If you have gathered three solar quotes and received three different system sizes, you are not imagining things. Many sales reps size a system based on a rough guess instead of the actual usage. Learning how to determine solar system size yourself removes the guesswork and protects you from paying for panels you do not need, or ending up with a system too small to offset your bill. This guide walks through the exact formula solar designers use, built around your kWh usage, Los Angeles sun hours, an efficiency buffer, and your offset goal. By the end, you will be able to determine solar system size accurately enough to challenge any quote that does not match the math.
Why Most Solar Quotes Get Your System Size Wrong
The Guesswork Method Too Many Installers Use
Some installers size a system based on your roof space alone, or a flat “average home” package, rather than your actual electricity habits. That approach ignores how much power you use in July compared to January, and it treats a single occupant in a condo the same as a family of five running central air. The result is a quote built to move product, not to match your home.
What Sizing Your System Wrong Actually Costs You
An undersized system leaves you paying SCE or LADWP for the gap every month, at rates now running around 34 to 35 cents per kWh, with peak time-of-use pricing pushing past 50 cents between 4 and 9 p.m. On a 20 percent shortfall, that can mean an extra $50 to $80 a month, still going to the utility. An oversized system costs you more upfront than necessary and can extend your payback period without adding meaningful savings, since excess production under NEM 3.0 is credited at a much lower rate than it once was. Getting the size right the first time avoids both problems.
The Real Data Behind How to Determine Solar System Size
Start With Your Actual kWh Usage, not a Rule of Thumb
Pull your last 12 months of utility bills and total your kWh usage, then divide by 12 to get a monthly average. California households use roughly 676 kWh per month on average, though yours could run higher with a pool, an EV, or window units working overtime. Skipping this step is the single biggest reason solar system sizing goes wrong.
Why Los Angeles Sun Hours Change the Math
Los Angeles gets about 5.6 peak sun hours per day on average, among the strongest solar conditions in the country. That figure matters because it converts your daily kWh need into the actual array size, measured in kilowatts, that your roof needs to produce to cover it.
The Step-by-Step Formula to Determine Solar System Size
Step 1 Through 3: Usage, Sun Hours, and Inefficiency Buffer
Start with your monthly kWh usage and divide by 30 for a daily average. Divide that daily figure by your local sun hours to get a baseline array size in kilowatts. Then add a buffer of about 20 percent to account for panel degradation and real-world losses, since lab-rated wattage rarely holds up perfectly on your roof.
For a home using 600 kWh a month: 600 divided by 30 is 20 kWh per day. Twenty divided by 5.6 sun hours is a 3.57 kW baseline array. Multiplied by 1.2 for the buffer, that comes to a 4.29 kW system.
Step 4 and 5: Offset Percentage and Panel Count
Next, decide how much of your bill you want solar to cover. Most homeowners target 100 percent offset, but a partial system, sized at 50 or 75 percent, is a valid way to start smaller and expand later. Once you have your final kW figure, divide it by your chosen panel’s wattage to get the panel count, a step covered in detail in our guide to how many 100-watt panels it takes to power an LA home, or if you are comparing higher-output equipment, our breakdown of 500-watt panel options.
Quick Reference Table: Usage to System Size
The table below shows how monthly usage translates into array size and panel count for a Los Angeles home at 5.6 sun hours, 100 percent offset, and a 20 percent efficiency buffer.
| Monthly kWh Usage | Daily kWh Usage | LA Sun Hours | Array Size Needed (kW) | Approx. Panels (400W) |
|---|---|---|---|---|
| 400 kWh | 13.3 kWh | 5.6 | 2.9 kW | 7 panels |
| 500 kWh | 16.7 kWh | 5.6 | 3.6 kW | 9 panels |
| 600 kWh | 20.0 kWh | 5.6 | 4.3 kW | 11 panels |
| 700 kWh | 23.3 kWh | 5.6 | 5.0 kW | 13 panels |
| 800 kWh | 26.7 kWh | 5.6 | 5.7 kW | 14 panels |
| 900 kWh | 30.0 kWh | 5.6 | 6.4 kW | 16 panels |
Find your row, and you have a realistic starting point before roof-specific adjustments.
Offset percentage changes those numbers directly, since a partial system is simply a smaller version of the same formula. The table below applies different offset targets to a home using 600 kWh a month, the closest match to the California average.
| Offset Target | Array Size Needed (kW) | Approx. Panels (400W) | Best Fit For |
|---|---|---|---|
| 100% offset | 4.3 kW | 11 panels | Homes eliminating most of their bill now |
| 75% offset | 3.2 kW | 8 panels | Homes easing into solar with room to add later |
| 50% offset | 2.1 kW | 5 panels | Homes on a tighter budget or with a smaller roof |
There is no wrong answer here. A 50 percent offset system still cuts your bill meaningfully, and most installers, including our team, can expand a system later if your usage grows or your budget opens up.
What Qcells Direct Does Differently When Sizing Your System
Factory-Direct Qcells Panels Built for LA Roofs
Every system we size uses American-made Qcells panels backed by a 25-year comprehensive warranty covering the panels, workmanship, and performance, not just one of the three. Because we work with Qcells directly, you get factory-direct pricing without the markup that comes from a middleman distributor.
Free On-Site Assessments That Correct the Math
A formula gets you close, but your roof still has the final say. Our CSLB-licensed consultants offer free virtual or on-site consultations to confirm shading, orientation, and structural details before finalizing your system size, and most approved projects move to installation within 3 to 4 weeks. That same team has helped guide more than 180 five-star reviewed installs, so the adjustments they make to your formula-based estimate come from pattern recognition across hundreds of LA roofs, not a single generic checklist.
Beyond the Formula: Roof, Shading, and Battery Considerations
Roof Orientation and Shading Adjustments
South-facing roof sections produce the most energy overall, but if your utility charges higher time-of-use rates in the evening, west-facing panels can offset more of your highest-cost hours even at slightly lower total output. Tree cover, chimneys, and neighboring rooflines can also cut production enough to justify sizing slightly above the baseline formula. A roof with multiple usable sections, split between south and west exposure, often ends up sized a bit larger than the formula alone suggests, simply to make up for the less efficient sections carrying part of the load.
Sizing for a Battery Under NEM 3.0
Under NEM 3.0, exported solar energy is worth far less than it used to be, which is why more Los Angeles homeowners are pairing their system with a battery instead of relying on export credits. If you are weighing that decision, our guide to how solar batteries work breaks down how storage changes both your sizing and your savings. Understanding your local SCE rate structure will also help you decide whether a full or partial offset makes more financial sense for your household.
Get Your System Size Right the First Time
Utility rates in Los Angeles keep climbing, and a system sized off a guess either leaves savings on the table or costs more than it should. Now that you know how to determine solar system size using your own usage, local sun hours, and offset goals, you are equipped to evaluate any quote you receive. If you would rather skip the spreadsheet, request a free consultation, and our team will confirm your exact system size using your actual roof and utility data.
How Homeowners Get Solar System Sizing Wrong Most Often
How do I determine solar system size without hiring an engineer?
You can determine solar system size yourself using your last 12 months of kWh usage, your local peak sun hours, and a 20 percent efficiency buffer. The formula in this guide gets most homeowners within a reasonable range, and a free consultation can confirm the final number.
What size solar system do I need for an average LA home?
Most Los Angeles homes land between a 4 kW and 6 kW system for full offset, depending on household size, pool or EV usage, and roof conditions. Use the reference table above to find the range closest to your monthly kWh usage.
Is it better to oversize or undersize a solar system?
Neither extreme is ideal. A slightly undersized system is generally the safer starting point since panels can be added later, while a significantly oversized system adds upfront cost without proportional savings under current net metering rules.
Does adding a battery change how I determine solar system size?
Yes. Under NEM 3.0, homeowners pairing storage often size their solar array to charge the battery fully during peak sun hours, which can mean a slightly larger array than a solar-only system would need.
How accurate are online solar calculators?
Online calculators are a reasonable starting point, but rely on averages rather than your actual bills and roof. Running the formula in this guide with your real usage data gets you closer, and a free on-site assessment closes the remaining gap.