How to Size a Rooftop Solar System From Your Electricity Bill
Turn your monthly bill into a system size in five steps: bill to daily load, worst-month sun hours, array math, roof check, and battery option — with worked examples at 2, 5, and 10 kW.
SolarQna Engineering · Engineering team · Sizing math matches the SolarQna calculator engine (PR 0.78 default)
Introduction
Most rooftop systems are sized wrong in exactly one place: the designer uses average sunshine instead of worst-month sunshine. This guide sizes from your electricity bill in five steps, so the system you buy covers your loads when the sun is weakest — not just when it is strongest.
You need one number to start: your average monthly consumption in kWh, from any recent bill. Everything else is arithmetic you can check.
Quick answer
Daily load (kWh) = monthly bill kWh ÷ 30. Array (kW) = daily load ÷ (worst-month peak sun hours × 0.78). A home using 450 kWh/month in a 4.5 sun-hour location needs roughly 4.3 kW. Round up to the nearest standard size and verify roof space before paying anything.
Design for your worst month, not your average
The one rule. Size for the month with the least sun — monsoon months in India (often 2–3.5 peak sun hours), winter in temperate regions. A system sized for the average runs short for months at a time. A system sized for the worst month has surplus the rest of the year.
Typical bands (verify your city with the Global Solar Atlas before buying): most of India 4–6 sun hours in good months; US sunbelt 5–7; Germany 2–4 in winter. Your DISCOM bill tells you consumption; only a solar resource map tells you sunshine.
Step 1 — Bill to daily load
Find your daily load
Take 2–3 bills, average the kWh (not the rupees), and divide by 30. A 450 kWh/month home uses 15 kWh/day. If you only know the bill amount, divide by your tariff per kWh first — tariffs change; units don't.
If you don't have bills
Build the load appliance by appliance with the Home Load Builder (Tools → Home Load Builder): it totals daily kWh, splits essential vs comfort loads, and hands the number straight into the sizing calculator.
Step 2 — Array math
The formula
Array kW = daily load kWh ÷ (worst-month sun hours × 0.78). The 0.78 performance ratio is our calculator default — it bundles wiring, inverter, temperature, and soiling losses into one checkable number.
Work it
15 kWh/day ÷ (4.5 × 0.78) = 15 ÷ 3.51 = 4.27 kW → buy 5 kW (standard sizes: 1, 2, 3, 5, 10 kW). Rounding up covers degradation (~0.5%/year in our 25-year model) without overspending.
Step 3 — Roof check
Space rule
Modern ~550 W panels need roughly 2.6 m² each including access gaps — a 5 kW system (9–10 panels) wants about 25 m² of shade-free roof. South-facing (northern hemisphere) at roughly your latitude angle is the default; east-west splits work with ~10–15% lower yield.
Shade kills strings
One shaded panel drags its whole series string. Map shade at 9 am, noon, and 4 pm before finalizing layout — or ask for module-level optimizers/microinverters on shaded roofs (see the inverter guide).
Step 4 — Battery: optional, sized separately
Grid-tied first
If your grid is reliable and net metering exists, skip the battery at first — it is roughly half the system cost and the grid is your night-time battery. Add storage later for outages.
Off-grid or backup
Size batteries for 2–3 cloudy days: battery kWh ≈ daily load × autonomy days ÷ usable depth (0.8 for LiFePO₄). Our 15 kWh/day home wanting 2 days needs ~37 kWh of LiFePO₄. Run the exact numbers in Tools → Battery Autonomy.
Worked examples at 2, 5, and 10 kW
2 kW — small home (150 kWh/month)
Daily load 5 kWh ÷ (4.5 × 0.78) = 1.4 kW → 2 kW (4 × 550 W panels, ~10 m²). Single-phase string inverter around 2–3 kW.
5 kW — family home (450 kWh/month)
Daily load 15 kWh ÷ 3.51 = 4.27 kW → 5 kW (9–10 panels, ~25 m²). The most common residential size; pairs with a 5 kW hybrid if backup is wanted later.
10 kW — large home / small commercial (900 kWh/month)
Daily load 30 kWh ÷ 3.51 = 8.5 kW → 10 kW (18–20 panels, ~50 m²). Check sanctioned load and net-metering caps with your DISCOM — several states cap residential exports.
What it costs (and what changes the price)
Indicative math, not a quote
Our economics model defaults to $1,000/kW turnkey — a 5 kW system ≈ $5,000 before support. Real quotes move with structure (flat vs shed), wiring runs, panel class, battery inclusion, and installer margin. Get three line-item quotes and decode them with the quote-decoder guide.
Support that actually applies
Grid-tied residential systems in India can claim PM Surya Ghar assistance (slabbed by kW, DCR modules required); off-grid systems cannot — the calculator's subsidy check enforces this. US residential systems use the 30% Section 25D credit. Amounts change; the subsidy check in Tools shows current modeled slabs with sources.
5 sizing mistakes that cost real money
- Designing for average sun. Covered above — the single most expensive error.
- Sizing the inverter to the average, not the peak. Motors surge 3–5× on startup; a freezer plus pump starting together trips an undersized inverter.
- Buying panels before the roof survey. Structure, shade, and sanctioned load change the design more than panel brand does.
- Mixing panel wattages on one string. The string inherits the weakest panel's current — keep strings uniform.
- Sizing batteries to nameplate, not usable capacity. A 10 kWh LiFePO₄ bank at 80% usable depth gives 8 kWh, not 10.
Frequently asked questions
Sizing questions answered from the calculator engine — open the full FAQ below for the checkable versions.
Key takeaways
- Bill kWh ÷ 30 = daily load; daily load ÷ (worst-month sun hours × 0.78) = array kW.
- Round up to standard sizes (1/2/3/5/10 kW); verify ~5 m² of clear roof per kW.
- Batteries are a separate decision: 2–3 days autonomy at 80% usable depth for LiFePO₄.
- Off-grid systems are excluded from grid-tied subsidies — verify before budgeting.
Sources
Methodology sources below — figures in this guide derive from our calculator defaults unless a source says otherwise.
Frequently asked questions
How many kW do I need for 500 units (kWh) per month?+
About 5 kW in most Indian locations. 500 ÷ 30 = 16.7 kWh/day; at 4.5 worst-month sun hours × 0.78 performance ratio that is 4.75 kW, rounded to the standard 5 kW size. Verify your city's worst-month sun hours before buying.
What is a peak sun hour?+
One hour of 1,000 W/m² equivalent sunshine. Five sun hours can mean ten weak hours or five strong ones — only the total energy matters for sizing. Always use your worst month's value, not the yearly average.
How much roof space does 1 kW need?+
Roughly 5 m² of shade-free roof per kW with modern ~550 W panels, including access gaps. A 5 kW system wants about 25 m² facing broadly south (northern hemisphere) at roughly your latitude tilt.
Can I add batteries later to a grid-tied system?+
Yes, easiest with a hybrid inverter bought upfront (AC- or DC-coupled storage retrofits exist but cost more). If outages are likely, buying hybrid now is cheaper than replacing a string inverter later.
Does shading on one panel really matter?+
Yes — in a series string, the shaded panel throttles the whole string to its reduced current. On partly shaded roofs, use module-level optimizers or microinverters, or split orientations across MPPTs.
Why 0.78 and not the nameplate rating?+
Nameplates are lab conditions. Wiring (~2%), inverter (~3–4%), heat (5–15% in hot climates), soiling, and mismatch stack to roughly 22% real-world loss. 0.78 is our calculator default and deliberately conservative.
Is net metering required for rooftop solar to pay back?+
Not required, but it shortens payback substantially where exports earn credit. Without it, size for daytime self-consumption and consider a small battery for evening loads instead of oversizing the array.
Sources
- Global Solar Atlas (World Bank / Solargis) — PVOUT and GHI maps by location (2026-10-10). globalsolaratlas.info
- MNRE — PM Surya Ghar: Muft Bijli Yojana — scheme guidelines (2026-10-10). pmsuryaghar.gov.in
- MNRE — Approved List of Models and Manufacturers (ALMM) (2026-10-10). mnre.gov.in
- BIS — IS 14286 — terrestrial PV module design qualification (2026-10-10). www.bis.gov.in
- CEA — Technical Standards for Connectivity of Distributed Generation (2026-10-10). cea.nic.in
- IEC — IEC 61730 — PV module safety qualification (2026-10-10). www.iec.ch
- SolarQna — PV sizing, battery autonomy, and net-metering calculator defaults (PR 0.78, 25-yr model) (2026-10-10). solarqna.com/en/tools