Solar panel count calculator
A home using 900 kWh a month with 400 W panels and 4.5 peak sun hours needs 21 panels, about an 8.4 kW system, at the default 80% efficiency. Each panel makes 1.44 kWh a day, and the count is rounded up. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Monthly kwh
- 900
- Panel wattage
- 400
- Peak sun hours
- 4.5
- System efficiency
- 0.8 item
- Days per month
- 30
- Daily kwh
- 30
- Panel kw
- 0.4
A solar panel count calculator turns your home electricity bill, the wattage of the modules you plan to buy, and the peak sun hours at your location into a clear whole-number array size. Enter average monthly kilowatt-hours from a recent bill, the rated watts of one panel, and daily peak sun hours for your climate; optionally refine system efficiency and the number of days in the billing month.
How solar panel count is estimated from monthly usage
Residential photovoltaic sizing often starts from energy, not roof area: the question people type into search is how many solar panels they need for a given monthly kilowatt-hour bill. This calculator answers that with the classic peak-sun-hour model. First the monthly usage is divided by days per month (default thirty) to obtain a daily energy target in kilowatt-hours.
Each panel is converted from watts to kilowatts by dividing by one thousand, then multiplied by peak sun hours and by a system efficiency factor that accounts for inverter losses, wiring, soiling, temperature, and mismatch. The resulting daily production per panel is the denominator; dividing daily demand by that figure yields an exact fractional panel count. Because modules are sold whole, the recommended count is the mathematical ceiling of that fraction, with a floor of one panel when demand is positive and production per panel is finite.
Peak sun hours are not clock hours of daylight: they are equivalent full-sun hours at one thousand watts per square metre, typically between about three and six for many mid-latitude homes depending on season, latitude, and average cloud cover. System efficiency defaults to zero point eight (eighty percent), a conservative overall derate used in many rule-of-thumb guides; you may raise it toward zero point nine for premium inverters and clean arrays or lower it when shading and heat are severe.
This is transparent shopping and planning math, not a full site survey with irradiance maps, string design, or structural load checks; treat the output as a starting array size and refine with a qualified installer when tilt, azimuth, shading, or code setbacks matter.
Practical tips, limits, and related searches
Pull monthly kilowatt-hours from twelve consecutive utility bills and average them so winter heating and summer cooling both influence the target, or size for the highest month if you want deeper self-consumption. Read panel wattage from the manufacturer datasheet STC rating, not the marketing headline of a kit that mixes multiple module sizes. Look up peak sun hours for your city from a trusted solar resource map or meteorological table for the annual average, then re-run with seasonal extremes if you care about winter shortfalls.
Leave system efficiency at the default eighty percent unless you have measured performance ratio data from a similar local system. Keep inputs inside the published absolute ranges so products cannot overflow into non-finite results the solver must reject. Prefer this capability when you already know bill energy and module wattage and only need a transparent whole-panel estimate; if your source is roof square footage alone, convert expected production another way or measure consumption first.
FAQ
What happens when monthly usage is zero or negative?
Monthly energy usage must be positive; a non-positive bill cannot define a real panel sizing target.
What are peak sun hours?
Peak sun hours are equivalent full-sun hours at 1000 W/m² irradiance, not clock hours of daylight. Typical residential sites often fall between about 3 and 6 depending on climate and season.
Why is system efficiency default 0.8?
An 80% overall factor is a common rule-of-thumb derate for inverter, wiring, soiling, temperature, and mismatch losses. Override it when you have a measured performance ratio for a similar system.
Is the solar panel count calculator free?
The browser tool is free and runs locally.
Which monthly kWh figure should I enter?
Average twelve consecutive utility bills so that both winter heating and summer cooling count. If you want to cover more of your own use, size for the highest month instead and compare the two panel counts.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/home/solar-panel-count \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"monthly_kwh":900,"panel_wattage":400,"peak_sun_hours":4.5}'const res = await fetch("https://api.kit.forhosting.com/home/solar-panel-count", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"monthly_kwh": 900,
"panel_wattage": 400,
"peak_sun_hours": 4.5
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/home/solar-panel-count",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"monthly_kwh": 900,
"panel_wattage": 400,
"peak_sun_hours": 4.5
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/home/solar-panel-count", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"monthly_kwh":900,"panel_wattage":400,"peak_sun_hours":4.5}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"monthly_kwh":900,"panel_wattage":400,"peak_sun_hours":4.5}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/home/solar-panel-count", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"monthly_kwh": 900,
"panel_wattage": 400,
"peak_sun_hours": 4.5
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "home.solar_panel_count",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Non-positive usage, wattage, or sun hours are rejected with a clear invalid_input error instead of inventing a free-energy system.
How to call the API and what comes back
Non-positive monthly usage, panel wattage, or peak sun hours is the primary hard error this capability advertises: zero or negative energy cannot define a real offset target, and zero wattage or sun hours cannot produce energy. Prefer sending numbers rather than pre-formatted strings when you control the client; when you must send strings, avoid unit suffixes or thousand separators that would fail the finite-number pattern. Send a JSON body with required monthly_kwh (aliases monthly_usage_kwh, usage_kwh, kwh_month, energy_kwh, monthly_usage, kwh), required panel_wattage in watts (aliases panel_watts, wattage, watts, panel_w, module_watts, W), and required peak_sun_hours (aliases sun_hours, psh, peak_hours, insolation_hours, hours). Optional system_efficiency is a number greater than zero and at most one (aliases efficiency, derate, performance_ratio, pr, system_derate; default zero point eight). Optional days_per_month converts monthly usage to a daily target (aliases days, month_days, billing_days; default thirty; must be positive and at most thirty-one). All required fields must be finite and strictly positive within the published absolute ranges so intermediate products remain finite in IEEE double precision on the Worker and in the browser. On success the response echoes rounded monthly_kwh, panel_wattage, peak_sun_hours, system_efficiency, and days_per_month; publishes daily_kwh, panel_kw, daily_kwh_per_panel, panels_exact, panels_needed and panel_count (same whole number), system_dc_kw, estimated_daily_kwh, estimated_monthly_kwh, unit as item, formula as the human-readable equation string, message as a short English summary, and steps as an ordered array of substitution lines. Failed validation—missing fields, non-finite numbers, non-positive required inputs, efficiency outside the open-closed interval, or values outside absolute limits—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item.
Practical tips, limits, and related searches
When teaching or writing tests, pair a typical nine-hundred kilowatt-hour month with four-hundred-watt modules and four-and-a-half peak sun hours, then include a deliberate zero or negative monthly usage to prove the invalid_input path. This landing targets people searching for a solar panel count calculator, how many solar panels they need, monthly kilowatt-hour to panel count conversion, or peak sun hours sizing online; it is not a roof-area layout tool, not a battery or hybrid inverter designer, not a net-metering tariff optimizer, and not a live weather service. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when you deliberately send zero usage to verify error handling. Round-trip checks are easy: recompute monthly divided by days, divide by panel kilowatts times peak sun hours times efficiency, take the ceiling, and confirm the published panels_needed matches within the same twelve-decimal rounding used for all numeric fields.
Integration use cases
Size a rooftop array from a utility bill
Homeowners enter average monthly kWh, chosen module wattage, and local peak sun hours to get panels_needed and system_dc_kw before requesting installer quotes.
Landlord multi-unit offset planning
Property managers feed aggregated meter usage into the API and attach panel_count, steps, and estimated monthly production to a transparent capital plan.
Golden fixtures for solar apps
Developers generate known-good panel-count cases plus deliberate non-positive usage rejections for unit tests and continuous integration.
Technical questions
How is the number of solar panels calculated?
Daily demand is monthly_kwh divided by days_per_month (default 30). Daily production per panel is (panel_wattage/1000) × peak_sun_hours × system_efficiency (default 0.8). panels_needed is the ceiling of daily demand divided by daily production per panel.