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How to Size a Genset for a Retail Store or Small Office
GensetPedia Technical Desk · · 8 min read
The take
Retail and small-office genset sizing is usually simpler than it looks — lighting and POS/computing load, HVAC (with its motor-starting surge), and refrigeration if applicable cover most of the connected load. The standard 20-25% commercial margin applies here, not the higher critical-load margin a hospital or data center would use — this isn't a life-safety installation, and oversizing for one adds cost without adding real value.
Retail and small-office sizing questions come up constantly, and the reassuring answer is that these are some of the more straightforward genset sizing calculations — far simpler than a hospital or data center's critical-load math.
The three load categories that cover most of it
- Power and lighting. POS systems, computers, general lighting, signage lighting, and small appliances — steady, predictable load that rarely surprises anyone in the calculation.
- HVAC. Split or ducted AC units, which contribute both a steady running load and a startup surge from the compressor motor — this is the category most sizing mistakes come from.
- Refrigeration, where applicable — grocery, pharmacy, F&B, and similar retail formats carry continuous refrigeration load that, unlike most retail load, needs to be treated as always-on and non-negotiable in any outage-duration planning.
Why HVAC starting surge is the detail people miss
A steady-state wattage total for a store's air conditioning looks manageable on paper. The number that actually determines genset size is the starting surge from the compressor motor kicking in — often several times the running wattage for a second or two — which is exactly the calculation our load calculator handles by sizing against the largest single motor's starting draw rather than just summing running watts. Skipping this step is the single most common way a retail sizing estimate comes in too small.
A worked example
A small retail unit with LED lighting, a POS system, a couple of computers, and a 2-ton split AC unit might have a connected running load in the 4-6 kW range. Once the AC compressor's starting surge is factored in — not just its running wattage — plus the standard 20-25% commercial margin, that commonly lands in the 15-20 kVA range once rounded to a standard genset size. Every site's actual equipment list will move this number, but it illustrates why the AC unit, not the lighting or POS load, is usually what decides the final size.
Why the margin here is lower than a hospital's
Retail and office sizing uses the standard 20-25% commercial safety margin, not the 30-40% critical-load margin covered in our hospital ICU sizing guide — that higher margin exists specifically for simultaneous critical-equipment startup risk and zero brownout tolerance, neither of which typically applies to a retail floor or office. Applying hospital-grade margins to a retail sizing calculation isn't extra safety, it's unnecessary cost.
Full backup vs. essential-only
For most single-unit retail stores and small office floors, the total connected load is modest enough that full backup is simpler and not meaningfully more expensive than trying to tier essential vs. non-essential circuits — that tiering complexity tends to pay off more at a larger commercial or multi-tenant scale, similar to the common-area-vs-per-flat distinction covered in our apartment society sizing guide.
Getting your actual number
Run your specific equipment list — including AC unit size — through our load calculator rather than relying on a general example, and confirm the result with a dealer before committing to a model.
Why it matters
Retail and office buyers often oversize out of general caution, driven by uncertainty about their own load rather than an actual technical requirement — which quietly adds capital cost and, per our fuel consumption guide, ongoing running-cost inefficiency from chronic light-loading. A clear-eyed load calculation, done once properly, avoids both the risk of undersizing and the cost of padding a number nobody actually checked.
Frequently asked questions
What size generator do I need for a retail store or small office?+
Most single retail units or small office floors fall in the 15-30 kVA range, covering lighting, POS/computing load, HVAC (including AC compressor starting surge), and refrigeration where applicable, with a standard 20-25% commercial safety margin. Run your specific connected load through our [load calculator](/genset-load-calculator) rather than assuming a number — actual load varies significantly by store size and AC tonnage.
Does a small retail store need three-phase power?+
It depends on the equipment, not just store size — a small shop running only lighting, POS systems, and window-unit AC may be fine on single-phase, while a store with larger split or ducted AC compressors or any three-phase-rated equipment needs three-phase regardless of overall load size. See our [single-phase vs three-phase guide](/blog/single-phase-vs-three-phase-genset) for how that decision is made.
What's the most commonly underestimated load in a retail sizing calculation?+
AC compressor starting surge. A running load calculation that only adds up steady-state wattage misses the several-times-higher current a compressor motor draws for a second or two on startup — this is exactly what a proper load calculator accounts for by sizing against the largest motor's starting surge, not just total running watts.
Should a small office size for full backup or just essential systems?+
Most small offices choose full backup for a modest office floor, since the total load usually isn't large enough to justify the complexity of a tiered essential-only setup — that tiering makes more sense at a larger commercial scale. Run your actual connected load through the calculator to see where your specific office falls.
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