In a recent webinar Elise Murphy, a system designer at Washington contractor Fire Mountain Solar, and Mara White, product manager for Outback Power, discussed tips for designing a solar storage system. Here’s a recap. You can watch the full webinar here.
Training
Before you even start the installation, it’s important to have proper training. There are quite a few good resources for information about installing solar and battery systems. For example, many manufacturers have product-specific training to learn about the components on the market and how they’re used in applications. NABCEP is also a good resource to find training opportunities, as well as obtain industry-recognized certifications. Also, Solar Energy International has both online and hands on training. Beyond formal training, it’s also a good idea to get advice and learn from professionals who are already in the industry.
Size the system
Caption: You can see in the pie chart some of the common household loads.
Once you have proper training you are ready to design and size systems.
Calculating loads is one of the first steps when working with a customer to size a system. A customer must decide what loads need to be backed up during a power outage. For example, a cooling system is not likely to be backed up but it’s common for customers to want to back up refrigerators, freezers and some lights.
Oversizing the system is especially important for battery-based systems, especially when designing for off-grid applications. There are always loads not anticipated, and factors not considered. It’s best to design for the worst case scenario (most power possibly needed) and what the budget can handle. This involves days of autonomy, meaning how many days worth of loads you want to be able to draw from your battery bank without charging them.
A detailed load analysis is critical to make sure the battery bank will be big enough to run all the loads. Load analysis involves determining what the loads are, how many there are, the wattage of each load and for how long each day each load will be used. This allows the designer to calculate the total ac Watts the system needs to provide, as well as the average Watt hours used each day. Customers are always surprised how detailed these calculations need to be, but if these calculations aren’t highly evaluated, you might end up oversizing or under sizing the system. A grid-tied battery back-up system will involve similar measurements for load analysis, but only includes loads you wish to back up, and not all the loads used on the grid.
Choosing components
Once the system size is determined, the next step is to select components. It’s important to choose quality components that will last. That doesn’t mean that more expensive products are always better, but the products should have good track records of reliability. Choose brands with good tech service, companies that will back you up when you run into problems down the road.
Caption: There are several different battery technologies common for use in renewable energy systems. These are the ones we’re finding primarily in the U.S.: lead acid is the most common on the market and it comes in both flooded and sealed, which is maintenance free.
Batteries
It is very important to choose the right battery technology and chemistry for your project. There are a variety of battery sizes and form factors (the physical shape of the battery and its terminal). One thing to consider is how batteries and inverters are installed—how are they strung together and how safety over-current devices are integrated. It’s important to consider the size, amp hours and the capacity of the batteries as well as how the components are integrated and installed.
One of the things to keep in mind is whether you need a deep-cycle or a float-use battery. Deep-cycle batteries will cycle frequently, even on a weekly or daily basis. They’re mostly used in self-consumption types of applications, where energy arbitrage or solar-generated electricity is used on a nightly basis. Float-use batteries are typically used more infrequently and sit “topped off.” These are more commonly used in grid connected power supplies for backup during infrequent power outages. Lithium-ion’s price is decreasing tremendously, also making it an option for renewable energy systems. These batteries can be completely discharged or left in the discharge state without damaging the battery.
Inverters
A key thing to look for in a solar inverter with energy storage is the ability to operate both on and off-grid seamlessly. This can be referred to as grid-hybrid.
Grid-hybrid inverters use an automatic transfer switch (ATS) for a seamless transitions. This allows the inverter to work with or without the grid and it won’t let the home owner actually know what’s powering the loads, meaning that no lights are going to flicker.
It’s also important that inverters are designed for grid-tied energy storage to ensure that the batteries are full, loads are supported and any excess power is able to be sold to the grid.
Programmability is also an important consideration for inverters. The inverter must be able to be reprogram on a daily bases for time of use. The inverter must also be able to be programmed to connect and disconnect from the grid on a schedule. For example, if a utility won’t allow net metering to occur during the day (say between 11a.m. and 2p.m.) the customer will still want to take advantage of solar production, even if they can’t sell it back. You will need to use power from the battery at that time of day instead, and take advantage of net metering outside of that block or period. Being able to draw from the utility if the batteries get too low or choosing how the loads are powered by the time of day are some of the key considerations when selecting a solar inverter for battery systems.
Develop site plan and layout
After selecting components, the last step is to develop a site plan and layout that tells where they components are going to fit. Make sure the system is code-compliant, which involves checking local building and electrical codes, as well as labor and industry requirements. You will also need to develop line diagrams and wiring plans of the proposed system.
These simple guidelines are a good start to thinking about the process of designing a solar installation with batteries. We highly recommend working with a professional in the industry to design the system or getting detailed training about the intricate steps of a battery based system. Having a battery based system whether it is off grid or grid hybrid can be very rewarding and allows you to have power even when the grid isn’t available.
