Solar Panel & Battery Storage Sizing Calculator

Design off-grid and backup power systems. Compute required PV array wattage, battery bank storage (Ah & kWh), MPPT charge controller amperage, and days of autonomy.

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Select a reference installation or enter custom metrics below.
kWh / day
Total 24-hour electrical demand (6.0 kWh = 6,000 Watt-hours).
Hours
Average daily solar generation window (US Sunbelt: ~5h, Northern: ~3.5h).
Number of days the battery bank must power loads without solar input.
Governs safe depth of discharge and round-trip efficiency.
Higher voltage drastically reduces wiring current and wire gauge thickness.
Nameplate wattage per solar module.
%
Standard solar derate: 18–22% (heat, dust, wiring, MPPT conversion).

Off-Grid Sizing & Equipment Specifications

Solar PV Array Size 1,667 W 5 × 400W Panels (2,000W Actual)
Total Battery Capacity 14.86 kWh 619 Ah @ 24V DC
MPPT Charge Controller 87 A Use 100A 150V MPPT Controller
Reserve Autonomy 2 Days 12.0 kWh usable energy reserve

System Engineering Bill of Materials & Parameters

Subsystem Component Calculated Requirement Engineering Rationale & Code Rules
Daily Solar Energy Harvest 7.20 kWh / day Accounts for 20% system derating across 4.5 PSH
Solar Panels Needed 5 Panels (400W ea) Total installed capacity: 2,000 Watts peak (STC)
Battery Usable Storage 12.00 kWh Pure consumable energy across 2 days of cloud cover
Battery Bank Gross Size 14.86 kWh (619 Ah) Protects cells by maintaining 85% max depth of discharge
MPPT Controller Rating 87 Amperes (NEC 1.25x) $I = (P_{text{array}} / V_{text{batt}}) times 1.25$ overcurrent rule
Recommended Inverter 3,000W Pure Sine Wave Supports surge startup loads at selected system voltage
Quick Overview & Direct Answer

Solar Panel & Battery Storage Sizing Calculator: Free Solar Panel & Battery Storage Sizing Calculator tool: size off-grid and grid-tied solar photovoltaic arrays, battery storage banks (Ah & kWh), MPPT.

Primary Use Solar panel wattage estimation, battery bank sizing, energy payback time
Input Formats Daily kWh usage, sun hours, panel wattage rating
Output Format System size in kW, recommended battery capacity, annual output
Processing Mode 100% Client-Side Solar Modeling
Access Tier Free, unlimited, no sign-up needed

The Science of Off-Grid Solar Photovoltaic & Storage Engineering

Designing an off-grid or hybrid energy storage system (ESS) requires precise mathematical balance between two independent energy equations: generation capacity (harvesting sufficient photons during sunlit hours to power daytime loads and replenish depleted cells) and storage autonomy (buffering enough chemical energy to sustain continuous appliance operation during nights, storms, and prolonged winter overcast).

An undersized solar array causes progressive battery deficit cycling, leading to irreversible lead sulfate crystallization or lithium low-voltage lockout. An undersized battery bank forces premature reliance on noisy fossil fuel generators. Understanding how Peak Sun Hours (PSH), Depth of Discharge (DoD), and system derating interact ensures predictable year-round power independence.

The Mathematical Formulas for PV Array & Battery Sizing

System sizing begins by determining gross daily consumption with loss factors:

Required Solar Array Wattage ($P_{text{array}}$):

$P_{text{array}} = frac{text{Daily Consumption (Wh)}}{text{Peak Sun Hours} times (1 - text{System Loss Factor})}$

Panel Count: $text{Number of Panels} = leftlceil frac{P_{text{array}}}{text{Individual Panel Wattage}} rightrceil$

Gross Battery Bank Capacity (Wh):

$text{Bank Capacity (Wh)} = frac{text{Daily Consumption (Wh)} times text{Days of Autonomy}}{text{Depth of Discharge} times text{Battery Efficiency}}$

Battery Bank Capacity (Amp-hours): $text{Bank (Ah)} = frac{text{Bank Capacity (Wh)}}{text{System Voltage (V)}}$

MPPT Charge Controller Amperage: $text{Controller Current (A)} = frac{P_{text{array}}}{text{System Voltage}} times 1.25text{ (NEC Safety)}$

Step-by-Step Worked Numerical Example

Consider an off-grid mountain cabin with typical modern conveniences (refrigerator, Starlink satellite terminal, LED lighting, laptops, and water pressure pump):

  • Daily Consumption: 6.0 kWh/day = 6,000 Watt-hours/day
  • Regional Insolation: 4.5 Peak Sun Hours (PSH)
  • Desired Autonomy: 2 days (48 hours of cloudy storm reserve)
  • Battery Chemistry: LiFePO4 (85% DoD, 95% round-trip efficiency)
  • System DC Voltage: 24 Volts
  • System Losses: 20% ($0.20$ derate, giving 80% net harvest efficiency)
  • Panel Specification: 400 Watt monocrystalline modules

Step 1: Calculate Required Solar PV Array:

$P_{text{array}} = frac{6,000text{ Wh}}{4.5text{ PSH} times 0.80} = frac{6,000}{3.6} approx mathbf{1,666.67text{ Watts}}$

$text{Number of Panels} = frac{1,666.67text{ W}}{400text{ W}} = 4.17 to mathbf{5text{ Panels}}; (mathbf{2,000text{ Watts actual installed}})$

Step 2: Calculate Battery Bank Capacity:

$text{Usable Energy Needed} = 6,000text{ Wh} times 2text{ days} = mathbf{12,000text{ Wh}}; (12.0text{ kWh})$

$text{Gross Battery Bank (Wh)} = frac{12,000text{ Wh}}{0.85text{ DoD} times 0.95text{ Eff}} = frac{12,000}{0.8075} approx mathbf{14,860.68text{ Wh}}; (mathbf{14.86text{ kWh}})$

$text{Battery Capacity (Ah at 24V)} = frac{14,860.68text{ Wh}}{24text{ V}} approx mathbf{619.2text{ Ah}}$

Step 3: Size the MPPT Solar Charge Controller:

Continuous Current = $frac{2,000text{ W installed}}{24text{ V}} = 83.33text{ Amps}$

NEC 1.25x Overcurrent Factor = $83.33 times 1.25 = mathbf{104.16text{ Amps}}$

Recommendation: Deploy a 100A MPPT controller or split into two 60A MPPT controllers on dual solar strings.

Why System Voltage Matters: 12V vs 24V vs 48V

A common novice mistake in solar design is attempting to power a large off-grid home using a 12V battery system. By Ohm's law ($P = V times I$), delivering 3,000 Watts at 12 Volts requires 250 Amperes of continuous current, necessitating expensive, thick 4/0 AWG welding cables to avoid dangerous voltage drop and fire hazard.

Stepping up to a 48V system reduces current by 75% to just 62.5 Amps for the exact same 3,000W load. This dramatically cuts copper cable costs, reduces heat dissipation, and unlocks compatibility with modern commercial hybrid inverters (such as Victron, Sol-Ark, and EG4).

Frequently Asked Questions

A Peak Sun Hour (PSH) is not simply an hour of daylight; it represents the equivalent amount of solar insolation that produces an irradiance of 1,000 Watts per square meter (1 kW/m²) for one hour. While a location may experience 12 hours of total daylight, solar angle and atmospheric scattering typically produce between 3.0 and 6.0 Peak Sun Hours of full-rated panel generation per day.

Required battery bank capacity in Watt-hours equals (Daily Consumption in Wh × Days of Autonomy) / (Depth of Discharge × Battery Efficiency). For a 48V system consuming 5,000 Wh/day with 2 days of autonomy using LiFePO4 (85% DoD, 95% efficiency): Required Bank = (5,000 × 2) / (0.85 × 0.95) = 12,384 Wh (12.38 kWh), which corresponds to 258 Amp-hours at 48 Volts.

A PWM (Pulse Width Modulation) controller mechanically clamps the solar panel's voltage down to match the battery voltage, discarding up to 30% of potential solar energy. An MPPT (Maximum Power Point Tracking) controller functions as a high-efficiency DC-to-DC buck converter, dynamically tracking the optimal voltage/current curve (Vmp/Imp) to deliver 20% to 35% more charging energy into the battery bank.

Under National Electrical Code (NEC 690.8) regulations, photovoltaic systems can produce current exceeding standard test conditions (STC) during cold, crisp winter days with intense cloud-edge lensing reflections. Sizing charge controllers and circuit breakers with a minimum 1.25x continuous duty safety factor prevents thermal tripping and equipment fires.

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