If you’re installing a grid-tied solar system for your home, chances are you’ll rely on a certified installer to figure out the technical stuff, including how big your system should be. But if you’re the type who wants to get to the bottom of how sizing works before anyone starts quoting you with numbers (especially when every extra kilowatt can add thousands to the total cost), you’re in the right place. In this guide, I’ll walk you through how to size your home solar system step-by-step, so you can double-check whether your installer’s proposal makes sense.
How to Size a Solar System in 5 Steps
Step 1: Know Your Average Monthly Energy Use (kWh)
A good approach to determining a solid baseline for your household energy use is by collecting your last 12 months of electric bills and checking the monthly kilowatt-hour (kWh) usage. From there, take the average of those 12 months.
Averaging smooths out those highs and lows, especially considering consumption spikes in summer when the AC runs all day, and again in winter with heating.
Now, you could also begin by looking at your budget or available roof space (we’ll cover those later). But averaging your last 12 months of energy use is arguably the most reliable method for estimating your solar system size since it’s based on actual data and not assumptions.
Step 2: Convert Monthly Use into Daily Average
Now that you’ve got your monthly average from Step 1, the next step is figuring out your daily usage. Simply divide your monthly average by 30.
Say your average monthly use is 900 kWh (which is roughly the average energy consumption for a U.S. household), then your average daily use amounts to:
900 kWh ÷ 30 days = 30 kWh per day
That means, if you’re planning to offset 100% of your usage, your solar system needs to be capable of producing 30 kWh per day as well.
In a sense, you now have a rough idea of what your system should cover daily, and thus its “size”. However, this doesn’t give the full picture because when we talk about solar system size, we refer to it in kilowatts (kW), not kilowatt-hours (kWh).
To close the loop, we’ll use your average daily usage alongside your location’s sun hours, as we’ll do in the next step.
Step 3: Account for Your Location’s Sun Hours
Average daily sun hours is a measure of how many hours per day the sun is strong enough to generate power. You can find your area’s average sun hours using this solar irradiance map from the National Renewable Energy Laboratory (NREL).
The goal is to use this number to get the kW size of your system. Let’s do some simple math. For instance, say, you’re installing a solar system for your house in Utah, which receives roughly 5.5 peak sun hours on average based on the NREL map above. Now use this formula to come up with your system size in kW: System Size (kW) = Daily kWh Needed / Daily Sun Hours So: 30 kWh / 5.5 sun hours = 5.45 kW Now, you’ve got a solid number for your home solar system size. But, we don’t stop there… Step 4: Account for Solar Panel Inefficiencies Not all of the energy your solar system could generate on paper actually makes it to your appliances. That’s because there are real-world inefficiencies you need to factor in, such as:
- Panel degradation over time: Solar panels naturally lose efficiency as they age. Most solar warranties hint at this, often guaranteeing around 80–85% output after 25 years.
- Shading throughout the day: Trees, chimneys, or nearby buildings can cast shadows on your panels at certain times, reducing their output.
- Roof orientation and tilt: If your roof isn’t angled or oriented in a way that gets the best sunlight, your system won’t produce at its full potential.
- Unpredictable weather: Even in sunny regions, cloudy or rainy days can lower how much sunlight your panels get.
Other Considerations for Sizing a Solar System
Budget
Even if your roof could fit 30 panels, your wallet might say otherwise. That’s why many homeowners start with their budget, then work backwards to figure out what kind of system they can afford.
Let’s say your total budget is $7,000. If you qualify for the 30% federal solar tax credit, your actual install budget is effectively:
$7,000 ÷ (1 – 0.30) = ~$10,000 total system cost before credit
Assuming an average installed cost of $3 per watt, that gets you a system around:
$10,000 ÷ $3/watt = ~3.33 kW system
If your home uses 30 kWh per day, and you live in a place with 5.5 sun hours, then:
3.33 kW × 5.5 sun hours = ~18.3 kWh/day
That setup would cover around 60% of that usage — not too shabby, especially when you’re trying to keep upfront costs down. And hey, nothing’s stopping you from expanding your system later on when the time’s right.
Roof Space
Remember earlier when we said you could fit 30 panels? That’s only if your roof layout has enough real estate — and I’m not exactly talking about your roof’s literal square footage…
Rather, what I mean is usable roof space. Sure, your roof might look big enough on paper, but once you factor in split levels, add angles, and roof obstructions like vents and chimneys, the actual area you can use for panels shrinks fast.
(I covered this in more depth in a previous article: How Many Solar Panels Can Fit on My Roof, in case you’re curious to learn more.)
If you really want your system size to match your actual needs, but your roof’s usable space is limited, you’ve got two solid workarounds:
- Use higher-efficiency panels to squeeze more power out of each square foot
- Consider a ground-mounted system if you’ve got the yard space
Geographic Location (Sunlight)
We already touched on this back in Step 3, but just in case it wasn’t clear: Where you live plays a huge role in how much sunlight your panels get — and that sunlight is what powers your system in the first place.
The more direct sunlight your panels receive, the more electricity they can generate.
That’s why someone in Arizona might need a smaller system than someone in Michigan to cover the same energy usage.
Battery Storage
If you’re planning to add a solar battery, say, for nighttime use, you’ll need to size your system a little larger to cover both your daytime consumption and the extra energy needed for charging.
Whether adding a battery is worth sizing up your system for is a rabbit hole for another day. But it’s worth mentioning that solar batteries tend to last a long time, and better yet, you could qualify for battery-specific rebates or incentives just by having one, which can make the math a whole lot more appealing.
Utility Arrangements
Depending on your local utility’s policies, your solar setup may interact with the grid in very different ways. And that interaction can directly affect how big or small your system should be.
If your utility offers true net metering, where any excess energy your panels produce is credited at the same retail rate, sizing your system all the way up to 100% coverage or more can make sense.
Now, if we’re talking about rate plans, that’s a whole other layer to consider when thinking about system size.
For example, if your utility uses Time-of-Use (TOU) rates, where electricity costs spike typically during mid-afternoon to evening hours, a battery may help you avoid those spikes. And as I mentioned earlier, adding a solar battery could influence whether you decide to size up your system just a bit more.
What Is a Good Size for a Home Solar System?
There’s no perfect number for the ideal solar system size. The “right” size is the one that actually fits your energy goals (whether you’re aiming for 100% offset or just partial coverage) and stays within your budget.
That said, if you’re looking for a ballpark range, know that the median size of home solar systems in 2023 is around 7 kW.
How Do I Factor in Future Changes in My Electricity Consumption, Such as Adding an EV?
If you’re buying an EV, expect an extra 3,000 to 3,800 kWh per year in usage, which translates to an additional 3 to 4 more panels.
In general, if you’re thinking about future upgrades like installing a heated swimming pool, converting to electric heating, or adding a second EV charger at home, keep in mind these three things:
- Add a buffer when sizing
- Install conduit and space for future expansion
- Choose an inverter that can accommodate future panel additions
We installers call this solar-ready planning, and it could save you from a costly redesign later.
Can I Add More Panels Later If I Initially Undersize My Solar System?
Yes, you can add more panels later on for future expansion. However, make sure that you have:
- An inverter that can handle the added capacity
- Enough unshaded roof space (or yard space, if you’re considering ground-mounted panels)
- Approval from your utility to increase system size
If you’re working with a solar installer, let them know upfront that you might expand. That way, they can design your system with that in mind.
Need Help Sizing Your Home Solar System?
Avail Solar has installed hundreds of home solar systems across the U.S., from Utah and Colorado to Nevada and Idaho. We’re dedicated to helping homeowners like you get the right-sized setup for your usage and budget, whether that means full coverage or just offsetting a portion of your energy needs. Request a quote today!