What Size Solar System Do You Actually Need?

Once you determine if your home is a good site for solar, your next question is probably “What size solar system is right for my home?” We custom design each of our solar systems around the following factors:

How Much Solar You NEED – your home’s energy consumption requirements tell us the upper limit of how much solar energy you will need to power your home.​

How Much Solar You SHOULD Get- once we know how much solar you need, we whittle that down to how much solar you should get. We build this recommendation off of:

  • Savings Goal - Do you want to maximize your ROI or do you want to reach net-zero energy use?

How Much Solar You CAN Get – 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:

  • Roof Size/Available Space - How much eligible space do you have on your roof or on the ground for solar?
  • Budget - How much do you want to spend on solar? Do you want our premium solar panels options our most cost-effective option?

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!

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 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 christmas 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.

Ballparking A Solar System Size through your Annual Usage

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 on-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.

Savings Goals = How Much You SHOULD Get

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.

Limiting Factors = How Much Solar You CAN Get

Available Area for Solar

Without enough eligible space to install solar panels, your solar savings will remain theoretical. We define eligible space as an area that:

  • Doesn’t have significant shading
  • Is free from roof obstructions like vents and dormers
  • Is not facing northward
Aerial view of a home solar system mounted on the roof

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 300 watts per panel) or 560 sq feet of eligible roof space.

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 and 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!

Budget

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 2019, our average residential solar system size is 8.2 kW which has an average price of $24,400 before incentives and $13,500 to $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 you your energy needs call for an 8 kw system, but your budget is closer to to $20k, we can work with you to scale your system down to closer to 6 kW — 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.

What's Next?

After reading this, you should have a good sense of what factors determine the perfect system size for your home.

  • What size do you NEED is determined by your energy usage.
  • What size you SHOULD get is refined by your savings goal - Maximum solar offset vs Maximum ROI.
  • What size you CAN get is defined by the eligible space you have available for a system -- and of course your project budget.

If you have questions about any of these items or you want help determining the perfect solar system size for you home, reach out to our solar educators, Mike or Davis, and let them know how we can help you on your solar journey.

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