Comprehensive Guide: Solar Power Budgeting and System Design + Step-by-Step Example

Posted on 07/01/2024 at 10:35 by Daniel Stieler, Phd


Originally published 4-6-18


Table of Contents

  1. Introduction
  2. Understanding Power Budget
  3. Daily Device Power Consumption
  4. Storage Element Self-Discharge
  5. Power Converter Efficiencies
  6. Available Illumination
  7. Charger Efficiency
  8. Tolerance to Downtime/Factor of Safety
  9. Example Application: Solar-Powered Medical Freezer
    1. Calculating Power Requirements
    2. Charge Controller Selection
    3. Battery Selection and Sizing
    4. Example: Trojan SCS225 Battery
    5. Determining Solar Panel Size
  10. Conclusion
  11. Next Steps


Imagine creating a device that never needs to be plugged in - harnessing the sun's power to run indefinitely, anywhere. 


It's possible with the right approach to solar power budgeting and system design. Whether you're developing a remote weather station, a solar-powered medical freezer, or a groundbreaking IoT device, understanding how to budget and design a solar power system properly is crucial. 


In this guide, we'll walk through the essential components of solar power planning, from calculating power consumption to selecting the correct battery and solar panel size. We'll even provide a real-world example to illustrate these principles. 


Ready to unlock the potential of solar power for your next project?


Let's dive in.


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Understanding Power Budget


Person holding a calculator at a desk with a laptop and notebook


A power budget consists of the following:


Daily device power consumption
Storage element self-discharge
Power converter efficiencies 
Available illumination 
Charger efficiency
Tolerance to downtime/factor of safety


Let’s explore each of these components in detail.


Daily Device Power Consumption

 woman analyzing a battery

First, we must determine a device's power consumption to quantify its required power.


Pro Tip: A straightforward way to determine power usage is to power the device with a full storage element (battery or capacitor) of known capacity (Watt-hours) and determine the length of time that the system can operate from that storage element. 


We will convert device power consumption to Watt-hours per day since that coincides with typical solar illumination cycles. 


To determine the number of Watt-Hours per day of power required to operate the device, use the following equation: 

watt hours of power to operate a given device formula

Storage Element Self-Discharge

A series of batteries at various states of discharge

In addition to the power consumed by your device, there will also be some parasitic discharge from the storage element. This discharge is based on the type and capacity of the storage element. 


Important: Self-discharge identification is essential for low-power draw applications and applications where a device may be dormant for extended periods.


Most self-discharge is a percentage of the capacity per day. Discharge rates also strongly depend on ambient temperature and will increase with temperature. 


Below is a table of discharge rates for common types of storage elements. 


Note: Even within a given battery chemistry, self-discharge rates can vary between manufacturers, so it is best to consult your battery manufacturer or test the battery to confirm discharge rates. Depending on the application, estimates of self-discharge may or may not be significant. 


self discharge rate by battery chemistry chart

Power Converter Efficiencies

Illustration of a woman with a lightbulb analyzing energy consumption and creation

When a device requires an input voltage that must be significantly different than the storage element’s voltage or stable over the entire range of a storage element, a power converter must be placed between the storage element and the device. 


Example: If a device operates on 5V but is connected to a 12V battery, then a 12V to 5V buck converter must be placed between the battery and the device. 


Converter efficiency can be measured by operating the system and measuring the voltage and current at the converter's input and output. 


To calculate the converter efficiency, use the following equation:


converter efficiency formula


The input is power supplied by the solar panel or storage element, and the output is the power consumed by the device to be powered.


Accounting for Converter Efficiency


Now that the converter efficiency is known, it needs to be considered in the daily device power consumption calculation. 


Use the following equation to account for converter efficiency in your device power consumption if the original measurement was not made with the converter included:

Adjusted watt hours formula accounting for converter efficiency

Available Illumination


Unfortunately, outdoor light is rarely consistent. However, the sun rises and sets daily, providing a cadence for studying the power consumption of outdoor devices. The same is valid for indoor devices. Often, the opening or closing of a business or other daily routine sets a schedule for available illumination. 


Factors Affecting Outdoor Light


Illustration of the sun behind clouds


Outdoor light varies widely based on several factors:


Cloud cover
Upper atmosphere particulates (often caused by forest fires) 
Reflection from ground cover (such as snow)
Solar panel orientation (facing south vs. north)
The location of the sun in the sky (time of day/year)


This makes for a very dynamic amount of available power and a complex system to design. All of these factors will shift both the solar spectrum and the intensity. 


Pro Tip: Regional average solar illumination data can be found using NREL’s PV Watts calculator at This calculator is an excellent tool for estimating illumination where you plan to use your solar product if it will be deployed outdoors in a non-shaded area. 


Designing for Year-Round Operation


Timeline of the months in a year


When sizing a system for continuous year-round operation, use the information for the worst-case month of the year. 


Indoor Deployment Considerations


If the product is deployed indoors or in a shaded area, a more in-depth analysis of hours of illumination and intensity will be required. The available light intensity can depend on factors like distance from the light source, angle to the light source, and potential for intermittent shadowing.


Note: Solar power generation is sometimes simplified when the product is deployed indoors due to the indoor environment's daily cadence. Indoors can be a much more reliable environment to operate in than outdoors.


Charger Efficiency


Hand turning knob with efficiency scale from black red green


A charge controller is often necessary for prolonged battery life and safety. However, it is optional in specific cases. 


Exception: A charge controller will not be needed when charging a lead-acid battery if the charge current in amps is less than 1/100th the battery's amp-hour capacity. 


Impact of System Efficiency


Adding a charge controller to the system will introduce another parasitic loss. Charge controller efficiencies range from 60% to 95%. 


Important: The panel size must be increased to account for this efficiency loss. The following section will discuss how to calculate the size of a panel.


Tolerance to Down Time/Factor of Safety


The available illumination, especially in outdoor applications, assumes the panel will see the average solar radiation daily to maintain operation without disruption. However, realistic world conditions are often less predictable.


Accounting for Weather Variability


Illustrated versions of various types of weather


Realistically, there will be overcast and sunny days, so a battery storage system must be large enough to provide power during extended periods of low solar radiation. 


To account for this variability:

Calculate your average daily power consumption
Multiply it by the desired number of run days between charges
Choose a storage element with a total watt-hour capacity close to that result

Sizing the Solar Panel


7-60W Rollable Solar Panels


The panel must be large enough to recharge the storage system in a reasonable amount of time during good solar radiation. 


To effectively charge the storage element, the solar panel must produce enough power to overcome:


Battery self-discharge
Daily power consumption
An additional amount based on the time required to charge a fully depleted system


Use the following calculation to determine the required panel size:


solar panel sizing formula


Key Takeaway: Developing and successfully executing a power budget requires attention to detail and extensive planning and execution. 


Example Application: Solar-Powered Medical Freezer


 Light blue freezer with a white snowflake in the middle


Now let’s apply what we learned to an example: using solar energy to power an Engel MHD13F-DM freezer and keep medical supplies frozen in a remote area of southern Africa. 


Calculating Power Requirements


According to the performance data: 

At 25°C ambient temperature (setpoint -5°C): 1.6 Ahr/hour at 12V
At 30°C ambient temperature: 2.5 Ahr/hour at 12V
Operating time: 24 hours per day


To calculate the daily power draw, use this formula:


daily power draw formula


Note: Since the freezer operates off a 12V battery system, we don't need to worry about power converter efficiency in this case.


Charge Controller Selection 

Illustration of a charge controller

We’ll choose a Galley Power GP2 series charge controller programmed to charge the AGM-type lead-acid battery. 


Key features:


Maximum power point tracking algorithm with 99.7% efficiency
Less than 4% loss in power conversion from solar to battery
Overall conversion efficiency: about 96%


To account for the efficiency loss in the charge controller, use the following formula:


efficiency loss in charge controller formula


So, depending on the ambient temperature, the system will require 480Whrs to 750Whrs of energy produced by the solar panel daily.


Estimating Solar Availability


Zimbabwe on a map


Using the PVWatts calculator from NREL for Zimbabwe:

Assume the solar panel is deployed flat on the ground (0° tilt angle)
June (least solar radiation): 4.45 kWh/m²/day
October (most solar radiation): 6.53 kWh/m²/day


Temperature ranges:

June: 11°C to 22°C
October: 15°C to 29°C


Important: The power draw must be quantified to optimize the system, as the freezer manufacturer's data is for consistent 25°C or 30°C ambient temperatures.


Zimbabwe solar radiation by month graph


Zimbabwe average temperature chart


The report given by the PVWatts calculator shows that June has the least solar radiation, with an average of 4.45kWh/m2/day. At this time of year, the maximum temperature is about 22C, with the minimum around 11C. 


October has the most solar radiation, averaging 6.53kW/m2/day. At this time of year, the maximum temperature is about 29C, and the minimum is 15C. 


The units for illumination are kW/m2/day, and the standard illumination for solar panels is 1kW/m2. In this case, a panel will receive an average of 4.45 hours of illumination per day in June and 6.53 hours in October. 


The information from the freezer manufacturing was for running at a consistent 25C or 30C ambient temperature, so the actual draw will need to be quantified to optimize the system.  


Battery Selection and Sizing


Illustration of two men analyzing battery capacity


For this application, we use a lead-acid battery as the storage element since the freezer is specially designed to pair with this type of battery. 


Battery Self-Discharge Consideration


Lead-acid battery self-discharge rate: 0.1-0.65% per day
Desired reserve battery power: approximately two days


Calculating Battery Size


Series of batteries getting incrementally larger


To calculate the minimum battery size, use the following formula:


battery size formula


We used the 750Whrs capacity in the calculation since that is the worst case. 


Note: We can neglect the self-discharge rate in this calculation as it's very low, and we're not relying on the battery to hold a charge for an extended time.


Converting to Amp-Hours


To determine the battery capacity in amp-hours:


Minimum Battery Capacity = 1500Wh / 12V = 125 


Important: The actual battery capacity will vary depending on the current draw. As the current draw increases, battery capacity will decrease.


Example: Trojan SCS225 Battery


Illustration of a blue car battery with a yellow lightning bolt in the middle


20-hour rate capacity: 130 Ah
5-hour rate discharge: 105 Ah


For example, a Trojan SCS225 specifies a 20-hour rate capacity of 130Ahr and a 5-hour rate discharge at 105Ahr. 


The discharge rate at each capacity can be calculated as follows:


battery discharge rate formula


Since the maximum current draw for this application is <3A, the battery capacity will be at least 130Ahr. 


Pro Tip: Battery discharge testing could determine a more accurate capacity.


Determining Solar Panel Size


We want the battery to fully charge with the solar panel in two days while operating the freezer. To calculate the solar panel size, use this formula:

solar panel sizing formula

Using our worst-case scenario:

Solar panel wattage formula to power freezer with ambient temp at 30C

Using a better-case scenario:

Solar panel wattage formula to power freezer with ambient temp at 25C


This shows that the panel size would need to be at least 268W, with fewer hours of illumination and temperature being the limiting situation. 


Real-World Considerations


The ambient temperature drops much lower for most of the day than the cooler operating temperature of 25°C or 30°C
Current consumption vs. ambient temperature drops quickly


Key Takeaway: Testing is always essential for system optimization. A PowerFilm standard 220W panel could easily support this freezer system based on the daily temperature cycle.




You can break solar power budgeting and design into manageable steps. By considering factors like power consumption, storage, efficiencies, and illumination, you can create reliable solar-powered systems for various applications.


Next Steps


Apply these principles to your next project. Start by assessing your power needs and local solar conditions, then experiment with different component combinations. Remember, optimization often comes through testing and refining your design.


Need help bringing your solar-powered vision to life? Our team of experts is here to assist you from the initial concept to the final implementation. Don't let technical challenges prevent you from harnessing solar energy's full potential.


Contact us today, and let's work together to create innovative, sustainable solutions that push the boundaries of what's possible with solar power.


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