Measure the actual inflow and delivered pump flow first; then check storage, cycling, discharge route, alarm and backup power as separate parts of one system.
1. Observe water levels and inflow safely
Record the clear pit diameter, pump-on level, pump-off level and the time required for the water to rise between them. Repeat during several wet conditions instead of relying on one dry-day observation.
Do not enter a pit or touch energized equipment in standing water. Use marked level observations and qualified help where electrical or confined-space hazards exist.
2. Separate storage from pump capacity
Storage between the control levels determines how long the pit can accept inflow before a start. Delivered pump flow determines how quickly the level falls while water continues to enter.
A larger nameplate flow is not enough if the real discharge head, pipe, fittings or check valve reduce delivery. Measure a level change or timed discharge at the installed condition.
3. Verify where the water goes
Confirm that the discharge is permitted, protected from freezing and directed away from the foundation without returning to the pit. Provide a check valve and accessible service points where required.
An undersized, blocked or frozen outlet can make a healthy pump appear inadequate. Inspect the full route, including the terminal and overflow path.
4. Size backup from wet-weather duty
Battery runtime depends on average duty during the outage, not only pump watts. Include inverter idle power and confirm that the inverter can start the motor without nuisance shutdown.
If consequences are serious, use a high-level alarm and an independent backup strategy. Test transfer, battery condition and alarm notification under controlled conditions.
5. Inspect and rehearse before the wet season
Clean debris, exercise floats, inspect the check valve and discharge, and compare current fill and drain times with the baseline record. Investigate a shorter cycle or longer run time.
Label isolation and power controls, keep the cover secure and document who responds to an alarm or extended outage.
Measurements that answer different questions
| Measurement | What it shows | Use |
|---|---|---|
| Level travel | Active pit storage | Time between starts |
| Wet-weather inflow | Incoming load | Required capacity and backup duty |
| Delivered pump flow | Installed performance | Drain time and margin |
| Pump input power | Electrical demand | Battery and inverter check |
Worked example
A small residential pit
Inside diameter 60 cm, 20 cm control travel, 12 L/min inflow and 80 L/min delivered pump flow.
Active volume = π × 0.60² / 4 × 0.20 × 1000 = 56.55 L Fill time = 56.55 / 12 = 4.71 min Drain time = 56.55 / (80 − 12) = 0.83 min Starts = 60 / (4.71 + 0.83) = 10.82 per hour
Compare the result with the pump maker’s cycling limits, then test the alarm and backup under the same wet-weather assumption.
What to have ready
- Pit diameter and measured control levels
- Wet-weather fill-time observations
- Delivered flow at the installed head
- Discharge route and freeze protection
- Alarm, backup power and test record
Common mistakes
- Using the pump’s free-flow rating as installed delivery.
- Sizing backup from pump watts but ignoring duty cycle and idle load.
- Ignoring water that returns through a failed check valve.
- Testing only during dry weather.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.