The goal: match production to consumption #
The purpose of this calculation is straightforward — figure out how many solar panels will generate roughly as much electricity as your household consumes in a year. “Roughly” is the key word. You won’t hit a perfect match, and you shouldn’t try to. For optimum performance, sizing to around 120% of your annual consumption is a good target: the extra headroom accounts for real-world losses, seasonal variation, and future load growth. That said, budget and available roof space may require you to size closer to 100% — both approaches are valid, and even a system sized at 80–90% of consumption delivers strong returns.
This process has four steps: find your daily energy need, determine your location’s solar resource, account for real-world losses, and work backward to a panel count.
Step 1: Determine your daily energy need #
From your electricity bills (covered in Topic 02), you should have your annual kWh consumption. Divide that by 365 to get your average daily need.
Example: 10,200 kWh/year ÷ 365 = 27.9 kWh/day
This is your target daily production. If you plan to add significant new loads before your system is installed — an EV, a heat pump, a hot tub — add an estimate for those now. It’s much easier to size correctly upfront than to expand later.
Step 2: Find your peak sun hours #
“Peak sun hours” (PSH) is not the number of daylight hours. It’s a measure of how much total solar energy hits your location per day, expressed as the equivalent number of hours of full-strength sunlight (1,000 W/m²).
A location that receives 5 PSH gets the same total solar energy as if the sun shone at full intensity for exactly 5 hours — even though actual daylight may be 10–14 hours.
How to find your PSH:
The most reliable source is the NREL PVWatts Calculator (pvwatts.nrel.gov), which uses decades of historical weather data. Enter your address and it will show you monthly and annual average solar resource data.
As a rough reference:
| Region | Typical annual PSH |
| Southwest US (AZ, NM, NV) | 5.5–6.5 |
| Southeast US (FL, GA, TX) | 4.5–5.5 |
| Midwest / Mid-Atlantic | 4.0–5.0 |
| Pacific Northwest / Northeast | 3.5–4.5 |
| Alaska / far north | 2.5–3.5 |
Use the annual average for system sizing unless you want to size for winter production specifically (which results in a larger system that overproduces heavily in summer).
Step 3: Account for system losses #
No solar system converts sunlight to usable AC electricity at 100% efficiency. Real-world losses come from several sources:
- Inverter efficiency: 93–97% depending on type
- Wiring losses: 1–3%
- Temperature derating: panels produce less power as they heat up in summer; typical loss is 10–25% on hot days
- Soiling: dust, pollen, and bird droppings reduce output; assume 2–5% unless you wash panels regularly
- Shading: even partial shading can significantly reduce output (more on this in Topic 03)
- Module mismatch: slight manufacturing variations between panels; typically 1–2%
PVWatts uses a default “derate factor” of about 86% (0.86), meaning it assumes your system will operate at 86% of theoretical peak efficiency. This is a reasonable starting point for most residential systems.
System efficiency factor to use in calculations: 0.80 to 0.85
Using 0.80 gives you a conservative (slightly larger) system; 0.85 gives you a lean estimate. For a DIY install where wiring runs may be longer and inverter selection is uncertain, 0.80 is the safer choice.
Step 4: Calculate the system size you need #
With your daily energy need, PSH, and efficiency factor in hand, calculate your required system size in kilowatts (kW DC):
Formula:
System size (kW) = Daily energy need (kWh) ÷ (PSH × efficiency factor)
Example: 27.9 kWh ÷ (5.0 PSH × 0.80) = 27.9 ÷ 4.0 = 6.97 kW
Round up to the nearest practical system size. In this case, a 7 kW system is the target.
Step 5: Convert system size to panel count #
Now divide your required system size by the wattage of the individual panels you plan to use.
Modern residential panels typically range from 380W to 450W. Higher wattage panels cost more per unit but require fewer panels for the same system size — relevant if your roof space is limited.
Formula:
Number of panels = System size (W) ÷ Panel wattage
Example: 7,000W ÷ 400W per panel = 17.5 → round up to 18 panels
Sanity-checking your result #
Before finalizing your panel count, run two quick checks:
Check 1: Do they fit on your roof? #
A standard 400W panel is roughly 68″ × 44″ (about 20.7 square feet). Eighteen panels would need approximately 375 square feet of unobstructed, south-facing roof space. Compare this against your available roof area from Topic 03.
If you don’t have enough space, you have two options: use higher-wattage panels to get more power per square foot, or accept a smaller system that covers less than 100% of your usage.
Check 2: Does PVWatts confirm your estimate? #
Run your final panel count through PVWatts using your actual panel specs and roof orientation. PVWatts will give you a monthly production estimate you can compare against your monthly consumption data. If the annual totals are within 10%, you’re well-sized. If PVWatts shows you significantly overproducing in summer and underproducing in winter, that’s normal for most locations and not a problem unless your utility has poor net metering rates.
Common sizing mistakes #
- Sizing to exactly 100% and stopping there. A 120% target is recommended for optimum performance — the extra capacity offsets real-world losses and seasonal dips that a 100%-sized system can’t fully cover. If budget or roof space is a constraint, sizing down to 90–100% is perfectly reasonable; just go in with realistic expectations about winter shortfalls and net metering credits. The mistake is treating 100% as a ceiling when your roof and utility terms would support going higher.
- Using peak-month consumption. If your July bill was 1,400 kWh because you ran the AC constantly, don’t size your system to cover that month — you’ll drastically oversize for the rest of the year.
- Ignoring future load growth. If you’re buying an EV next year, adding a battery, or planning a home addition, factor those loads in now.
- Not accounting for shading. A system sized perfectly on paper can underperform by 20–40% if there’s a tree or chimney casting shadows on even a few panels. Get an accurate shading assessment (Topic 03) before finalizing your count.
Quick reference formula summary #
| Step | Formula |
| Daily kWh target | Annual kWh ÷ 365 |
| Required system size | Daily kWh ÷ (PSH × 0.80) |
| Panel count | System size in W ÷ panel wattage |
Summary #
Calculating your panel count is arithmetic, not guesswork — but only if the inputs are right. Start with accurate consumption data from 12 months of bills, use your location’s real PSH from PVWatts, apply a conservative efficiency factor, and cross-check with a full PVWatts simulation before purchasing equipment. A few hours of calculation now can save you from a system that leaves money on the table or falls short of your goals.
