Off Grid Solar System Sizing Calculator

The amount of battery storage you need is based on your energy usage, measured in kilowatt-hours (kWh) over time.

Example: 1,000 watts x 10 hours per day = 10 kWh per day

Sizing Your Battery Bank

The exact math for sizing your battery system is based on daily power usage and battery type. For 10 kWh per day, here are some examples:

Lead Acid Sizing

10 kWh x 2 (for 50% depth of discharge) x 1.2 (inefficiency factor) = 24 kWh

Lithium Sizing

10 kWh x 1.2 (for 80% depth of discharge) x 1.05 (inefficiency factor) = 12.6 kWh

Battery capacity is specified in kWh or amp hours.

Example: 24 kWh = 500 amp hours at 48 volts → 500 Ah x 48V = 24 kWh

Consider rounding up to cover inverter inefficiencies, voltage drop, and other losses. Based on this example, you may want 600-800 amp hours of capacity, depending on your needs.

How To Calculate Solar Battery Bank Size

Our calculator helps you find the ideal battery bank size, watts per panel, and charge controller. When building an off-grid system, size it based on the month with the least sunlight.

Step 1: Determine Your Daily Energy Usage

Use your electric bill to find monthly kWh usage, then divide by 30 to get daily usage in watt-hours.

Step 2: Estimate Days Without Sun

Find the average number of cloudy days per year in your area. This will help you size the bank adequately.

Step 3: Estimate Lowest Temperature for Battery Bank

Check your area's average low temperatures to determine battery capacity accurately.

How to size an off-grid solar system

Calculate Energy Usage:

Assess the total daily energy consumption in watt-hours (Wh) or kilowatt-hours (kWh) based on the electrical loads in your home. This can include lighting, appliances, electronics, and other devices that will rely on the solar system.

For off-grid systems, precise load calculation is crucial as there is no grid backup. Review past energy bills if available or use load estimation methods for new installations​.

Account for Seasonal Variations:

Consider the seasonal sunlight variations in your location. Winter months generally produce less solar power due to shorter days and lower solar irradiance. Using resources like PVWatts or local solar insolation data can help estimate monthly production changes​​.

Determine Solar Array Size:

Divide your daily energy needs by the average daily sun hours to estimate the size of the solar array. For instance, if you need 5 kWh daily and receive 4 peak sun hours, the array size would be 5,000 Wh / 4 hours = 1,250 W of panels.

Adjust for inefficiencies (e.g., losses from inverters, shading, wiring) by increasing the array size by around 10-20%​​.

Battery Storage Sizing:

Calculate battery capacity to provide sufficient backup during cloudy days or periods of high usage. Most off-grid systems aim for 2-3 days of autonomy (storage for cloudy days).

Battery capacity can be estimated by multiplying daily energy usage by the number of days of autonomy needed. Then, adjust for the battery’s depth of discharge (DoD). For example, if you need 10 kWh/day and want two days of autonomy, with a lead-acid battery DoD of 50%, you'd need around 40 kWh (10 kWh x 2 days ÷ 0.5 DoD).

Consider lithium batteries for better efficiency and deeper discharges if budget allows​​.

Inverter Sizing:

Choose an inverter that can handle the peak load (maximum wattage drawn at one time) and has a sufficient surge capacity for devices that need higher startup power.

Off-grid inverters should be robust and suited to manage varying load demands​​.

Charge Controller Sizing:

Select a charge controller with a current rating that matches the output of the solar array to ensure safe charging of the battery bank. MPPT (Maximum Power Point Tracking) controllers are more efficient than PWM (Pulse Width Modulation) and help optimize power extraction, especially in low-light conditions​.

Include a Safety Margin:

Account for power losses from factors such as heat, dust on panels, wiring resistance, and component inefficiencies. Adding a 10-25% safety margin in your calculations can help ensure consistent power availability.

Design for Expandability:

Consider future energy needs or potential upgrades. This could mean opting for a larger inverter or modular battery storage that can be expanded as demand grows​​.

These steps provide a foundational approach for designing a reliable off-grid solar system that aligns with both energy needs and environmental conditions. For a precise design, consult with a solar professional or use specialized solar design software.