What size solar system do I need for a shed?

Determining the right size solar system for a shed depends on several factors, including the size of the shed, your energy needs, solar resource in your area, and your budget. With some basic information about your shed and energy use, it’s easy to estimate the solar array size that will meet your needs.

How Much Power Does My Shed Need?

The first step is calculating your power needs. Consider what you’ll use electricity for and how much power those items require. Some common uses and power needs in a shed include:

• Lighting – 60-100 watts per standard light bulb
• Power tools – vary greatly but can range from 300-1500 watts
• Electric heater – typically 1500-2000 watts
• Mini fridge – 50-100 watts
• Radio/stereo – 50-100 watts
• Security cameras and devices – 20-100 watts
• Phone chargers – 5-10 watts

Add up the wattage of the items you plan to run in the shed. This will give you an idea of your power needs. Remember to factor in that some devices use more power at startup than when running continuously.

Average Solar Hours Per Day for Your Location

The amount of sun your location receives impacts solar system sizing. Areas with more annual sunlight can utilize smaller solar arrays compared to less sunny regions. Check solar insolation maps to find the average peak sun hours for your location.

Most of the United States ranges from 4-6 peak sunlight hours per day. Use this number when doing solar size calculations.

Sizing Your Solar Array

With your power needs and sunlight hours, you can now estimate the solar array size you need:

1. Calculate your daily power usage

• Add up the wattages of items you plan to run
• Multiply each item’s wattage by the estimated hours per day used
• Add up the totals for your estimated daily Wh (watt hours) usage

2. Factor in system inefficiencies

• Multiply your daily Wh number by 1.3 to account for system inefficiencies and losses

3. Divide by your peak sun hours

• Take the Wh per day number and divide it by your location’s peak sun hours.

This gives you the minimum recommended solar array wattage size to meet your energy needs.

Solar Array Sizing Example

Here is an example solar sizing calculation for a shed:

• 5 x 60W lights x 3 hours/day = 900 Wh
• 1500W heater x 2 hours/day = 3000 Wh
• 100W mini fridge x 5 hours/day = 500 Wh
• Total Wh/day = 900 + 3000 + 500 = 4400 Wh/day

• 4400 Wh/day x 1.3 = 5720 Wh/day (adjusted for system inefficiencies)

• 5720 Wh/day / 5 sun hours = 1144 watts (minimum solar array size)

For a shed with this usage, a solar array of at least 1,144 watts is recommended. Round up to allow for future energy increases.

Sizing the Inverter

Solar panels produce direct current (DC) electricity. To use this electricity, it must be converted into alternating current (AC) with an inverter. Size your inverter based on the total wattage of your solar array.

To choose your inverter:

• Add up the max wattages of your solar panels
• Multiply by 1.2 or 1.3 to allow a buffer
• Choose an inverter that can support the calculated wattages

Oversizing your inverter slightly prevents overworking it and allows for potential panel upgrades. Many solar installations use a 3,000-5,000W inverter.

Sizing the Battery Bank

For sheds running fully on solar power, batteries are needed to store energy for use at night or on cloudy days. Larger battery banks provide more stored energy.

To size your battery bank:

• Take your adjusted Wh/day usage calculated above
• Multiply by the number of days of autonomy needed
• Divide by the depth of discharge and system voltage

Typical factors for shed solar systems:

• 2-5 days of autonomy
• 50% depth of discharge
• 12V or 24V system voltage

Using the example above, for a 5720 Wh/day usage, 3 days autonomy, 50% discharge depth and 24V system, the calculated battery capacity would be:

• 5720 Wh/day x 3 days = 17,160 Wh storage needed
• 17,160 Wh / 0.5 DoD = 34,320 Wh
• 34,320 Wh / 24V = 1430 Ah battery bank

Given the parameters, a battery bank of around 1430 Ah at 24V would be recommended. You can achieve this using multiple smaller batteries wired in parallel.

Key Factors That Impact Solar System Sizing

When determining solar system size, keep the following factors in mind:

• Energy usage – The more power you use, the larger the system needs to be.
• Sunlight – Less sun means more panels are needed.
• Panel wattage – Higher watt panels require fewer panels for the same output.
• Panel efficiency – High efficiency panels produce more electricity.
• Autonomy time – More days of backup battery storage requires a bigger battery bank.
• Efficiency losses – Allow extra capacity for system inefficiencies.

Estimating Your Shed’s Solar Array Size

Use the worksheet below to estimate the size of solar system you need. Fill in your own usage, sun hours and days of autonomy.

Appliance	Wattage	Hours/day	Wh/day
Lights
Tools
Heater
Refrigerator
Other
Total

Total Wh/day: _____ x 1.3 = _____ Adjusted Wh/day

Adjusted Wh/day: _____ / _____ Sun hours per day = _____ Minimum solar array size

Battery capacity: Adjusted Wh/day x _____ Days autonomy / 0.5 DoD / _____ System voltage = _____ Battery capacity

Expanding Your Solar System

One benefit of solar power is that it’s modular and expandable over time. You can begin with the minimum system size needed for your current use and add more panels in the future as your power demands grow.

When installing your original solar array, leave room to add more panels. Plan ahead for panel locations that will be optimally oriented and angled as your system expands.

Additional solar panels, batteries, and a larger inverter can be added down the road. New components are connected into your existing system, so expanding is a straightforward process.

Getting Help Designing Your Shed Solar System

While it’s possible to estimate your solar system size yourself, it’s recommended to have a qualified solar installer or designer help plan your system. They will consider details and regulations you may miss and design a robust system sized just right for your energy usage and location.

Be sure to get multiple quotes from solar providers. Compare costs, experience, equipment and warranties. DIY shed solar kits are also available if you’re comfortable self-installing.

Investing in a properly-sized solar system for your shed provides clean, renewable power for years to come. Accurately calculating your needs using sun hours, power usage and days of autonomy ensures your solar system will be ready to take your shed off-grid.