How To Size A Solar Battery Bank
*Two Phase Trojan Battery Bank
When installing an off-grid or hybrid PV array, sizing thebattery bank can be a complicated process, and oftentimes, research leads tomore questions than answers. There are multiple specifications to a battery andthe meanings of these terms and ratings can lead to college calculusflashbacks. Luckily, there is a way to determine the size of battery you shouldbe selecting for your solar system.
The first step is to determine the size of your solar array,this can be easily determined by collecting 6 months to one year of your utilitybills, and calculating your average kilowatt hours.
Example:
Total 13,375 kWh
13,375 kWh/6 months = Average 2,229.68kwh or 2.3kWh
The next step is determining what voltage system you willwant. This voltage is often determined by the overall voltage of your solarpanels. Typically, for a solar set up, a 12-volt system is the most desirable.For the purposes of determining the remaining calculations, we will use a12-volt set-up.
For Amp Hour ratings, most PV compatible batteries use a C20rating. The C20 rating means that the battery has been completely dischargedover a period of 20 hours. There are also C100 ratings that mean the batteryhas been completely discharged over a period of 100 hours. Typically, whenlooking at solar retailer sites, the rated Amp hours listed in the producttitle is the C20 rating typical for the industry.
One-Phase Battery Bank System
Two-Phase System
*Trojan T105 AGM Battery
Comparing The Pv Size Estimation To A Simulated Result
Since this is a rough estimate, how does it compare against an actual, comprehensive design for a home with the same characteristics?
Using the same conditions as above, a PV system design software found that the required system size to be 4 kW, which is almost identical to the answer from the estimation conducted above.
Although the answers are very close, its important to note that this may not always be the case. For instance, when there is shading on the panels, a significant reduction in power output can occur. Although a shading term is included when calculating a derate factor, it can fail to accurately capture the effect that shading has on a PV systems power output. Therefore, expect the results to be less close when modeling a location with shading.
What Roadblocks Are There To Filling Your Roof With Solar Panels
Besides the obvious , the other obstacle preventing you from maxing out your roof with panels are limitations set by your Distributed Network Service Provider, or DNSP.
You can see all of the DNSP rules for each state here.
But, to use an example, here in South Australia, SA Power Networks is our only DNSP.
They have set system size limits of:
- Single-phase homes: 10 kW inverter limit, 5 kW export limit.
- Three-phase homes: 30 kW inverter limit, 15 kW export limit.
Meaning if you have a single-phase home in SA, you could have up to 13.3 kW of solar panels on your roof with a 10 kW inverter. Remember you can oversize an inverter by 33%. But youd be export limited to 5 kW
Export limitations mean your inverter will intelligently ramp down solar power production to ensure a maximum of 5 kW is sent to the grid at any one time.
Some homeowners ask me If my energy needs are low-to-moderate, isnt it pointless to install a large system if Im export limited?
The answer, surprisingly, is no. You lose less generation than youd expect with export limitation due to a variety of factors.
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How Much Do Solar Panels Cost Overall
Using a solar cost calculator is an excellent way to figure out specific numbers for your circumstances. Each home consumes a different amount of electricity based on various factors. Having said that, if you take a 2,000 square foot home, the average solar panels cost equates to somewhere between $15,000 and $40,000.
Lets look at an example to get a better idea of how much solar panels cost. The average home in the United States is 1,500 square feet with a monthly electricity bill of $100. This means that the house needs a 6-kilowatt solar panel system with between 15 and 18 350-watt solar panels.
The estimated cost for a system of this size would be about $18,000. If the system saved you $2,500 per year on energy expenses, the payback period would be just over seven years until you would break even.
It Pays To Shop Around
With a big investment like solar, you want to look at multiple options to ensure youre getting a good deal. By signing up on the EnergySage Solar Marketplace, you can receive multiple quotes from pre-screened installers to compare. These quotes will include production estimates, as well as production ratios. Our quote comparison table will also flag production estimates that are outside the normal range for your area. If youd prefer to start with an initial estimate of costs and savings associated with solar, our Solar Calculator can be a good place to start.
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Energy Usage = How Much You Need
We always start with energy usage when sizing a solar system. For existing homes we prefer to look at your previous 12 months of electric bills so we can establish a good sense of your energy usage patterns over the course of a year. If we arent able to get a full 12 month view of your bills, we can estimate your monthly usage based on your peak winter and peak summer usage while factoring in whether you use natural gas or electricity for heating.
For new construction homes, our building science team can create an energy model for your home that predicts your future energy usage .
Once we have a sense of your electricity consumption needs, well talk with you about any events on the horizon that might impact your usage. Are you thinking about getting an electric vehicle? Are your kids moving out soon? Are you going to get a pool with your end of year bonus? Are you contemplating mining Bitcoin or starting a hydroponic farm? Well take all of these changes into account to appropriately size your system up or down.
How Solar Installers Calculate Solar Production Estimates
When youre evaluating your solar options, there are a lot of factors to take into account. Price, of course, is key, whether youre paying out of pocket or financing monthly payments with a lease or loan. You should also think carefully about the installation company you choose and the type of equipment going on your roof. But arguably the most important data points are the amount of electricity your panels will produce, and how much theyll save you in the long run.
When a solar installer starts to work with you, he or she will highlight a production estimate as part of their quote, which is the basis for your long-term savings. Heres how your installer estimates your solar panel systems electricity production.
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How Many Solar Panels Do You Need For Specific System Sizes
In our long example at the beginning of this piece, we determined that an 8 kW system would probably cover the average energy use for an American household if you live in an area with a production ratio of 1.6, which might be a realistic number for homes in most parts of California. Lets extend that a little further, and look at a few more examples. In the table below, weve compiled some solar panel estimates for common system sizes seen on the EnergySage Marketplace. Again, the big caveat here is that were using 1.6 as the production ratio of choice. For California shoppers, this might actually be realistic, but for folks in the Northeast or areas with less sun, these estimates might be a bit high on the production end and low on the number of panels needed.
How many solar panels do I need for my house? System size comparison
| System size |
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Why Bigger Is Better Even If You Believe You Wont Need A Larger System
Even with feed-in tariffs dropping and export limitations for single phase homes in certain states, maxing out your roof with solar panels is a smart move for two reasons:
Batteries and electric cars need lots of solar generation to reliably charge, and then you need more solar electricity to offset the energy needs of the rest of your home.
Im about to install more solar panels on my south-facing roof because my home now has two electric cars and a Tesla Powerwall battery. My existing 6 kW system struggles to generate enough energy for them all.
Ill finish by saying while I regularly hear from homeowners who wish they put on more solar panels when they had the chance , I have never heard anyone complain theyve installed too much solar power.
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And Finally Lets Do Out The Math
We have our three main assumptions now how do those numbers translate to an estimated number of solar panels for your home? The formula looks like this:
Number of panels = system size / production ratio / panel wattage
Plugging our numbers in from above, we get:
Number of panels = 10,649 kW / 1.3 or 1.6 / 320 W
which gives us between 20 and 25 panels in a solar array, depending on which production ratio we use . 25 panels each at 320 W results in a total system size of 8 kW, which is right around the average for EnergySage shoppers looking for a solar installer. Tada!
The Home And Site Evaluation
In the design phase, the solar installer will evaluate your home to determine the best solar panels, inverter, racking and other components. SunPower dealers often recommend our SunPower Equinox platform, an integrated solar system that combines SunPower® solar panels, micro-inverters and other components all covered under SunPower’s industry-leading Combined Power and Product Warranty. Other installers will design systems with components that may come from several different manufacturers, all with different warranty terms and services.
As part of the evaluation, your solar installer will make an appointment to inspect your attic, your roof and your home’s electrical system. He or she will determine the best location to mount the hardware that runs your system . During the inspection, the installer will evaluate your property to ensure that the installation is code compliant and will recommend upgrades if required. For example, if you have an older home, your electrical panel may need to be updated, or when inspecting your attic or roofing, the installer may discover issues that require repair before your system is installed.
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How Much Electric Usage Do You Want To Offset
When developing an on-farm solar electric system, the goal is to offset a portion or all of your farm’s annual electric demand. In Ohio, a policy tool called net metering supports the development of on-farm energy systems. Net metering is a billing arrangement where customers who produce their own electricity can receive a credit on their electric utility bills for any extra electricity produced by the customer that flows back onto the electric utility’s distribution system. The excess generation credits are allowed to accumulate until applied against future charges on the customer’s electric bill. As of 2013, 43 states have incorporated some type of net metering policy. Ohio’s net metering policy applies to the states’ investor-owned utilities and has no specified capacity limit. According to Ohio Administrative Code a net-metered system should be designed to offset part of or all of the customer’s electric requirements, but not exceed usage on a 12-month basis. Rural electric cooperatives and municipal electric utilities are not required to offer net metering . While not required by law, many of Ohio’s rural electric cooperatives and municipal utilities do offer net metering, but may have capacity limits on the system size. A capacity limit would limit the amount of electricity a consumer is allowed to produce and flow back to the utility’s distribution system. Contact your rural electric cooperative or municipal utility to find out specific details on their programs.
How Many Hours Of Sunlight Can You Expect In Your Area
The peak sunlight hours for your particular location will have a direct impact on the energy you can expect your home solar system to produce. For example, if you live in Phoenix you can expect to have a greater number of peak sunlight hours than if you lived in Seattle. That doesnt mean a Seattle homeowner cant go solar it just means the homeowner would need more panels.
The Renewable Resource Data Center provides sunlight information by state and for major cities.
Now multiply your hourly usage by 1,000 to convert your hourly power generation need to watts. Divide your average hourly wattage requirement by the number of daily peak sunlight hours for your area. This gives you the amount of energy your panels need to produce every hour. So the average U.S. home in an area that gets five peak sunlight hours per day would need 6,000 watts.
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What Is Solar Energy
Simply put, solar is the most abundant source of energy on Earth. About 173,000 terawatts of solar energy strike the Earth at any given time – more than 10,000 times the world’s total energy needs.
By capturing the sun’s energy and turning it into electricity for your home or business, solar energy is a key solution in combating the current climate crisis and reducing our dependence on fossil fuels.
Average System Size And Power Output By Location
We know that real-world output for a given system size is affected by several factors and is constantly fluctuating. But dont worry – its possible to account for weather conditions in your area based on decades of historical data, which allows us to calculate average solar power output for a given solar system size at any specific location.
Average solar system size in selected states| State |
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*Daily kWh generation figures can see large variations between seasons, and even from day to day.
Do note that the figures above are just averages. The amount of power generation will vary, especially if weather conditions where you live are different from the state average, or if your roof has a sub-optimal orientation or angle.
Dont worry though, you can still come up with accurate system size and production figures for your specific home – just use one of the three calculations methods we discussed earlier.
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Solar System Sizing And Production Estimates
After researching your local utility’s requirements surrounding interconnection and net metering, you will have a better sense of the maximum system sizes allowed given any restrictions currently in place set by the authority having jurisdiction or the utility. Some utilities do not allow more solar put into the electric grid than what the residence uses on an annual basis. Thus, some utility companies have a rule of thumb for maximum solar sizing.
For instance, California electric utilities require solar systems not to exceed 2 watts per square foot of conditioned space without an exceptional load justification. As another example, in Massachusetts, there is a “cap” on net metering for each utility for certain types of projects. Systems that are 10 kilowatts and under, however, are exempt from the cap, so many residential solar photovoltaic systems in Massachusetts are 10 kW or less.
As an example, the required solar load size in California is based on building electricity use, where the new code requires solar systems to offset 100% of electricity consumption for mixed-fuel homes. The new California Title 24, Part 6 building energy code also dictates that all-electric homes generate as much solar power as an equivalent mixed-fuel home would.
To calculate the size of the solar system to generate a predetermined percentage of electricity needed, there are various energy software programs available, including but not limited to:
Calculate Your Kwh Usage
To determine your homes energy usage more accurately, use our home appliances power consumption table to find out how many kWh your appliances would use per month.
If your utility provides a favorable net metering policy, the energy your system generates can be banked with the utility as a credit that can be used later. Not all utility companies give you credit check with your local provider.
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