Generator Sizing Calculator

ANA›Life Services Authority›National Calculator Authority›Generator Sizing Calculator

.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Generator Sizing Calculator

Estimate the minimum generator size needed to power your electrical loads, accounting for power factor and a recommended safety margin.

Total Connected Load (Watts)

Sum of all appliance/equipment wattages running simultaneously.

Motor Starting Factor

Multiplier for motor inrush current (typically 1.25–2.0; use 1.0 if no motors).

Power Factor (0.1 – 1.0)

Typical value: 0.8 for mixed loads; 1.0 for purely resistive loads.

Safety Margin (%)

Recommended 20–25% headroom above calculated load.

System Voltage (V)

Single-phase: 120 or 240 V; Three-phase: 208, 480 V, etc.

Phase

Single-Phase Three-Phase

Calculate

function genCalc() { const resultDiv = document.getElementById('gen-result'); resultDiv.style.display = 'block'; resultDiv.className = 'calc-result';

const watts = parseFloat(document.getElementById('gen-total-watts').value); const startFactor = parseFloat(document.getElementById('gen-starting-factor').value); const pf = parseFloat(document.getElementById('gen-power-factor').value); const marginPct = parseFloat(document.getElementById('gen-safety-margin').value); const voltage = parseFloat(document.getElementById('gen-voltage').value); const phase = parseInt(document.getElementById('gen-phase').value);

// --- Validation --- if (isNaN(watts) || watts 3) { resultDiv.className = 'calc-result calc-error'; resultDiv.innerHTML = '⚠ Motor Starting Factor must be between 1.0 and 3.0.'; return; } if (isNaN(pf) || pf 1.0) { resultDiv.className = 'calc-result calc-error'; resultDiv.innerHTML = '⚠ Power Factor must be between 0.1 and 1.0.'; return; } if (isNaN(marginPct) || marginPct 100) { resultDiv.className = 'calc-result calc-error'; resultDiv.innerHTML = '⚠ Safety Margin must be between 0% and 100%.'; return; } if (isNaN(voltage) || voltage 15000) { resultDiv.className = 'calc-result calc-error'; resultDiv.innerHTML = '⚠ System Voltage must be between 100 V and 15,000 V.'; return; }

// --- Core Calculations --- // Step 1: Apply motor starting factor to account for inrush const adjustedWatts = watts * startFactor;

// Step 2: Convert W → kVA using power factor // kVA = W / (PF × 1000) const kvaBase = adjustedWatts / (pf * 1000);

// Step 3: Apply safety margin // kVA_required = kVA_base × (1 + margin/100) const kvaRequired = kvaBase * (1 + marginPct / 100);

// Step 4: kW required (real power) // kW = kVA × PF const kwRequired = kvaRequired * pf;

// Step 5: Required current
// Single-phase: I = (kVA × 1000) / V
// Three-phase: I = (kVA × 1000) / (√3 × V)
const sqrt3 = Math.sqrt(3);
const current = phase === 1
? (kvaRequired * 1000) / voltage
(kvaRequired * 1000) / (sqrt3 * voltage);

// Step 6: Suggest standard generator size (round up to nearest common size) const standardSizes = [1,1.5,2,2.5,3,3.5,4,4.5,5,6,7,7.5,8,9,10,12,12.5, 15,17.5,20,22,25,30,35,40,45,50,60,75,80,100, 125,150,175,200,250,300,350,400,500,600,750,1000]; let recommended = standardSizes.find(s => s >= kvaRequired); if (!recommended) recommended = Math.ceil(kvaRequired / 50) * 50;

// --- Output --- resultDiv.innerHTML = ` ### ⚡ Generator Sizing Results

ParameterValue Connected Load${watts.toLocaleString()} W (${(watts/1000).toFixed(2)} kW) After Starting Factor (×${startFactor})${adjustedWatts.toLocaleString()} W Power Factor${pf.toFixed(2)} Base Apparent Power${kvaBase.toFixed(2)} kVA Safety Margin Applied (${marginPct}%)+ ${(kvaRequired - kvaBase).toFixed(2)} kVA Minimum Generator Size${kvaRequired.toFixed(2)} kVA / ${kwRequired.toFixed(2)} kW Recommended Standard Size${recommended} kVA Estimated Full-Load Current (${phase === 1 ? 'Single' : 'Three'}-Phase, ${voltage} V)${current.toFixed(1)} A

ⓘ Always consult a licensed electrician before purchasing or installing a generator.

`; }

#### Formulas Used

1. Adjusted Load (W) Adjusted Load = Connected Load (W) × Motor Starting Factor

2. Base Apparent Power (kVA) kVAbase = Adjusted Load (W) ÷ (Power Factor × 1000)

3. Required kVA with Safety Margin kVArequired = kVAbase × (1 + Safety Margin ÷ 100)

4. Required Real Power (kW) kW = kVArequired × Power Factor

5. Full-Load Current Single-phase: I = (kVA × 1000) ÷ V Three-phase: I = (kVA × 1000) ÷ (√3 × V)

#### Assumptions & References

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References

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