Solar Battery Sizing: How to Calculate Storage Needs for Off-Grid Living

Reading time: ~11 min read

Off-grid solar systems live or die by the battery bank. Undersize it and your clients face frequent generator run-time, premature battery degradation from deep cycling, and dissatisfaction with system performance. Oversize it and you inflate capital costs beyond what the load profile justifies. Correct battery sizing requires a systematic approach: calculate daily energy consumption, determine autonomy requirements, apply depth-of-discharge and temperature derating factors, and add system efficiency losses. This guide walks through each step with worked examples for different home sizes.

PES Supply stocks the battery modules, charge controllers, and hybrid inverters needed to build properly sized off-grid systems. Browse our Battery Storage collection for LiFePO4 modules from EG4, Fortress Power, Battle Born, and Victron, or explore complete Energy Storage Systems for all-in-one hybrid inverter packages.

Step 1: Calculate Daily Energy Consumption (Load Audit)

The foundation of every battery sizing calculation is an accurate daily load audit. This means listing every electrical load the battery bank must support, its wattage, and the hours of operation per day. The product of wattage and hours gives daily watt-hours (Wh), which sum to total daily energy consumption in kilowatt-hours (kWh).

Load Audit Worksheet Template

Load	Watts	Hours/Day	Daily Wh	AC or DC
Refrigerator (efficient)	150	24 (compressor cycles ~8h)	1,200	AC
LED Lighting (8 fixtures)	10 each	5	400	AC or DC
Well Pump (1/2 HP)	1,000	1	1,000	AC
Washing Machine	500	1	500	AC
Laptop / Router / Modem	100	8	800	AC
TV (LED, 50")	100	4	400	AC
Phone / Device Charging	50	4	200	AC
Ventilation Fans	40	12	480	AC
Total Daily Load			4,980 Wh (~5.0 kWh)

For a load audit to be accurate, use actual nameplate wattage or measured draw with a clamp meter or kill-a-watt device. Refrigerator compressors do not run 24 hours — they cycle on roughly 30-50% of the time, so multiply rated wattage by an effective duty cycle. Well pumps and motors have surge currents 3-6 times their running wattage; size the inverter for surge, but calculate battery load on running wattage.

Seasonal variation matters. Heating and cooling loads can double or triple daily consumption in peak summer or winter months. Always size for the worst-case season the system must handle without generator backup.

Step 2: Determine Days of Autonomy

Autonomy days define how long the battery bank can sustain the load without any solar recharge. This is the buffer for cloudy weather, storms, or periods of low solar production. The number of autonomy days depends on climate, the client's tolerance for generator use, and whether a backup generator is present.

Recommended Autonomy by Application

Application Type	Autonomy Days	Rationale
Weekend cabin (backup generator present)	1–2 days	Generator covers extended cloud periods
Full-time off-grid, mild climate	2–3 days	Standard off-grid design with moderate solar variability
Full-time off-grid, harsh winters	3–5 days	Low winter solar production requires deeper buffer
Critical loads (medical, communications)	3–5+ days	Maximum reliability; redundancy is non-negotiable

Each additional autonomy day directly increases battery bank size and cost. For systems with a reliable backup generator, 2-3 days of autonomy is typically the sweet spot — enough to ride through normal weather variability without excessive capital investment.

Step 3: Apply Depth of Discharge (DoD) Factor

Depth of discharge is the percentage of the battery's total capacity that can be safely used before recharging. DoD varies by chemistry and directly affects how much nominal capacity you need to purchase to deliver the required usable energy.

Battery Chemistry	Recommended DoD	Usable Capacity Multiplier
Lithium Iron Phosphate (LiFePO4)	80–95%	1.05–1.25×
Lead-Acid (Flooded)	50%	2.0×
Lead-Acid (AGM/Gel)	50–60%	1.67–2.0×
Saltwater / Sodium-Ion	Up to 100%	1.0–1.1×

The usable capacity multiplier tells you how much nominal capacity you need relative to the energy you must deliver. For example, if you need 10 kWh of usable energy with lead-acid at 50% DoD, you must purchase a 20 kWh battery bank. With LFP at 90% DoD, you need approximately 11.1 kWh of nominal capacity — nearly half the lead-acid requirement.

This is why LFP has become the dominant chemistry for off-grid installations. The lower DoD multiplier means smaller banks, fewer modules, less weight, and lower upfront cost when measured against usable energy delivered.

Step 4: Apply Temperature Derating

Battery capacity is rated at 25°C (77°F) under standard test conditions. Real-world operating temperatures deviate from this benchmark, and battery chemistry responds differently to temperature extremes.

Temperature Derating Factors

Battery Temperature	Lead-Acid Capacity	LFP Capacity
-10°C (14°F)	~65% of rated	~75% of rated (discharge only)
0°C (32°F)	~75% of rated	~85% of rated
10°C (50°F)	~87% of rated	~92% of rated
25°C (77°F) — Standard	100% of rated	100% of rated
35°C (95°F)	~102% of rated (short-term)	~100% of rated
45°C (113°F)	~100% but accelerated aging	~98% but accelerated aging

Cold temperatures reduce usable capacity, while high temperatures accelerate calendar aging and shorten overall lifespan. For battery banks installed in unconditioned spaces in cold climates, apply a temperature derating factor of 0.75-0.85 to account for winter capacity loss. For installations in temperature-controlled environments (garages, basements, utility rooms maintained near 20-25°C), derating may be minimal (0.95-1.0).

Important caveat for LFP: while discharge is permitted down to -20°C, charging below 0°C can cause irreversible lithium plating on the anode. Systems installed in cold environments should specify LFP batteries with integrated self-heating elements or include a thermostatically controlled battery heater.

Step 5: Account for System Efficiency Losses

Energy is lost at multiple conversion stages between the solar array and the loads. The standard system derating factor of 0.8 accounts for:

• Inverter efficiency: 90-96% for pure sine wave inverters under typical loading
• Wiring and connection losses: 2-5% depending on conductor sizing and run lengths
• Charge controller efficiency: 95-99% for MPPT controllers
• Battery round-trip efficiency: 92-98% for LFP, 70-85% for lead-acid

Multiply the daily load by a system efficiency factor to account for these losses. For LFP systems with modern MPPT controllers and properly sized wiring, an overall efficiency factor of 0.85 is conservative. For lead-acid systems, use 0.75 to account for lower round-trip efficiency and Peukert losses at higher discharge rates.

The Complete Sizing Formula

Combining all factors, the battery bank sizing formula is:

Battery Bank Capacity (kWh) = (Daily Load (kWh) × Autonomy Days) / (DoD × Temperature Derating × System Efficiency)

Let's break down each variable:

• Daily Load: Total energy consumption from your load audit (Step 1)
• Autonomy Days: Required backup days without solar recharge (Step 2)
• DoD: Decimal depth of discharge (0.90 for LFP, 0.50 for lead-acid) (Step 3)
• Temperature Derating: Decimal capacity factor at operating temperature (Step 4)
• System Efficiency: Decimal overall system efficiency (Step 5)

Worked Sizing Examples

Example 1: Small Off-Grid Cabin (1-2 occupants)

Scenario: A 400 sq ft cabin used on weekends and occasionally for week-long stays. No air conditioning. Propane heating and cooking. A backup generator is present.

• Daily Load: 3.0 kWh (refrigerator, LED lights, water pump, device charging, small TV)
• Autonomy Days: 2
• Chemistry: LFP (LiFePO4)
• DoD: 0.90
• Temperature Derating: 0.90 (unconditioned shed, mild climate)
• System Efficiency: 0.85

Calculation: (3.0 × 2) / (0.90 × 0.90 × 0.85) = 6.0 / 0.6885 = 8.7 kWh nominal

Recommended System: One EG4 LifePower4 48V 100Ah battery (5.12 kWh) plus one EG4 14.3kWh WallMount battery, or two EG4 PowerPro wall-mount units totaling approximately 10-12 kWh. Alternatively, two Fortress Power eFlex 5.4 kWh batteries (10.8 kWh total) provide comfortable headroom.

Example 2: Average Full-Time Off-Grid Home (3-4 occupants)

Scenario: A 1,800 sq ft home occupied year-round. Wood/propane heating, efficient electric well pump, standard appliances, no central air conditioning. Located in a temperate climate with moderate winter solar resource.

• Daily Load: 8.0 kWh (refrigerator, freezer, well pump, LED lighting, washing machine, laptops, TV, ventilation fans, occasional microwave use)
• Autonomy Days: 3
• Chemistry: LFP (LiFePO4)
• DoD: 0.90
• Temperature Derating: 0.95 (conditioned garage, maintained above 10°C)
• System Efficiency: 0.85

Calculation: (8.0 × 3) / (0.90 × 0.95 × 0.85) = 24.0 / 0.7268 = 33.0 kWh nominal

Recommended System: A rack-mount bank of seven EG4 LifePower4 48V 100Ah batteries (5.12 kWh each = 35.8 kWh total), or three Fortress Power eFlex 5.4 kWh modules stacked with a Fortress Power Avalon HV Pro ESS (29.4 kWh + expansion). A Sol-Ark 15K or EG4 18kPV hybrid inverter pairs well with this bank size.

Example 3: Large Off-Grid Home (5+ occupants)

Scenario: A 3,500 sq ft home with a full complement of modern appliances, including an energy-efficient refrigerator and chest freezer, well pump, septic pump, electric dryer (occasional), LED lighting throughout, home office equipment, and a mini-split heat pump for supplemental heating/cooling. Located in a climate with cold winters and significant snow cover reducing winter solar production.

• Daily Load: 15.0 kWh
• Autonomy Days: 4
• Chemistry: LFP (LiFePO4)
• DoD: 0.85 (slightly conservative to extend cycle life for daily cycling)
• Temperature Derating: 0.90 (conditioned space, but winter dips below 10°C at times)
• System Efficiency: 0.85

Calculation: (15.0 × 4) / (0.85 × 0.90 × 0.85) = 60.0 / 0.6503 = 92.3 kWh nominal

Recommended System: A commercial-scale rack configuration with twelve to fourteen EG4 LL-S 48V 100Ah batteries (5.12 kWh each, ~66-72 kWh) supplemented by additional modules, or a BYD Battery-Box Premium HVS/HVM system scaled to 90-100 kWh. Pair with dual Sol-Ark 15K inverters or an Outback Power Radian system for the required surge and continuous power capacity.

Example 4: Same Home with Lead-Acid (For Comparison)

Using the same large home parameters but with flooded lead-acid at 50% DoD and 0.75 system efficiency:

Calculation: (15.0 × 4) / (0.50 × 0.90 × 0.75) = 60.0 / 0.3375 = 177.8 kWh nominal

This illustrates why lead-acid is rarely specified for full-time off-grid homes: you need nearly double the nominal capacity to deliver the same usable energy, resulting in a larger footprint, heavier weight, and ultimately higher cost when replacement cycles are factored in.

Sizing Worksheet Summary

Use this worksheet for every off-grid battery sizing project:

Parameter	Value	Notes
1. Daily Load (kWh)	_______	From load audit (Step 1)
2. Autonomy Days	_______	Based on climate and backup (Step 2)
3. Energy Required (kWh)	_______	Line 1 × Line 2
4. DoD (decimal)	_______	0.90 for LFP, 0.50 for lead-acid (Step 3)
5. Temperature Derating	_______	0.75–1.0 based on operating temp (Step 4)
6. System Efficiency	_______	0.85 for LFP, 0.75 for lead-acid (Step 5)
7. Combined Derating Factor	_______	Line 4 × Line 5 × Line 6
8. Required Battery Capacity (kWh)	_______	Line 3 ÷ Line 7
9. Battery Voltage	_______	48V for most off-grid; 51.2V for LFP systems
10. Required Ah at System Voltage	_______	(Line 8 × 1000) ÷ Line 9

Additional Sizing Considerations

Inverter Surge Capacity

Well pumps, air compressors, and power tools have surge currents 3-6 times their running wattage. Size the inverter (not the battery bank) for the largest combined surge. The battery bank must be capable of delivering this surge current without voltage sag that triggers a low-voltage disconnect. LFP batteries with integrated BMS typically handle 1C continuous and 2C peak discharge rates without issue.

Charge Controller Sizing

The solar array must be large enough to fully recharge the battery bank on a typical solar day, even during winter months with reduced sun hours. As a rule of thumb, size the array to deliver at least 1.2 times the daily load plus charging losses during the shortest solar day of the year. Use MPPT charge controllers rated for the array voltage and current, with sufficient headroom for cold-weather voltage spikes.

Future Expansion

Specify battery systems with modular expansion capability. LFP rack-mount systems (EG4 LifePower4, Pylontech US series, BYD Battery-Box) allow adding modules as loads grow. Communicate expansion plans to clients so conduit, racking, and charge controller capacity are specified with headroom from day one.

Products and Support

PES Supply offers battery storage solutions from 169 authorized brands with over 50,000 SKUs. Our inventory includes:

• Battery Storage Collection — Stackable residential ESS, rackmount commercial units, and LiFePO4 batteries from EG4, Fortress Power, Sol-Ark, Battle Born, Victron, BYD, and more
• Energy Storage Systems Collection — All-in-one hybrid inverters with integrated charge controllers from Sol-Ark, EG4, Outback Power, Schneider Electric, and MidNite Solar

All products ship with full manufacturer warranties. Standard delivery is 7-10 business days. Our technical support team can assist with system sizing verification, component compatibility, and commissioning support for off-grid projects of any scale.

Conclusion

Proper battery sizing is the difference between an off-grid system that performs reliably for a decade and one that frustrates the client from year one. The formula is straightforward: calculate daily loads accurately, select autonomy days appropriate to the climate and backup strategy, apply chemistry-specific DoD and temperature derating factors, and add system efficiency losses. The worked examples in this guide provide a framework, but every installation is unique. When in doubt, consult the battery manufacturer's sizing guidelines and work with a qualified system designer to verify your calculations.

Shop This Article

Ready to build your system? PES Supply stocks 50,000+ SKUs from 169 authorized brands, with delivery in 7-10 business days. Here are the products mentioned in this article:

• EG4 100Ah WallMount Battery Paralleling Cable Kit EG4PARACABLE100A
• BYD 144 TOPCon Cells Black/Silver 580w Solar Panel - NLBK-36 555-580W
• Fortress eFlex Max - 5.4kWh 51.2V Battery
• Pelio Cable Set - Wall Mount Kit for Up to 3 Pelio Units
• Sonnen ECOLX20SS-15 8.0KW Energy Storage System (20.0kwh Capacity)

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