To size your first off-grid solar system, you must first calculate your total daily energy use in watt-hours (Wh). Next, determine the peak sun hours for your specific location during the worst month of the year. Finally, divide your daily energy use by your peak sun hours and account for system inefficiencies (typically 25% loss) to find the total solar panel wattage required. Your battery bank should then be sized to store enough energy to cover 1 to 3 days of use without sunlight.
Planning an off-grid solar system from scratch can feel overwhelming. Whether you are powering a remote cabin, an RV, or a backyard shed, the math relies on a few core principles. This guide walks you through the exact steps to size your panels, batteries, inverter, and charge controller so you can build a reliable system without overspending.
Step 1: Calculate Your Daily Energy Use
The foundation of any off-grid system is your load profile. If you guess how much power you need, you will either buy too much equipment or end up with dead batteries in the middle of the night.
List every appliance you plan to run. For each device, find its power draw in watts (usually printed on a sticker on the back or bottom of the device) and estimate how many hours per day it will run.
Multiply the watts by the hours to get your daily energy use in watt-hours (Wh).
Watts × Hours = Watt-hours (Wh) per day
For example, a 60-watt laptop charger running for 4 hours a day uses 240 Wh. Add up the watt-hours for all your devices. Because inverters (which convert battery power to standard wall power) are not 100% efficient, add a 10% to 15% buffer to your total AC load to account for conversion losses.
Step 2: Determine Your Peak Sun Hours
A "peak sun hour" is not just any hour the sun is up. It is an hour where the intensity of the sunlight reaches 1,000 watts per square meter. Six hours of weak morning or late afternoon sun might only equal two peak sun hours.
To build a system that works year-round, you must design around your worst-case scenario. Look up the peak sun hours for your specific location during the winter months using a solar insolation map or database like the National Renewable Energy Laboratory (NREL) PVWatts calculator.
If you design your system based on summer sun, you will not generate enough power in December.
Step 3: Size the Solar Panel Array
Once you know your daily energy use and your peak sun hours, you can calculate how many solar panels you need.
Array Size (Watts) = Daily Use (Wh) ÷ Peak Sun Hours ÷ System Efficiency
Solar panels rarely operate at their perfect laboratory rating due to heat, dust, wiring resistance, and charge controller losses. A safe rule of thumb is to assume a 75% efficiency rate (or a 0.75 multiplier).
If you need 2,000 Wh per day and get 4 peak sun hours: 2,000 Wh ÷ 4 hours ÷ 0.75 = 666 Watts of solar panels required.
You could meet this requirement with two 350W panels or seven 100W panels.
Step 4: Size the Battery Bank
Your solar panels only generate power while the sun is shining. Your battery bank must store enough energy to run your loads at night and during cloudy days.
Usable Capacity (Wh) = Daily Use (Wh) × Days of Autonomy
"Days of autonomy" refers to how many days your system can run without any solar input. For a weekend cabin, 1 to 2 days might be enough. For a full-time off-grid home, 3 to 5 days is standard.
Next, you must account for Depth of Discharge (DoD). You cannot safely drain most batteries to 0%.
- Lead-acid batteries should only be drained to 50% to preserve their lifespan.
- Lithium iron phosphate (LiFePO4) batteries can safely be drained to 80% or even 100%.
Total Battery Capacity (Wh) = Usable Capacity ÷ DoD
If you need 4,000 Wh of usable capacity and use lithium batteries (80% DoD): 4,000 Wh ÷ 0.80 = 5,000 Wh total battery capacity required.
Crucial Factors Many Beginners Overlook
When sizing a system for the first time, it is easy to focus only on the basic math and miss real-world constraints that can cause a system to fail.
Surge Watts vs. Running Watts Devices with electric motors or compressors—like refrigerators, well pumps, and air conditioners—require a massive spike of power to start up. A fridge that runs on 150 watts might need 600 to 1,000 watts for a split second when the compressor kicks on. Your inverter must be sized to handle the combined running watts of all your active devices plus the highest surge wattage of any single motor starting up.
Battery Temperature Limits Batteries are highly sensitive to temperature. Lead-acid batteries lose significant capacity in freezing weather. More importantly, standard lithium (LiFePO4) batteries cannot be charged when the battery core temperature drops below freezing (32°F / 0°C). Doing so will permanently destroy the battery. If your batteries will be stored in an unheated space, you must buy self-heating lithium batteries or build an insulated, temperature-controlled battery box.
Phantom Loads Inverters consume power simply by being turned on, even if nothing is plugged into them. A large 3,000W inverter might draw 30 to 50 watts continuously. Over 24 hours, that "phantom load" consumes 720 to 1,200 Wh—which could be more than half the daily energy budget of a small cabin. Always factor inverter idle draw into your daily load calculations, or plan to physically turn the inverter off when not in use.
Illustrative Worked Example: The Weekend Cabin
Let's walk through a realistic sizing scenario for a small off-grid hunting cabin used primarily on weekends.
1. The Load Profile
- LED Lights: 4 bulbs × 10W × 4 hours = 160 Wh
- Laptop: 50W × 3 hours = 150 Wh
- Small 12V Fridge: Runs 24/7, consumes roughly 400 Wh per day
- Phone chargers: 2 phones × 10W × 2 hours = 40 Wh
- Total Daily Use: 750 Wh
Because the fridge is 12V DC, we only need a small inverter for the laptop. We will add a 15% buffer to the laptop load for inverter inefficiency: 150 Wh × 1.15 = 172 Wh. Adjusted Total: 772 Wh per day.
2. Peak Sun Hours The cabin is located in Ohio. In December, the location receives only 2.2 peak sun hours per day.
3. Solar Array Sizing 772 Wh ÷ 2.2 peak sun hours ÷ 0.75 efficiency = 467 Watts. Decision: Two 250W panels (500W total) will safely cover the winter requirement.
4. Battery Sizing The owner wants 2 days of autonomy in case of a rainy weekend. Usable capacity needed: 772 Wh × 2 days = 1,544 Wh. Using a 12V LiFePO4 battery (80% DoD): 1,544 Wh ÷ 0.80 = 1,930 Wh total capacity. To convert Wh to Amp-hours (Ah) for a 12V battery: 1,930 Wh ÷ 12V = 160 Ah. Decision: One 12V 200Ah lithium battery provides plenty of buffer.
Selecting the Right Charge Controller
The charge controller sits between your solar panels and your battery, regulating the voltage to prevent overcharging.
Always choose an MPPT (Maximum Power Point Tracking) charge controller over a cheaper PWM (Pulse Width Modulation) controller. MPPT controllers are up to 30% more efficient because they actively convert excess solar voltage into usable charging current.
To size the controller, divide your total solar array wattage by your battery bank voltage. For example, a 500W array charging a 12V battery: 500W ÷ 12V = 41.6 Amps. You would need a charge controller rated for at least 50 Amps to provide a safe margin.
Frequently Asked Questions
Can I mix different sizes or brands of solar panels? It is highly discouraged. Mixing panels with different voltage and current ratings will drag the performance of the entire array down to the lowest common denominator. If you must expand a system later, use a separate charge controller for the new panels and connect that controller to the same battery bank.
Do I need a 12V, 24V, or 48V battery bank? For small systems under 1,200W of solar, 12V is standard and makes it easy to find compatible DC appliances. For medium systems (1,200W to 3,000W), 24V is better because it halves the amperage, allowing for thinner, cheaper wiring. For whole-home systems over 3,000W, 48V is required to keep amperage at safe levels and utilize large hybrid inverters.
How do I know if my inverter is big enough for my refrigerator? Check the refrigerator's compressor rating. A standard fridge might draw 150W while running but require a 1,000W to 1,200W surge to start the compressor. You must buy an inverter with a continuous rating that covers your other loads, plus a "surge" or "peak" rating that exceeds the refrigerator's startup requirement.
Can I use car batteries for my off-grid solar system? No. Car batteries are "starter batteries" designed to deliver a massive amount of current for a few seconds to start an engine. If you slowly drain them day after day (deep cycling), they will permanently degrade within a few months. You must use true "deep cycle" batteries, such as LiFePO4 or deep-cycle AGM/Gel lead-acid batteries.
What happens to the solar power when my batteries are fully charged? The charge controller automatically detects that the battery is full and stops sending current. The solar panels will simply sit idle in the sun, generating voltage but no actual power (current), which is perfectly safe and normal.
Next Steps
Before buying any equipment, finalize your load list and run your numbers through the WattSizing calculator. Double-check your peak sun hours for your specific zip code, and decide where you will safely store your battery bank.
