Off-Grid System Sizing



Written by Reme Meck

One of the biggest challenges in modern solar system design is how to properly design a stand-alone solar system. Unlike a grid-tied system, a solar system with batteries consists of three separate systems all working together to create, store, and deliver energy for your electric loads. You need to ensure each component is properly sized for your specific energy needs and environmental conditions. This will explore sizing a stand-alone solar system based on an average daily load in watt hours that will be calculated from your usage habits.


Load Estimation

For everything that uses electricity in an “off-grid” solar system, there needs to be an estimate of the average number of hours per day it will be turned on and drawing power, as well as the number of watts it will use. By multiplying watts times hours, the total amount of energy that it will need for the day is found. For an entire house, there are a lot of things drawing power at once, a lot of calculations that need to be done to determine the total load. A good way to keep track of everything in is to use Affordable Solar's Off-Grid Load Estimator. This online tool lets you enter any number of loads into the calculation, their size in watts, and hours/day in use. It adds all of them and gives the number of watt-hours that need to be delivered every day by the system.



Figure 1: Affordable Solar’s Off-Grid Load Estimator

If the power/energy of an electric load is unknown, it can be determined using a “Kill A Watt” electricity usage monitor, easily purchased online and sometimes available locally as well. The “Kill A Watt” monitor can be used to find out how much power is being drawn at any specific moment, or how much energy is being used over time in watt hours. It is an excellent tool to use for finding your average daily electric load, and highly recommended for any battery based system sizing.



Figure 2: The Kill A Watt EZ, available in most major hardware stores

While they may sound similar, ‘Power’ and ‘Energy’ describe two different things. Power is the amount of electricity being used or created at any moment in time, while Energy is the amount of electricity that is used over time. Watts are used to describe Power, and Watt-hours are Energy. This distinction is important for sizing the three discrete components for a battery-based solar system. The solar array and battery components are both sized based on the amount of energy that is required, while the inverter and power panel are sized based on the amount of power that will be used at once.


Solar Array Sizing

To size the solar array based on the average daily electric load obtained earlier, you will need to know how much sunlight hits your site on a daily basis. If you have not read “Sizing Grid-Tied Solar Arrays”, it is recommended that you do now because this will directly reference its sections on sun hours and system efficiency.

The section titled “Sun Hours” can be used to find the available solar resource for a location for any orientation. Calculating the number of sun hours your site will receive, PVWatts outputs a table of results with values for every month as well as an average for the year. At this point, you will need to know if you plan on using a source of back-up power in addition to the solar array. An example of a source of back-up power would be a generator or the grid. This is a very important question to ask, because it greatly affects the eventual size of the solar array. If you are using a source of back-up power for your system, then you can size the solar array based on the average daily sun hours for the entire year just like a grid tied solar array. If you will not have anything besides the solar array, you need to use the month that has the lowest value of sun hours for the year. In most cases, this is December.



Figure 3: PVWatts Results table showing monthly values for solar radiation (sun hours)

There is a very good reason that having back-up power lets you get away with a much smaller solar array. If you don’t have anything backing up the solar system, then the only thing keeping the batteries charged are the solar panels. Since the batteries are the critical component of any off-grid system, it is very important that their charge be maintained or you risk over cycling them, drawing them down too far, or otherwise overworking them so that they prematurely age and require replacement in 2-5 years rather than 10-15. Without something external to bail out the solar system when the days are short, you need to size the solar array for the absolute worst case scenario or you’ll find yourself without enough power in winter. Winter is a challenging time for any battery-based solar system, for more tips on making it through read “Suriving Winter With a PV System”.

With the daily load in watt hours and the solar resource in sun hours, the solar array size can be calculated. The equation, from “Sizing Grid Tied Solar Arrays”, is:



The system efficiency factor η is usually estimated between 0.65 – 0.7 for stand-alone systems (0.55 – 0.6 for systems with a non-MPPT charge controller). You may notice that this is much lower than the ones used for sizing grid tied solar arrays. This is because of the addition of a battery bank and charge controller, and the differences in inverters between grid tied and off-grid systems. Charge controllers will lose between 1-5% of the power that goes through them, more if they are not an MPPT controller. Most stand-alone solar arrays will use an MPPT controller, however. Battery-based inverter/chargers typically have lower efficiencies as well, 88 – 93% versus 94 – 98% for grid tied inverters. For more information on solar system derating, read “Sizing Grid Tied Solar Arrays”. For more information on MPPT charge controllers and their benefits over conventional charge controllers, read “Maximum Power Point Tracking (MPPT) Charge Controllers”.


Battery Bank Sizing

Sizing the battery bank for your daily electric load is very simple. The equation is:



If the average daily load in Watt hours has been calculated already from the section “Load Estimation”, then the only factors that need to be obtained are the days of autonomy and the maximum depth of discharge.

The days of autonomy represents the longest amount of time the off-grid solar array will have to deliver power for your loads while you are unable to charge the batteries. This is due to cloud cover and storms that can block the sun for days at a time. With a grid tied solar array, there wouldn’t be any issues because the electric grid is there as the ultimate free battery bank. However, since this is a stand-alone system, the batteries have to be sized to handle the possibility that there won’t be any other power available for days at a time. For most systems, a value of 2-3 days will cover almost all scenarios and give you a sufficiently large battery bank. If you live somewhere with consistent cloud cover or long winters, however, you may want to design a larger battery bank to accommodate them, one that can deliver power for 4 days or longer.

The maximum depth of discharge of a battery bank can be tricky thing to figure out. It comes from properties of the deep cycle lead-acid batteries that are used in renewable energy applications. A complicated discussion on these properties could follow here, but suffice to say that the less you use batteries, the longer they last. By setting an arbitrary point for when you stop draining batteries, you help preserve them and they will hold a charger for longer than if you fully discharge them. The longest-lasting and most robust battery banks are never discharged more than 20-30%. Most systems are designed to a 50% depth of discharge, which is enough to give you 7-10 years of life from your batteries.

Another reason to set a maximum depth of discharge is the chemistry of deep cycle batteries. As you drain a battery, the acid in each cell becomes less and less acidic, and its freezing point rises. A fully charged flooded battery might not freeze in 0 degree weather, but a fully discharged one most certainly will.



Figure 4: Depth of Discharge vs. Electrolyte Freeze Point

As a consequence of this, you need to be aware of where you plan on locating the batteries for a stand-alone system. If they will be somewhere where it could reach 0 degrees or less and if they are discharged below 60% they will freeze and you will have to buy a whole new bank of batteries. For more information on battery banks and their maintenance, read “Extending Battery Life”.


Inverter Sizing

Sizing a battery-based inverter for a stand-alone solar system is a simple question. Unlike in grid tied solar arrays, a battery-based inverter is physically providing the power for your electric loads. So, if you have 3000 watts of lights you need to run at once with your stand-alone system, you will need an off-grid inverter rated for 3000 watts or higher. When sizing an inverter for your loads, it’s best to come up with a “worst case scenario” where the most appliances, lights, and other loads will be running at once. The inverter needs to be able to handle your highest continuous power load.

In addition to continuous loads, sometimes there are “surge loads” that your inverter will need to be able to accommodate. A surge load occurs when starting an electric motor or pump, usually for air conditioning units, well pumps, or shop tools. These surges can be 2-5 times higher than the continuous load, and the inverter has to have extra capacity to start it. Fortunately, most modern battery-based solar inverters have a “surge rating” for 1-10 seconds that is twice as high as their continuous rated output.

For more information on battery-based inverters, read the article “How to Choose an Inverter”.