This is a technical guide for those with a basic understanding of solar and off-grid inverters. For less technical information, see the basic guide to selecting a home grid-tie or off-grid solar battery system. Solar and battery storage systems should always be installed by a licensed electrical professional.
Basic Steps to Designing An off-grid Solar System
Before purchasing any equipment required for a solar battery (hybrid) or off-grid power system, it is very important to understand the basics of designing and sizing energy storage systems. As explained below, the first part of the process is to use a load table or load calculator to estimate the amount of energy needed to be generated and stored daily. If you cannot develop a load table, a professional solar installer or system designer should be consulted.
Step 1 – Estimate the loads
The most important part of designing any off-grid solar or battery system is calculating how much energy is required per day in kWh. For grid-connected sites, detailed load data can often be obtained directly from your electricity retailer or by using meters to measure the loads directly. For off-grid or stand-alone power systems, always start by using an off-grid load calculator (load table) for summer and winter. The load table can also be used to estimate surge loads, power factors, and the maximum demand required to size an appropriate off-grid inverter.
Step 2 – Battery sizing
Battery capacity is measured in Ah (Amp-hours) or Wh (Watt-hours). Lead-acid (deep-cycle) batteries are sized in Ah, while lithium battery capacity is generally measured in kWh (kilowatt-hours). After using a load calculator to estimate the average daily loads in kWh, you need to determine the number of days of autonomy (continuous days without sunshine) you require the battery to last. Typically, two days is the minimum for lithium battery systems, while the less efficient lead-acid batteries are generally sized for three or more days.
Additionally, all loss factors must be considered to ensure the battery size is adequate to meet the loads, including inverter losses, temperature derating for lead-acid batteries, maximum allowable depth of discharge (DoD) and round-trip efficiency. Also, consider battery type and chemistry, battery voltage range, and maximum battery charge rate (C rating), as explained in Section 6 – Battery Selection and Sizing.
Step 3 – Solar array sizing
A correctly sized solar array is required to charge the battery while also supplying the loads. As explained in more detail below, you must ensure the solar array is large enough while taking into account local conditions, including average solar irradiance throughout the year (peak sun hours), shading issues, panel orientation and tilt angle, cable losses, and temperature derating (loss factors). Our Photonik solar calculator can help estimate solar generation throughout the year, depending on the panel orientation, location and shading losses.
Step 4 – Inverter selection
After steps 1 to 3 have been established, you can select a suitable solar inverter or MPPT Solar Charge Controller to match the solar array depending on the panel and string length, which will determine the string voltage. Always use a string voltage calculator to calculate the maximum and minimum string voltages to ensure the voltage does not exceed the input rating. Next, the primary hybrid or off-grid inverter must be selected to meet the continuous and surge loads, taking into account temperature derating and other loss factors explained in more detail below.
Step 5 – Backup generation source
After your solar system is sized correctly and you have estimated a suitable battery capacity, you need to consider a backup generation source such as a diesel generator, especially if you live in a temperate (colder) location. Even though a backup generator may only be needed for occasional use in winter, it needs to be sized correctly to power the loads and recharge the battery simultaneously, as explained in section 8. However, emerging technologies such as vehicle-to-load (V2L) can offset generator runtime in certain situations, as explained in our guide to using V2L in off-grid systems.
