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Solar Sizing Guide: Size Your Off-Grid System Right
Formulas that work. Components that match. Systems that do not fail. This solar sizing guide shows you how to size every component correctly.
For DIY Patriots Who Refuse to Trust Contractors With Their Independence
Solar sizing guide quick formulas:
- Panels: (Daily Ah × Voltage × 1.3) ÷ Peak Sun Hours
- Batteries: (Daily Ah × Days Autonomy) ÷ Depth of Discharge
- Inverter: Running load × 1.25 continuous; largest motor × 3-7x surge
- Charge controller: (Panel watts × 1.25) ÷ Battery voltage
Key insight: 70% of DIY systems fail from mismatched components. Size them together, not separately. This solar sizing guide teaches you how.
You paid $15,000 for a contractor-installed system. First heat wave, the batteries melted. Contractor blamed the batteries. Battery company blamed the contractor. Nobody fixed anything. You learned the hard way: the only person you can trust with your energy independence is yourself.
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Get Free CalculatorSolar Sizing Guide: Why 70% of DIY Systems Fail
Not bad components. Mismatched components.
The solar industry sells you panels and gadgets. They do not care if parts work together. Undersized batteries. Mismatched inverters. Wire too thin for the load.
Each part might be quality. Together they are useless. Like building an engine with motorcycle carburetor and truck pistons.
This solar sizing guide teaches you to size components as a system. When parts match, systems work for years without failure. According to the U.S. Department of Energy, proper system design significantly impacts long-term performance and reliability.
The Five Components That Must Match
Solar Panels: Convert sunlight to electricity.
Charge Controller: Regulates power flow to batteries.
Battery Bank: Stores energy for later use.
Inverter: Converts DC to AC power.
Wiring: Connects everything safely.
Size one wrong and the whole system fails. This solar sizing guide covers each component.
WATTSON'S SIZING TRUTH: "Match your components or watch them die."
I paid a contractor $15,000 for a system. He undersized the batteries to pad his margins. First summer, they melted. He never came back. That failure pushed me to get solar trained. Now I size my own components. Never trusted another "expert" with my energy independence.
Solar Sizing Guide Step 1: Calculate Your Energy Needs
Forget utility bills. Monthly averages kill off-grid systems. You need daily consumption in amp-hours (Ah).
This solar sizing guide uses amp-hours because batteries are rated in Ah. Watts tell you instantaneous power. Amp-hours tell you storage capacity.
Essential Loads (Must Run)
- Refrigerator: 40-60 Ah/day
- LED lighting: 10-20 Ah/day
- Water pump: 5-15 Ah/day
- Communication devices: 5-10 Ah/day
Comfort Loads (Nice to Have)
- Washing machine: 15-25 Ah/day
- Television: 10-20 Ah/day
- Microwave: 20-40 Ah/day
Heavy Loads (Power Hogs)
- Electric heat: 100-300 Ah/day
- Air conditioning: 200-500 Ah/day
- Power tools: Variable, high surge
DAILY LOAD FORMULA
Total Daily Ah = (Essential × 1.0) + (Comfort × 0.7) + (Heavy × 0.3)
Multipliers reflect realistic usage. Essentials run 100%. Comfort loads 70%. Heavy loads only when needed.
Solar Sizing Guide Step 2: Panel Array Sizing
Your array must generate enough to meet daily needs, recharge batteries, account for losses, and handle seasonal variations.
The National Renewable Energy Laboratory provides solar resource maps showing peak sun hours by location. Use these for accurate solar sizing guide calculations.
PANEL SIZING FORMULA
Panel Watts = (Daily Ah × System Voltage × 1.3) ÷ Peak Sun Hours
Variables: Daily Ah = your consumption. System voltage = 12V, 24V, or 48V. 1.3 = loss factor. Peak sun hours = your location (4-6 for most US).
EXAMPLE CALCULATION
- Daily need: 200 Ah
- System: 48V
- Location: 5 peak sun hours
- Calculation: (200 × 48 × 1.3) ÷ 5 = 2,496 watts needed
Panel Configuration Options
Series: Increases voltage. Better for long wire runs and MPPT controllers.
Parallel: Increases current. Better for partial shading situations.
Series-Parallel: Best of both for larger arrays.
CRITICAL: SIZE FOR WINTER
December sun is weakest. Your power needs do not care about weather. Summer-sized systems fail in winter. Always use your location's worst-case peak sun hours for solar sizing guide calculations.
Every watt you generate is a watt you do not buy from the utility. Every properly sized component is one less failure point. The time you invest in this solar sizing guide pays dividends for decades. Your family's comfort during the next grid failure depends on what you learn today.
Solar Sizing Guide Step 3: Battery Bank Sizing
Battery bank is your insurance. Size it wrong and you ration power like a third-world country during cloudy weather.
BATTERY SIZING FORMULA
Battery Ah = (Daily Ah × Days of Autonomy) ÷ Depth of Discharge
Days of Autonomy: How many days without sun? Minimum 3-5 days.
Depth of Discharge: Lead-acid = 50% max. LiFePO4 = 80% safe.
EXAMPLE CALCULATION
- Daily need: 200 Ah
- Autonomy: 3 days
- Battery: LiFePO4 (80% DOD)
- Calculation: (200 × 3) ÷ 0.8 = 750 Ah bank needed
Battery Chemistry Comparison
| Type | Usable Capacity | Lifespan | Maintenance |
|---|---|---|---|
| Lead-Acid | 50% (needs larger bank) | 3-5 years | Monthly required |
| LiFePO4 | 80% usable | 10-15 years | Zero maintenance |
System Voltage Selection
12V: Small cabins under 1,000W. Simple but high current.
24V: Medium homes 1,000-3,000W. Good balance.
48V: Large installations over 3,000W. Most efficient, thinner wires.
For detailed battery comparisons, see our Lithium vs Lead Acid Guide.
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Get Free CalculatorSolar Sizing Guide Step 4: Inverter Sizing
Inverter converts DC battery power to AC household power. Size wrong and surge loads will not start. Or you waste money on oversized equipment.
INVERTER SIZING FORMULAS
Continuous Watts = Total Running Load × 1.25
Surge Watts = Largest Motor Start × 3-7 multiplier
Motor Start Multipliers
- Refrigerator compressor: 3-4x running watts
- Well pump: 4-6x running watts
- Air compressor: 5-7x running watts
- Circular saw: 2-3x running watts
PURE SINE WAVE ONLY
Modified sine wave is cheaper but problematic. Makes motors run hot. Causes interference with electronics. Damages sensitive equipment. False economy that costs more long-term. Pure sine wave. No exceptions.
For inverter recommendations, see our Best 48V Inverter Guide.
WATTSON'S INVERTER WISDOM: "Size for surge, not just running."
Watched a neighbor's inverter die trying to start his well pump. Running watts looked fine on paper. Surge demand said otherwise. His 3,000W inverter could not handle the 6,000W startup. Now he runs a 5,000W unit with 10,000W surge. Lesson cost him $800.
Solar Sizing Guide Step 5: Charge Controller
Charge controller regulates power from panels to batteries. The traffic cop that prevents overcharging and damage.
MPPT vs PWM
| Type | Efficiency | Best For |
|---|---|---|
| PWM | 75-80% | Small systems under 400W only |
| MPPT | 92-97% | Any system over 400W |
MPPT SIZING FORMULA
Controller Amps = (Panel Watts × 1.25) ÷ Battery Voltage
Example: 2,400W array at 48V: (2400 × 1.25) ÷ 48 = 62.5A. Choose 80A controller (next size up).
Controller Sizing Tips
- Always round up to next standard size
- Plan 25% extra capacity for expansion
- Multiple smaller controllers often better than one large
- Match input voltage to panel string voltage
Solar Sizing Guide Step 6: Wiring and Fusing
Wrong wire gauge causes voltage drop, power loss, and fires. Improper fusing kills people. This is not optional.
Voltage Drop Limits
- DC circuits: 3% maximum
- AC circuits: 5% maximum
- Critical loads: 2% maximum
Common DC Wire Sizes
| Wire Size | Max Amps | Typical Use |
|---|---|---|
| 10 AWG | 30A | Short panel runs |
| 8 AWG | 40A | Medium runs |
| 6 AWG | 55A | Longer runs |
| 4 AWG | 70A | Main battery connections |
| 2 AWG | 95A | High current |
| 1/0 AWG | 125A | Inverter connections |
Fusing Rules
- Fuse rating = 125% of circuit current
- Install within 7 inches of positive terminal
- DC-rated fuses only (AC fuses fail on DC)
- Every positive conductor needs protection
REQUIRED FUSE LOCATIONS
- Battery bank positive terminal
- Solar panel combiner box
- Each branch circuit
- Inverter DC input
- Charge controller input
For detailed wiring guidance, see our Wire Sizing Guide.
Solar Sizing Guide: Complete System Example
This solar sizing guide example shows how components work together for a 1,200 sq ft cabin.
ENERGY AUDIT RESULTS
- Daily consumption: 250 Ah @ 48V
- Peak load: 3,000W continuous, 6,000W surge
- Location: 4.5 peak sun hours (winter)
Component Sizing Results
Solar Array (using solar sizing guide formula):
- Required: (250 × 48 × 1.3) ÷ 4.5 = 3,467W
- Selected: 3,600W (12 × 300W panels)
Battery Bank:
- Required: (250 × 3) ÷ 0.8 = 938 Ah
- Selected: 960 Ah LiFePO4 @ 48V
Inverter:
- Continuous: 3,000W × 1.25 = 3,750W
- Surge: 6,000W required
- Selected: 4,000W continuous, 8,000W surge pure sine
Charge Controller:
- Required: (3,600 × 1.25) ÷ 48 = 94A
- Selected: 100A MPPT controller
Wiring:
- Battery to inverter: 4/0 AWG
- Solar to controller: 10 AWG
- All circuits properly fused per NEC
How do I calculate solar panel size for off-grid?
+Use this solar sizing guide formula: (Daily Ah × System Voltage × 1.3) ÷ Peak Sun Hours. For 200Ah at 48V with 5 sun hours: (200 × 48 × 1.3) ÷ 5 = 2,496 watts minimum.
What size battery bank do I need?
+Formula: (Daily Ah × Days of Autonomy) ÷ Depth of Discharge. For 200Ah daily, 3 days autonomy, LiFePO4 (80% DOD): (200 × 3) ÷ 0.8 = 750Ah minimum.
How do I size an inverter correctly?
+Size for continuous watts (running load × 1.25) AND surge watts (largest motor × 3-7x). Well pump at 1,000W running needs 4,000-6,000W surge capacity. Always choose pure sine wave.
MPPT or PWM charge controller?
+MPPT for anything over 400 watts. MPPT delivers 92-97% efficiency versus PWM's 75-80%. Size: (Panel Watts × 1.25) ÷ Battery Voltage.
Why do DIY solar systems fail?
+70% fail from mismatched components. Common mistakes: undersized batteries, wrong wire gauge, mismatched voltages, inadequate fusing. This solar sizing guide teaches you to avoid these errors.
What system voltage should I choose?
+12V for small cabins under 1,000W. 24V for medium homes 1,000-3,000W. 48V for large installations over 3,000W. Higher voltage means lower current and thinner wires.
How many days of battery autonomy do I need?
+Minimum 3 days for most locations. 5-7 days for cloudy climates or critical loads. More autonomy means larger battery bank but better weather protection.
What wire size do I need for solar?
+Depends on current and distance. Keep voltage drop under 3% for DC. Common sizes: 10 AWG for 30A, 6 AWG for 55A, 1/0 AWG for inverter connections over 100A.
Should I size for summer or winter?
+Always size for winter conditions. December sun is weakest but power needs stay constant. Summer-sized systems fail in winter. Use worst-case peak sun hours.
How do I account for system losses?
+Apply a 1.3 multiplier (30% safety margin) to calculations. This covers wire losses, controller inefficiency, charging losses, temperature effects, and panel degradation.
Conclusion: Size It Right, Build It Once
This solar sizing guide gives you the formulas that work. Component sizing is not glamorous. No flashy marketing. No miracle solutions. Just engineering discipline applied to energy independence.
Properly sized components work together for years without failure. When the grid dies, your system keeps running.
The grid will fail again. When it does, you will have two options: stumble in the dark like the unprepared masses, or flip on your lights powered by a correctly sized system.
Use this solar sizing guide. Do the math. Or use our calculator. Either way, size your components correctly the first time.
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