How many panels and kWh storage do you need off-grid?

Getting off-grid solar sizing right means balancing three critical factors: your actual energy consumption, available sunlight, and storage capacity. Too few panels leave you powerless during cloudy days. Too much storage drains your budget without meaningful benefits. This guide provides the exact calculations you need to size both components correctly.

Start with your actual energy consumption

Accurate load assessment forms the foundation of proper off-grid sizing. Most homeowners dramatically underestimate their daily consumption, leading to undersized systems that fail during critical periods.

Calculate your daily kWh requirements

List every electrical device you plan to power, including:

• LED lighting: 8-12W per bulb × hours of use
• Refrigerator: 150-400W × 8-12 hours daily runtime
• Electronics: laptops (45-65W), phones (5-10W), TV (100-200W)
• Water pump: 500-1500W × 2-4 hours daily
• Power tools: 800-1500W × actual usage hours

A typical off-grid cabin consumes 3-8 kWh daily, while a full household may require 15-25 kWh. According to Department of Energy data, the average American home uses 30 kWh daily, but off-grid systems typically operate more efficiently due to conscious energy management.

Account for seasonal variations

Winter heating can triple your energy consumption. Electric space heaters consume 1000-1500W continuously. Heat pumps, while more efficient, still require 2000-5000W during peak operation. Size your system for winter demands, not summer minimums.

Convert energy needs to solar panel capacity

Solar panel sizing depends on your location's peak sun hours and system efficiency losses. The basic formula requires several key inputs.

Peak sun hours by location

Peak sun hours represent the equivalent hours of 1000W/m² solar irradiance your location receives daily. These vary significantly:

Region	Winter Peak Sun Hours	Summer Peak Sun Hours
Southwest US (Arizona, Nevada)	4.5-5.5	6.5-7.5
Southeast US (Florida, Georgia)	3.5-4.5	5.5-6.5
Northeast US (New York, Maine)	2.5-3.5	4.5-5.5
Northwest US (Washington, Oregon)	1.5-2.5	5.0-6.0

Always use winter values for off-grid calculations. Summer excess can charge batteries for cloudy periods, but winter shortfalls create immediate power failures.

System efficiency factors

Real-world solar systems lose 20-25% of theoretical output due to:

• Inverter losses: 8-12%
• Battery charging losses: 10-15%
• Wiring losses: 2-3%
• Temperature derating: 5-10%
• Dust and shading: 2-5%

The practical sizing formula becomes:

Required PV Capacity (W) = Daily kWh ÷ Peak Sun Hours ÷ 0.75 × 1000

For a 5 kWh daily load in a location with 3 winter peak sun hours:

Required PV = 5 ÷ 3 ÷ 0.75 × 1000 = 2,222W or approximately 2.2kW

Size battery storage for reliability

Battery capacity determines how many days you can operate without solar input. This autonomy period varies based on your risk tolerance and local weather patterns.

Autonomy days calculation

Most off-grid systems target 3-5 days of autonomy. Longer periods require exponentially larger battery banks with diminishing returns. According to IRENA research, optimal autonomy periods depend on regional weather patterns and backup generation availability.

Calculate required battery capacity using:

Battery Capacity (kWh) = Daily kWh × Autonomy Days ÷ Depth of Discharge

Battery technology considerations

LiFePO4 batteries allow 80-90% depth of discharge, while lead-acid systems should not exceed 50% for longevity:

Battery Type	Usable Capacity	Cycle Life	Cost per kWh
LiFePO4	90%	6,000-8,000	$400-600
AGM Lead-Acid	50%	500-800	$150-250
Flooded Lead-Acid	50%	1,000-1,500	$100-180

For a 5 kWh daily load with 3 days autonomy using LiFePO4:

Required Capacity = 5 × 3 ÷ 0.9 = 16.7 kWh

Real-world sizing examples

These examples demonstrate complete sizing calculations for different applications, based on actual installations and performance data.

Remote cabin scenario

Daily loads:

• LED lighting: 200W × 4 hours = 0.8 kWh
• Refrigerator: 180W × 10 hours = 1.8 kWh
• Electronics: 100W × 6 hours = 0.6 kWh
• Water pump: 800W × 1 hour = 0.8 kWh
• Total: 4.0 kWh daily

Location: Colorado mountains (3.0 winter peak sun hours)

PV requirement: 4.0 ÷ 3.0 ÷ 0.75 × 1000 = 1,778W

Recommended: 2.0kW array (eight 250W panels)

Battery sizing for 4 days autonomy with LiFePO4:

4.0 × 4 ÷ 0.9 = 17.8 kWh battery bank

Off-grid farm operation

Agricultural applications often require higher power for equipment and irrigation. A typical setup might include:

• Barn lighting and outlets: 2.0 kWh
• Water pumping: 3.0 kWh
• Grain handling equipment: 4.0 kWh
• Electric fencing: 0.5 kWh
• Total: 9.5 kWh daily

For Midwest location (2.8 winter peak sun hours):

PV requirement: 9.5 ÷ 2.8 ÷ 0.75 × 1000 = 4,524W

Recommended: 5.0kW array with 42 kWh LiFePO4 battery bank

System integration and monitoring

Proper component matching ensures optimal performance and longevity. Undersized charge controllers or inverters create bottlenecks that waste your investment.

Charge controller sizing

MPPT charge controllers should handle 125% of your array's short-circuit current. For a 2kW array producing 8.3A at 24V:

Required controller: 8.3 × 1.25 = 10.4A minimum

Choose a 15A or 20A controller for safety margin and future expansion.

Inverter capacity

Size inverters for your largest simultaneous loads plus 20% margin. Inductive loads like pumps and refrigerators require 3-5x their running current during startup.

A 1500W water pump needs at least a 2000W inverter to handle startup surge. Pure sine wave inverters prevent equipment damage and reduce electromagnetic interference.

Avoiding common sizing mistakes

Based on field experience with hundreds of installations, these errors cause the majority of off-grid system failures.

Undersizing for winter conditions

Systems sized for summer performance fail during winter months when heating loads increase and solar production drops. Always calculate based on worst-case scenarios, not average conditions.

Ignoring surge requirements

Motor-driven appliances require 3-7 times their running power during startup. A 500W well pump may need 2500W for 2-3 seconds. Undersized inverters shut down during these surge events.

Battery bank imbalance

Mixing battery ages, types, or capacities reduces overall performance and lifespan. Replace entire banks simultaneously, not individual batteries.

Conclusion

Accurate off-grid sizing requires methodical calculation of energy consumption, solar resources, and storage requirements. Start with precise load analysis, use conservative efficiency factors, and size for worst-case weather conditions. The extra investment in proper sizing prevents costly system failures and ensures reliable power when you need it most.

Remember that off-grid systems require ongoing maintenance and monitoring. Install proper system monitoring to track performance and identify issues before they cause failures. With careful planning and quality components, your off-grid solar system will provide decades of reliable service.