Example Load Plan
Use this as a starting point for typical backyard irrigation and lighting.
| Item | Watts | Qty | Motor | Suggested surge factor |
|---|---|---|---|---|
| Surface water pump | 400 | 1 | Yes | 3.0 |
| Grow light strips | 120 | 2 | No | 1.0 |
| Pond aerator | 60 | 1 | Yes | 2.5 |
| Garden controller | 15 | 1 | No | 1.0 |
Formula Used
How to Use This Calculator
- Add every device you want powered in the garden.
- Enter watts from the nameplate or product manual.
- Set quantity, then mark motor loads like pumps.
- Use a surge factor for motors, usually 2–4.
- Choose battery voltage and realistic inverter efficiency.
- Add headroom if you expect future expansion.
- Optionally add battery Ah and DoD for runtime.
- Press Calculate, then download CSV or PDF.
Load profiling for garden backup
Garden power plans fail when only “typical watts” are counted. Build a load list from nameplates, manuals, or meter readings, then group devices by duty cycle. An irrigation pump may run 15–30 minutes per zone, while lighting may run 8–12 hours. This calculator totals continuous watts and highlights surge exposure.
Surge behavior of pumps and motors
Inductive loads draw a short starting current that can be 2–6× the running watts. Small centrifugal pumps often land near 3×, while older or high‑head pumps can exceed 4×. Using a surge factor avoids nuisance shutdowns. If your pump lists “starting watts” or “LRA,” enter that value directly and set surge factor to 1 for best accuracy.
Headroom and thermal derating
Real installations experience heat, cable losses, and future expansion. A 20–30% headroom setting is common for mixed garden loads. In hot sheds, inverter output can derate as internal temperature rises, so margin protects reliability. When you plan to add a second pump or extra lighting later, increase headroom rather than replacing equipment.
VA, power factor, and inverter ratings
Some inverters are sold in VA, not watts. Apparent power (VA) equals watts divided by power factor. Resistive loads sit near PF 1.0, while motors may be closer to 0.7–0.9. This calculator converts your recommendation to VA so you can match product labels. When PF is unknown, using 0.8 is a practical conservative default for motor‑heavy setups.
Battery current and runtime planning
Battery voltage strongly affects current. For the same AC load, 24V draws about half the DC current of 12V, reducing cable size and heating. Runtime uses usable battery energy: V × Ah × depth of discharge, then applies inverter efficiency. Treat the runtime figure as a planning estimate; real results vary with temperature, battery age, and pump pressure changes. After installation, verify with a plug-in watt meter. If readings differ by over 10%, adjust surge factor or headroom and re-check cable sizing and battery voltage. for seasonal changes.
FAQs
Should I size for watts or VA?
Use both. Match the inverter’s watt rating to running watts and its VA rating to your calculated VA, especially when pumps or motors reduce power factor.
What surge factor should I use for a water pump?
Start with 3.0 for small pumps. If you see startup trips, increase toward 4–5. If the manual lists starting watts or LRA, use that value directly.
Why does battery voltage matter?
Higher voltage lowers DC current for the same load. That reduces cable heating and voltage drop, helping the inverter start pumps more reliably.
How accurate is the runtime estimate?
It is a planning estimate. Temperature, battery age, wiring losses, and changing pump pressure can shift runtime. Use it to compare options, then validate with real measurements.
Do I need extra headroom if I add solar later?
Solar affects charging, not instantaneous inverter output. Add headroom when you expect more loads, longer run time demands, or high heat that can reduce inverter capacity.
Can I run sensitive controllers and timers?
Yes, but prefer a pure sine inverter and keep wiring tidy. For outdoor enclosures, protect electronics from moisture and heat, and use proper grounding and fusing.
Practical Notes
- Prefer pure sine inverters for pumps and controllers.
- Keep cable runs short and use correct wire gauge.
- Heat reduces performance; allow airflow around equipment.
- If your load is mostly motors, increase headroom.