Interactive Calculator · 2026
Solar Water Heater Size Calculator
Size a solar water heater from your household hot-water demand. Enter the number of people, how much hot water each uses per day and your system yield — get the recommended solar tank in LPD, the collector area in m² and the number of standard panels to install.
Daily hot-water demand → nearest standard solar tank size
Your household
People drawing hot water from the system daily.
A bucket bath is ~15–20 L hot; a shower ~25–35 L. 25 L is a typical Indian default.
Depends on climate and collector type — ETC (evacuated tube) and warmer, sunnier regions sit higher; FPC (flat plate) and cooler regions lower. ~60–70 L/m² is a common India-wide figure.
Daily demand vs recommended tank
The tank is rounded up to the nearest standard size so it covers a full day's hot water.
Your household needs about 100 L of hot water a day, so a 100 LPD solar tank with roughly 1.43 m² of collector (about 1 panel) is a sensible fit.
Solar always needs a backup — an electric element in the tank or a separate geyser — for cloudy and monsoon days when the collector cannot fully heat the store.
How this is calculated
- Daily hot-water demand = persons × per-person = 4 × 25 = 100 L/day.
- Recommended solar tank = next standard size ≥ demand, from 100 / 150 / 200 / 250 / 300 / 500 LPD = 100 LPD.
- Collector area = demand ÷ system yield = 100 ÷ 70 = 1.43 m².
- Standard panels = ⌈collector area ÷ 2⌉ = ⌈1.43 ÷ 2⌉ = 1 (≈ 2 m² per standard panel).
Indicative sizing for concept planning. A south-facing, unshaded roof is required, and subsidies may apply — check current MNRE and state schemes. Confirm actual capacity, collector type (ETC vs FPC) and yield with a qualified installer or manufacturer data before procurement.
Frequently asked questions
- How does the solar water heater size calculator work?
- It multiplies the number of persons by the hot water each uses per day to get your daily demand in litres, then rounds up to the nearest standard solar tank size — typically 100, 150, 200, 250, 300 or 500 LPD. It divides the same demand by your system yield to estimate the collector area in square metres, and assumes roughly 2 square metres per standard panel.
- What inputs do I need and what values are sensible for India?
- You need the number of people drawing hot water daily, hot water per person per day, and your system yield. A bucket bath is roughly 15 to 20 litres and a shower about 25 to 35 litres, so 25 litres per person is a typical Indian default. A yield near 60 to 70 litres per square metre suits most regions; evacuated tube collectors and sunnier areas sit higher.
- Do I still need a backup with a solar water heater?
- Yes. Solar output drops on cloudy and monsoon days, so almost every system keeps a backup — an electric element inside the tank or a separate geyser — to guarantee hot water when the collector cannot fully heat the store. Also confirm you have a south-facing, unshaded roof, and check current MNRE and state subsidy schemes before you buy. Treat these figures as indicative and verify with a qualified installer.
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