Enter your system details
Example data
| Scenario | Battery setup | Limits | Load | Estimated runtime |
|---|---|---|---|---|
| Pond pump overnight | 12 V 100 Ah × (1S, 2P) | DoD 50%, health 95%, temp 90% | 120 W, 60% duty, 10 W overhead | ≈ 6h 25m |
| Grow lights backup | 12 V 200 Ah × (2S, 1P) | DoD 80%, health 90%, temp 100% | 300 W, 100% duty, 15 W overhead | ≈ 8h 35m |
| Irrigation controls | 12 V 50 Ah × (1S, 1P) | DoD 50%, health 100%, temp 100% | 20 W, 20% duty, DC load | ≈ 30h 00m |
Examples are simplified and should be validated with real measurements.
Formula used
- Bank voltage:
Vbank = Vbattery × series - Bank capacity:
Ahbank = Ahbattery × parallel - Nominal energy:
Wh = Vbank × Ahbank - Usable DC energy:
Whusable = Wh × DoD × Health × Temp - Average load:
Wavg = (Wload × Duty) + Overhead - Delivered energy (AC loads):
Whdelivered = Whusable × Efficiency - Runtime:
Hours = Whdelivered ÷ Wavg
How to use this calculator
- Enter the battery voltage and capacity for one unit.
- Set how many batteries are in series and how many strings are in parallel.
- Type your running watts and estimate the duty cycle for cycling equipment.
- Choose AC via inverter or direct DC, then set efficiency and overhead if needed.
- Apply conservative limits for discharge, health, and temperature.
- Press calculate, review runtime and current, then export CSV or PDF.
Battery bank energy in watt-hours
Runtime begins with stored energy. A 12 V, 100 Ah battery holds about 1,200 Wh. Wire two in series for 24 V at 100 Ah, still 2,400 Wh. Wire two strings in parallel for 24 V at 200 Ah, giving 4,800 Wh. This calculator separates series and parallel so voltage and capacity are treated correctly. Cold temperatures can reduce available capacity by 10–30% in sheds.
Depth of discharge and battery health
Usable energy is rarely 100%. Many lead‑acid banks are planned around 50% depth of discharge to extend life, while lithium systems may allow 80–90% depending on the model. Aging also reduces capacity; a 90% health factor means a 100 Ah label behaves like 90 Ah. Using these limits prevents optimistic runtime estimates.
Inverter efficiency and real AC output
If your load is AC, inverter losses matter. Typical efficiencies range from 85% to 95% at moderate power. For example, 2,000 Wh usable DC energy at 90% efficiency delivers about 1,800 Wh to the load. Small loads can be less efficient because of standby draw, so the calculator includes an optional fixed inverter overhead.
Load profile, duty cycle, and startup surge
Garden equipment often cycles. A pond pump might run 60% of the hour, and irrigation timers may run only minutes. Average watts equals rated watts multiplied by duty cycle. Startup surge affects inverter sizing more than runtime; however, logging surge helps you spot loads that could trip protection or waste energy during frequent starts. Short field tests help tune duty cycle and overhead assumptions quickly.
Planning for safe currents and cable losses
Higher load on lower voltage increases current. A 600 W load on a 12 V bank at 90% efficiency draws roughly 56 A. High current can heat cables and reduce voltage at the inverter, shortening runtime. When currents look high, consider moving to 24 V or 48 V, increasing conductor size, and keeping leads short.
FAQs
1) Why does series wiring change voltage but not amp-hours?
Series adds battery voltages end-to-end, so voltage increases. The same current flows through each battery, so amp-hours stay the same for the string.
2) What depth of discharge should I use for planning?
For lead-acid banks, 50% is a common conservative limit. Many lithium systems can plan 80–90%, but always match your battery specification and desired lifespan.
3) How do I estimate duty cycle for a pump?
Measure on/off time over an hour. If it runs 15 minutes each hour, duty cycle is 25%. Use the average over typical weather and water demand patterns.
4) Should inverter overhead be included for short runtimes?
Yes. Standby draw can be significant for small garden loads. If your inverter stays on, overhead is continuous and can noticeably reduce runtime.
5) Why does cold reduce runtime?
Battery chemistry delivers fewer usable amp-hours at low temperatures. Cold also increases internal resistance, causing voltage sag under load, which can trigger cutoffs earlier.
6) Can I use this for direct DC loads?
Yes. Select direct DC load to avoid inverter efficiency and overhead. The calculator will estimate runtime from usable battery energy and your average DC watts.