How to Calculate Solar Needs: Real Load Analysis That Actually Works (2025)

Two years ago, I confidently built a “backup solar system” without doing the math. I eyeballed it - “400W of panels and a couple batteries should be plenty.” Three days into our first power outage, my system was dead and I was running an extension cord to my neighbor’s generator. Embarrassing and expensive lesson learned.

The problem wasn’t the equipment quality - it was that I had no idea how much power I actually used or how much my solar system could realistically provide. I’d confused theoretical specs with real-world performance.

Since then, I’ve properly sized two solar systems (mine and my brother’s) and helped neighbors avoid my expensive mistakes. Here’s the step-by-step process that actually works, with real examples from my current system.

The Solar Sizing Reality Check

What I wish someone had told me:

• Marketing watts ≠ real-world watts - panels rarely produce rated power
• Efficiency matters more than total capacity - losses add up quickly
• Weather kills solar production - cloudy days drop output by 80%+
• Starting loads are huge - refrigerators need 3x running power to start
• Battery math is backwards - you can only use 50-80% of capacity safely

My systematic approach (learned through mistakes):

1. Calculate actual daily energy use
2. Size panels for worst-case weather
3. Choose inverter for startup loads, not just running loads
4. Size batteries for 2-3 days without sun
5. Add 20-30% safety margin to everything

Step 1: Calculate Your Actual Daily Energy Use

The Load Audit That Actually Works

My process (with Kill-A-Watt meter and spreadsheet):

Your Load Audit Worksheet

Appliance	Watts (W)	Hours/Day	Daily Watt-Hours (Wh)
Refrigerator	150 W	8	1,200 Wh
LED Lights	72 W	6	432 Wh
WiFi Router	25 W	24	600 Wh
Laptop Charger	65 W	4	260 Wh
Phone Charger	15 W	2	30 Wh
Your Total:			2,522 Wh

My essential loads:

• Refrigerator: 150W running, 450W startup, 8 hours/day actual runtime = 1,200 Wh/day
• LED lights: 12W each × 6 bulbs × 6 hours = 432 Wh/day
• WiFi router + modem: 25W × 24 hours = 600 Wh/day
• Laptop charging: 65W × 4 hours = 260 Wh/day
• Phone charging: 15W × 2 hours = 30 Wh/day
• Coffee maker: 800W × 0.25 hours = 200 Wh/day (yes, coffee is essential)

Total daily load: 2,722 Wh/day (2.7 kWh/day)

Reality check: This is just basic survival loads. No TV, no microwave, no electric heat. A normal house uses 30 kWh/day.

Step 2: Size Solar Panels for Real Conditions

Peak Sun Hours Reality

My location (Colorado, good solar state):

• Summer: 5.5 peak sun hours/day average
• Winter: 3.5 peak sun hours/day average
• Cloudy days: 0.8-1.5 peak sun hours

Why I size for winter: If my system works in December, it works year-round. Summer gives me surplus for other loads.

Panel Sizing Calculation

My formula (accounts for real-world losses):

For my 2,722 Wh/day system:

My actual panel setup: 1,200W (four 300W panels) to have cushion for expansion and bad weather days.

Step 3: Choose the Right Inverter

The Startup Load Problem

My expensive lesson: Bought a 1000W inverter for my “800W” loads. When the refrigerator tried to start while the coffee maker was running, the inverter shut down on overload.

My inverter sizing method:

1. List all simultaneous loads: What might run at the same time?
2. Add starting loads: Use 3x multiplier for motors, 1x for resistive loads
3. Size inverter for peak: Choose inverter 1.5x your calculated peak load
4. Check surge rating: Must handle highest single motor starting load

My setup: 3000W inverter for my calculated 2000W peak simultaneous load (with starting loads). Sounds like overkill, but it prevents shutdowns.

Step 4: Size Batteries for Autonomy

The 2-3 Day Rule

Why 2-3 days matters: Most weather systems pass in 2-3 days. Longer than that and you’re in extended storm territory where solar won’t help anyway.

My battery calculation: Daily load: 2,722 Wh Target autonomy: 3 days Total needed: 2,722 × 3 = 8,166 Wh (8.2 kWh)

Depth of Discharge Reality

Battery type matters:

• Lead-acid: Can only use 50% safely = need 2x capacity
• AGM: Can use 50-60% safely = need 1.8x capacity
• LiFePO₄: Can use 80-95% safely = need 1.1x capacity

My battery sizing (LiFePO₄ system):

Convert to amp-hours:

My actual setup: 400 Ah LiFePO₄ at 48V (19.2 kWh total) because I wanted room for future loads and some bad weather margin.

Real-World Example: My Complete System

System Specs and Performance

Panel array: 1,200W (4 × 300W monocrystalline) Batteries: 400 Ah LiFePO₄ at 48V (19.2 kWh total) Inverter: 3000W pure sine wave Charge controller: 60A MPPT

Actual performance:

• Summer: System fully recharged by 2 PM, surplus power available
• Winter: System recharged by 4-5 PM on sunny days
• Cloudy days: Can run 2-3 days on battery alone
• Load growth: Added TV and workshop tools, still works fine

Total System Cost (2023 prices)

Panels: $600 (4 × 300W at $150 each) Batteries: $2,400 (400 Ah LiFePO₄) Inverter: $400 (3000W pure sine wave) Charge controller: $300 (60A MPPT) Wiring/safety equipment: $300 Total: $4,000 for whole system

Cost per usable kWh: $208 per kWh (vs. $300-500 for commercial battery systems)

Common Sizing Mistakes I See

Under-sizing Problems

Not enough panels: System never fully recharges batteries Weak inverter: Shuts down when multiple loads start Small batteries: Can’t make it through one cloudy day Wrong battery type: Lead-acid when you need daily cycling

Over-sizing Waste

Massive panels: More than you can use or store Huge inverter: Wastes power in standby mode Too many batteries: Expensive and hard to maintain

Seasonal Adjustments and Load Management

Summer vs. Winter Reality

Summer surplus strategy:

• Run larger loads during peak production hours
• Charge electric tools and devices
• Pre-cool house before evening
• Consider adding seasonal loads (fans, pool pumps)

Winter conservation mode:

• Time coffee maker and high loads for solar production hours
• Use LED lights exclusively
• Minimize inverter idle loads
• Have backup heating plan that doesn’t require electricity

Load Management Tools

Smart switches and timers: Automatically turn on loads when batteries are full Battery monitors: Show real-time production and consumption Load dump controllers: Send excess power to water heating or space heating

Solar sizing isn’t about buying the biggest system you can afford - it’s about building the right system for your actual needs. Start with measuring your real loads, size for worst-case conditions, and add safety margins. You can always expand later, but you can’t easily fix an under-sized system during an outage.