Water Barrel Thermal Calculator

Number of barrels

Total thermal mass scales with barrel count.

Volume per barrel (L)

1 L ≈ 1 kg of water.

Initial water temperature (°C)

Measured at start of the period.

Ambient temperature (°C)

Use average air temperature over the period.

Exposure duration (hours)

Daytime heating or night cooling window.

Solar heat gain per barrel (W)

Set 0 for shade/night conditions.

Use dimensions to estimate area

Area affects heat loss and gain.

Barrel diameter (m)

Typical 200 L drum is ~0.58 m.

Barrel height (m)

Used with diameter to estimate surface area.

Area per barrel (m²)

Enter exposed heat‑exchange area directly.

Exposed fraction (0–1)

Use 0.7 if partially sheltered.

Insulation R-value (m²·K/W)

Higher reduces heat loss; 0 means uninsulated.

Outside heat transfer coefficient (W/m²·K)

Typical: 5–12 depending on wind.

Calculation uses water heat capacity and a simple loss model.

Example Data Table

Scenario	Barrels	Volume (L)	T₀ (°C)	T_a (°C)	Hours	Solar (W/barrel)	R (m²·K/W)	h (W/m²·K)	Notes
Sunny patio	2	200	22	16	6	140	0.10	8	Dark barrel absorbs more heat.
Greenhouse buffer	4	120	24	18	10	70	0.25	6	Partial shielding lowers exposed fraction.
Cold night check	3	200	18	-2	8	0	0.15	10	Use freeze estimate for protection planning.

Use these as starting points, then adjust for your location.

Formula Used

Water mass

m = N × V_L × ρ

ρ ≈ 1 kg/L for water.

Thermal capacity

C = m × c_p

c_p ≈ 4186 J/(kg·°C).

Overall U-value

U = 1 / (R + 1/h)

R is insulation resistance; h is outside transfer.

Overall conductance

UA = U × A

A is total exposed area of all barrels.

Temperature prediction (first‑order model)

T(t)=T_a+(T₀-T_a-Q/UA)e^-UA·t/C+Q/UA

Q is total constant heat input (W), t is seconds, and C is J/°C. If UA is near zero, the tool uses T(t)=T₀+Q·t/C.

How to Use This Calculator

Enter the number of barrels and the water volume per barrel.
Measure the current water temperature and estimate average ambient temperature.
Set the exposure duration to match your day or night window.
Add solar heat gain for sunny periods, or set it to zero.
Choose area mode: use dimensions or enter area directly.
Adjust exposed fraction, insulation R-value, and heat transfer coefficient.
Press Calculate to view results above the form.
Use Download CSV or Download PDF for reports.

FAQs

1) What does solar heat gain mean here?

It represents average heating power absorbed by one barrel from sun. Use a lower value for light colors, shade, or cloudy conditions.

2) How do I estimate the solar gain value?

Start with 50–150 W per barrel for mild sun. Calibrate by observing water temperature rise on a similar day, then adjust until predictions match.

3) What insulation R-value should I use?

Uninsulated barrels can be near 0. Thin foam wraps may be 0.1–0.3 m²·K/W. Higher values slow losses and make night buffering stronger.

4) Why is the outside heat transfer coefficient important?

Wind increases heat transfer and cools barrels faster. Calm indoor air might be near 5 W/m²·K, while breezy outdoor conditions can exceed 12 W/m²·K.

5) Is this accurate for real gardens and greenhouses?

It’s a practical estimate. Real conditions include radiation, varying wind, and changing sun. Use it for comparisons and planning, then fine‑tune with observations.

6) What does the time constant tell me?

It indicates how quickly temperature approaches a new balance. Larger time constants mean steadier temperatures, which is helpful when you want gentle night‑time buffering.

7) How should I use the freeze estimate safely?

Treat it as a conservative planning signal. If sensitive plants are involved, add safety margin, consider multiple nights, and verify with a thermometer during the coldest hours.