Well Pump HP Sizing: What Size Pump Do I Need?

Sizing Factors

1. Well Depth and Pump Setting Depth

Well depth is the total drilled depth of the well, but the pump setting depth is where the pump actually sits — typically 10-20 feet above the bottom of the well. The pump needs to lift water from its setting depth all the way to the surface and then push it through the plumbing system to your pressure tank. A well drilled to 400 feet with a pump set at 380 feet requires a pump that can handle at least 380 feet of vertical lift plus friction and pressure.

In areas like Julian, Palomar Mountain, or the higher elevations of Anza, we commonly drill wells 400-700 feet deep. These wells typically require 1.5 HP to 3 HP submersible pumps to deliver adequate flow. By contrast, wells in lower Temecula or Escondido valley floors may only be 150-250 feet, needing just 3/4 HP to 1 HP.

2. Static Water Level

The static water level is where water naturally sits in your well when the pump isn't running. This is your starting point for lift calculations — not the total well depth. If you have a 300-foot well but the static water level is at 80 feet, the pump only needs to lift water from 80 feet (plus drawdown), not 300. Your well driller measures this during well completion and records it on your well log.

Static water levels change seasonally in many Southern California areas. During drought years, we've seen water levels drop 20-50 feet in some Riverside County wells, which means a pump that was properly sized five years ago may now be undersized. If you're noticing declining pressure during dry months, this is often the cause — not a failing pump.

3. Drawdown

Drawdown is how far the water level drops below static when the pump is running at full capacity. A well with a static level at 100 feet and a drawdown of 40 feet has a pumping water level of 140 feet. This pumping water level — not the static level — is what you must use for sizing calculations. Wells with high drawdown (50+ feet) need more powerful pumps than wells with stable water levels.

Drawdown varies hugely depending on aquifer type. Fractured-rock wells in the mountains often have significant drawdown (30-80 feet or more) because water flows through narrow fractures. Alluvial wells in valley floors typically have low drawdown (5-20 feet) because water moves freely through sand and gravel. Your well's pump test — conducted by the driller when the well was completed — documents this critical number.

4. GPM Requirements

Gallons per minute (GPM) is how much water your pump needs to deliver during peak demand. The standard rule of thumb is 1 GPM per fixture for simultaneous peak use. Count every faucet, shower, toilet, dishwasher, washing machine, and hose bib in your home. A typical 3-bedroom, 2-bathroom home has about 10-12 fixtures and needs 8-12 GPM. Larger estates with guest houses, pools, or agricultural use may need 20-50+ GPM.

Important: your well's production rate also matters here. If your well only produces 5 GPM (common in fractured-rock areas like Julian or Warner Springs), you can't size a pump for 12 GPM — it will outpace the well and pull air. In low-production wells, the strategy is to pump slowly into a storage tank and use a booster pump for household pressure. This is common in rural San Diego County and we install these systems frequently.

5. Pressure Requirements

Most residential pressure tanks operate on a 40/60 PSI cycle (pump kicks on at 40 PSI, shuts off at 60 PSI). Some homeowners prefer 50/70 for more consistent pressure, especially in multi-story homes or properties with long runs from the well to the house. Higher pressure settings increase the Total Dynamic Head calculation and may push you into a larger HP pump.

Constant pressure systems (like the Franklin SubDrive or Grundfos SQFlex) use variable frequency drives to maintain a steady pressure regardless of demand. These systems can sometimes allow a smaller HP motor because they don't need to hit a high cutoff pressure — they just maintain a steady 55 PSI. However, VFD systems cost more upfront ($800-1,500 for the drive alone).

6. Pipe Size, Length, and Friction Loss

Every foot of pipe between your pump and your pressure tank creates friction that the pump must overcome. This friction loss depends on pipe diameter, pipe material, flow rate, and total length including horizontal runs. Using 1-inch pipe with 10 GPM flowing through it creates about 7.5 feet of friction loss per 100 feet of pipe. Upgrade to 1.25-inch pipe and that drops to about 3 feet per 100 feet. On deep wells, this difference can add up to 20-30+ feet of additional head — enough to bump you from a 1 HP to 1.5 HP pump.

We always recommend 1.25-inch drop pipe on wells deeper than 200 feet. The cost difference is minimal ($1-2 more per foot), but the reduction in friction loss and improved pump efficiency pays for itself many times over in pump longevity and lower electricity bills.

HP Comparison

Note: These are guidelines. Actual sizing requires TDH calculation.

How to Calculate

Total Dynamic Head (TDH)

TDH = Static Head + Friction Loss + Pressure

Components

• Static Head: Vertical distance from pumping water level to pressure tank
• Friction Loss: Resistance from pipe (lookup in tables)
• Pressure: Convert PSI to feet (PSI × 2.31)

Example Calculation

• Well depth: 200 feet
• Static water level: 50 feet
• Drawdown: 20 feet
• Pumping level: 70 feet
• Vertical lift to tank: 80 feet (70 + 10 above ground)
• Friction loss: 15 feet (200 ft of 1" pipe at 10 GPM)
• Pressure: 50 PSI × 2.31 = 116 feet
• TDH = 80 + 15 + 116 = 211 feet

Match to Pump Curve

• Each pump has a performance curve
• Find intersection of TDH and desired GPM
• Choose pump that operates in efficient range

Pump Types

Submersible Pumps

• Most common for deep wells
• Sits underwater in well
• Quiet, efficient, reliable
• Available 1/2 HP to 10+ HP

Jet Pumps (Shallow Well)

• For wells under 25 feet
• Sits above ground
• Easy to service
• Typically 1/2 to 1 HP

Jet Pumps (Deep Well)

• Two-pipe system
• Jet assembly in well, pump above ground
• Works 25-100+ feet
• Typically 3/4 to 1.5 HP

2-Wire vs 3-Wire

• 2-Wire: Control box in motor (down in well)
• 3-Wire: Control box above ground
• 3-wire easier to diagnose/repair control issues
• Both available in various HP

Problems with Oversized Pumps

Short Cycling Destroys Motors

An oversized pump fills the pressure tank far too quickly, causing the pump to turn on and off every few minutes instead of running for longer, healthier cycles. Each startup draws 3-5 times the running amperage, creating a massive surge of heat in the motor windings. A properly sized pump might cycle 4-6 times per hour; an oversized pump can cycle 20+ times per hour. At that rate, a pump that should last 10-15 years may burn out in 3-5. We've pulled pumps from wells in Ramona and Fallbrook that were only 2-3 years old because someone installed a 1.5 HP motor where a 3/4 HP was correct.

Well Damage and Sand Production

When a pump pulls water faster than the aquifer can replenish it, the water level drops below the pump intake. This creates turbulent flow conditions near the well screen or perforations, which can pull fine sand and sediment into the well. Over time, this sand erodes the pump stages, clogs the check valve, and fills the bottom of the well — reducing its effective depth. In severe cases, an oversized pump can actually collapse the formation around the screen, permanently reducing well yield. We've cleaned out wells in Anza that had 30-40 feet of sand packed in the bottom because of years of overpumping.

Higher Energy Costs

A 1.5 HP motor draws about 10-11 amps at 230V; a 3/4 HP motor draws about 6-7 amps. If the 3/4 HP would have been correct, you're paying 50% more in electricity every hour the pump runs — and the short cycling means it runs more often. Over a year, this can add $200-400 to your electric bill versus a properly sized pump.

Mitigation (If You Already Have an Oversized Pump)

If replacing the pump isn't immediately feasible, a cycle stop valve (CSV) can help by throttling the pump output to match demand, reducing short cycling. A larger pressure tank (85+ gallon) also extends the time between cycles. A constant pressure system with VFD is the best solution short of replacing the pump itself. But ultimately, replacing with the correct HP is the only real fix — band-aids just delay the inevitable motor failure.

Problems with Undersized Pumps

Symptoms of an Undersized Pump

The most obvious sign is low pressure when multiple fixtures run simultaneously — the shower pressure drops noticeably when someone flushes a toilet or runs the dishwasher. You may also notice the pump running constantly without ever reaching the cut-off pressure, or taking an unusually long time to fill the pressure tank. In extreme cases, the pump simply can't maintain usable pressure during morning or evening peak usage.

Why Undersized Pumps Fail Early

An undersized pump runs at or near full capacity for extended periods — sometimes continuously for hours. Submersible pump motors are cooled by the water flowing past them; when the motor runs at maximum for too long, it generates excess heat that the water can't adequately dissipate. The motor windings degrade, insulation breaks down, and eventually the motor shorts out. We typically see undersized pumps fail in 4-6 years instead of the normal 10-15 year lifespan. The cost of one premature pump replacement ($2,000-5,000 installed) far exceeds the price difference between a 3/4 HP and a 1 HP pump at initial installation (usually just $100-200 for the motor upgrade).

Solutions for Undersized Pumps

The best solution is replacing with a properly sized pump based on current TDH and GPM calculations. If your household water needs have grown (added bathrooms, pool, irrigation) since the original installation, this is especially important. For situations where the well itself can't produce enough GPM, a storage tank system is the answer: the undersized pump fills a 1,000-2,500 gallon storage tank slowly over time, and a separate booster pump delivers water to the house at the pressure and flow rate you need. This is a common setup on low-yield wells throughout rural San Diego and Riverside counties.

We service all major pump brands including Franklin Electric, Grundfos, Goulds (Xylem), and Sta-Rite (Pentair). Our trucks carry common parts and components for same-day repairs.

Frequently Asked Questions