Once you determine if your home is a good site for solar, your next question is probably “What size solar system do I need to run my house?” We custom design each of our solar systems around the following factors:
Your home’s energy consumption requirements tell us the upper limit of how much solar energy you will need to power your home.
There’s not much we can do about overcoming set constraints and we’ll never push you beyond what you are comfortable with, but there are a few ways we can work around specific limitations when it comes to:
By the end of this post, you’ll know how each of these factors impact your system size and should be able to approximate the solar system size 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 aren’t 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 (if you are building your home with one of our 200+ NC Builder partners, then we’ve probably already generated this model).
Once we have a sense of your electricity consumption needs, we’ll 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? We’ll take all of these changes into account to appropriately size your system up or down.
Based on your annual electricity and monthly consumption pattern, we can ballpark a general system size for you. To do this, we use a rule-of-thumb number for solar production in NC to estimate your needed system size. Based on our experience, our rule of thumb is that 1 kilowatt (kW) of solar installed in NC will produce 1,300 kilowatt hours (kWh) per year. So if your home uses 12,000 kWh per year, we’d estimate you need around a 9.2 kW solar system to meet 100% of your energy needs (12,000/1,300 = 9.2).
Remember, this is just an initial rough estimate. We always refine your system size with more accurate annual production estimates based on the exact solar potential of your site after we perform a full site evaluation of your home, discuss your goals of going solar, and work through any limitations if encountered. With that said, this graph shows how this rough estimation translates to solar kW and number of solar panels.
This graph shows the balance we are trying to reach between monthly usage for an average NC home using electric heating (a home with natural gas heating would not have a winter spike in usage) and monthly solar production. Overlayed on top of the home’s energy usage is the production from two solar systems — one that offsets 100% of the home’s annual usage and one that minimizes the amount of energy that will be forfeited to Duke (any excess solar production from your solar system accumulates until the end of May, when it is forfeited to Duke).
Based on our more detailed comparison of monthly usage vs solar production we might refine our recommended system size for this home from 9.2 kW to 6.5 kW if maximizing your ROI is your main goal. I say might, because our solar designers will weigh your cost of forfeiting some production to your utility vs the cost efficiencies of having a larger system which produces more total savings.
The two primary factors that limit what size solar system you can add to your home is the physical space to install the solar panels (either on your roof, or ground mounted in your yard) and your budget for making the switch to power up with the sun.
Without enough eligible space to install solar panels, your solar savings will remain theoretical. We define eligible space as an area that:
Roof space needed for a solar system
If a space meets all those criteria, then it is ready to shine. For a roof mounted solar system, each panel takes up an area of approximately 18 square feet. So for the 100% energy offset 9.2 kW solar system we have been using as an example, we would need 31 panels (if we assume 350 watts per panel) or 470 sq feet of eligible roof space (100 sq ft less than 2 years ago!).
For a ground mounted solar system, an eligible space is one that is free from significant shading and close enough to your home or electric meter that trenching is feasible and cost effective (i.e., within approx 500 ft). Ground mounted panels can take up more area than roof mounted panels if there are multiple rows because we have to account for shading created between rows of panels (this is creatively called inter-row shading). For a south facing system, tilted to 30 degrees (to optimize production), the effective area taken up by the panels (accounting for inter-row shading) would be close to 60 square feet for the same 18 square foot panel!
Your budget is an obvious and important criteria for your system size. We’ll work with you to find a system that balances what you can spend with your optimal system size. The good news is that there is some flexibility in price and how we design your system to maximize your budget and your home’s solar production. For example, if you are limited by space you might need more efficient, higher wattage panels that take up less room. However, while you will need less panels to reach the same system size (and production output), these panels are more expensive. On the flip side, you might have more than enough space allowing you to opt for a higher quantity of lower wattage and lower cost panels to reach the same production numbers. So while there is flexibility between these options, by far, the biggest driver of your system’s price is the overall system size. For a full breakdown of how much solar costs and the variables that can impact the bottom line, check out our guide to home solar cost.
For simplicity, let’s look at some averages for solar system cost and size. In 2021, our average residential solar system size is 7.9 kW which has an average price of $23,000 before incentives and $17,000 after incentives. This graph will give you a reference for how that average price scales up and down as system size changes. As a note, this illustration represents solar system pricing without battery storage.
So if your energy needs call for an 8kW system, but your budget is closer to to $15k, we can work with you to scale your system down to closer to 7kW — and we’re more than happy to show you all of the different panel options and quantities combinations we can use to land at your budget.
After reading this, you should have a good sense of what factors determine the perfect system size for your home.
If you have questions about any of these items or you want help determining the perfect solar system size for you home, schedule a call with one of our solar educators, and let them know how we can help you on your solar journey.
This article was originally published August 1, 2019 and has since been updated for 2021.
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