systems for families who cannot afford to have the lights go out.
Table of Contents
Phase 1: The Daily Load Audit Worksheet
The load audit is a list of everything that draws power. You must use the nameplate wattage (or a Kill-A-Watt meter) for every device.
| Appliance | Watts (W) | Hours/Day (h) | Daily Load (Wh) |
|---|---|---|---|
| Refrigerator (DC or Efficiency) | 60W (Cycling) | 8h aggregate | 480 Wh |
| LED Lighting (6 Bulbs) | 60W | 4h | 240 Wh |
| Starlink / Internet | 50W | 24h | 1,200 Wh |
| Water Pump (Startup Surge) | 1,200W | 0.5h | 600 Wh |
| Laptop Charging | 85W | 3h | 255 Wh |
| TOTAL DAILY LOAD | 2,775 Wh |
Note on Inverter Efficiency: Always add 15% to your total load to account for inverter idle draw and conversion losses. (2,775 Wh × 1.15 = 3,191 Wh).
Phase 2: Sizing the Battery Bank
Once you have your daily load, you must decide your "Days of Autonomy"—how long your system runs without sun.
Formula: (Daily Load × Autonomy) ÷ Usable Depth of Discharge (DoD) = Bank Capacity.
For LiFePO4 (80% DoD) with 2 days autonomy: (3,191 Wh × 2) ÷ 0.80 = 7,977 Wh (approx 8kWh battery bank).
"Statistical analysis from the National Renewable Energy Laboratory (NREL) suggests that off-grid systems designed with less than 2.0 days of autonomy experience a 40% higher probability of critical battery failure within the first three years of operation due to excessive cycling during overcast periods."
— NREL, Reliability of Standalone PV Systems, 2020
Phase 3: Sizing the Solar Array
Your array must be large enough to refill your daily load during the shortest days of winter. This depends on your "Peak Sun Hours."
Formula: (Daily Load ÷ Winter Sun Hours) × 1.25 Loss Factor = Array Wattage.
Example (Ohio - 2.5 Winter Sun Hours): (3,191 Wh ÷ 2.5) × 1.25 = 1,595 Watts of Panels.
🦍 WATTSON'S SIZING RULE: 'SIZE FOR DECEMBER, NOT JULY.' "A lot of guys build a system that works great in the summer and leaves them in the dark in January. I call December the 'Off-Grid Tax Man.' He comes every year and takes 60% of your production. If your math doesn't account for the short days and low sun, you'll be spending your winter evenings with a headlamp and a generator."
Phase 4: Sizing the Inverter and Charge Controller
The inverter is sized by your Peak Load (everything running at once), not your daily totals. The charge controller is sized by your Array Current (Amps).
For a system with a well pump, you need a high-surge inverter. Standard 3,000W inverters might trip when a 1HP pump kicks on.
Get the Direct Path to Independence
The Solar Buyer Checklist includes a simplified sizing worksheet and the exact specs you need to take to a vendor. Download it now to avoid being oversold. Get the Free Solar Buyer Checklist →
Real-World Example: The 2-Bedroom Cabin
A veteran in the Ozarks wanted to run a 2-bedroom cabin. He did his audit and found 4kWh of daily load.
- Load: 4,000 Wh + 15% loss = 4,600 Wh.
- Battery: 4,600 Wh × 2 days ÷ 0.80 = 11,500 Wh bank.
- Solar: 4,600 Wh ÷ 3.5 Sun Hours (Winter) = 1,314W × 1.25 = 1,642W of Panels.
By following the math, he built a system that has run for four years without a single low-voltage disconnect.
The rancher in West Texas who needs to power his cattle well. The veteran in Michigan who is tired of guessing with 12V kits. The father in Tennessee who wants to know exactly what he is buying. This guide is for them.
Frequently Asked Questions
How do I calculate the solar system size I need?
First, calculate your daily watt-hour usage (Watts × Hours). Divide that by your local winter peak sun hours, then multiply by 1.25 to account for efficiency losses. This gives you the minimum solar array wattage required to maintain independence year-round.What is the average size of an off-grid solar system?
A standard small-to-medium off-grid home typically requires 2,000W to 4,000W of solar panels and 15kWh to 30kWh of battery storage. However, "average" is a dangerous word off-grid. Your specific loads determine the size, and a 10% sizing error can equal a 100% loss of power in winter.Can I start with a small system and expand it later?
Yes, but it is expensive. Expanding a system later usually requires a larger inverter and charge controller from the start. It is better to "over-size" your wiring and electronics today so you can add panels and batteries tomorrow without rewiring the entire core.How many solar panels do I need for a 2,000 sq ft home?
Square footage doesn't draw power; appliances do. A 2,000 sq ft home using efficient DC lighting and propane for heating might need only 3,000W. The same home with electric heating and a standard refrigerator might need 12,000W. Perform a load audit to get your real number.Why is winter the limiting factor for solar sizing?
In winter, you have fewer daylight hours and the sun sits lower in the sky (the atmosphere absorbs more energy). Most locations lose 50–70% of their daily solar production in December compared to June. If you size for summer, your system will fail for three months of every year.Math is the only thing that protects you.
The grid is a safety net. Off-grid, you are the net. If your math is loose, your independence will be temporary. Perform the load audit. Use real wattage numbers. Size for the darkest day of the year. When you build on a foundation of math, you build a system that won't leave you in the dark.
🦍 WATTSON ON SIZING: "I've never met a man who regretted having too much power in December. But I've met hundreds who wished they hadn't listened to a salesman who sized their system for the Arizona sun when they live in Pennsylvania. Trust the math, not the pitch."
You are a builder of things that last.
You didn't do all this work to have a system that trips every time you use the microwave. Mathematical sizing is the difference between a project and a power plant. Get your numbers right, then go to work.
"Have a question about specific appliance wattages or sun hours for your zip code? Our AI Guide handles those calculation details." Ask Wattson's AI Guide →