A small-business solar system should be sized from measured electricity use, operating schedules, critical loads, and local production conditions—not from roof area alone. This practical solar system sizing calculator guide shows how to estimate panel capacity, battery storage, inverter size, and future expansion needs for offices, shops, workshops, farms, and other commercial properties.
Overview
Commercial solar sizing has three separate questions: how much energy the business uses, how much solar power the site can produce, and how much equipment is needed to serve loads at any given moment. Keeping these questions separate prevents common design mistakes, such as installing enough solar panels for annual energy use but choosing an inverter that cannot handle a motor starting, or buying a large battery that cannot support the business’s peak demand.
Use the worksheet below as an initial estimate rather than a final design. A qualified installer or engineer should verify electrical service capacity, roof structure, equipment compatibility, fire and building requirements, utility interconnection rules, and local permitting before equipment is purchased.
Quick commercial solar sizing worksheet
- Average daily energy use: ______ kWh/day
- Design-day energy use: ______ kWh/day
- Peak simultaneous load: ______ kW
- Largest motor or startup load: ______ kW or surge requirement
- Usable peak-sun-hours assumption: ______ hours/day
- System performance factor: ______
- Required battery runtime: ______ hours
- Planned battery depth of discharge: ______%
Record the source of every input. Utility bills, interval data, equipment nameplates, submeter readings, and operating schedules are more useful than broad assumptions based only on building size.
How to estimate
1. Calculate daily energy use
Start with electricity consumption in kilowatt-hours (kWh). If monthly utility bills are available, divide the usage for each month by the number of days in that month. Keep the monthly results separate so seasonal changes are visible.
Average daily use = monthly kWh ÷ days in the billing period
For a more detailed estimate, list major loads and multiply each load’s power by its operating time:
Daily energy = load power in kW × operating hours per day
For example, a 2 kW refrigeration load that operates for an estimated 10 hours contributes 20 kWh before duty-cycle adjustments. If the compressor cycles rather than running continuously, use measured consumption or a defensible duty-cycle assumption. Add lighting, computers, HVAC, pumps, refrigeration, tools, chargers, and other significant loads separately.
2. Convert energy use into panel capacity
A simple solar panel calculator uses daily energy demand, available solar resource, and a performance factor for wiring, temperature, inverter conversion, dust, shading, and other losses.
Required solar array size in kW = daily kWh ÷ (usable peak-sun-hours × performance factor)
Use a conservative production assumption for the design period that matters most to the business. A grid-tied business focused on annual bill reduction may compare monthly production with monthly consumption. An off-grid operation or a business that needs dependable backup should examine the least favorable season and include a backup generator or other contingency where appropriate.
Once the array size is estimated, divide it by the planned panel wattage:
Panel count = required array watts ÷ panel watts
Round up to a whole panel and then check string voltage, current, roof layout, setbacks, shading, and the inverter’s permitted DC input range. Panel count alone does not confirm that an array is electrically compatible.
3. Size the inverter for power, not daily energy
The solar inverter must support the highest expected simultaneous load, with suitable capacity for short startup surges from motors, compressors, pumps, and power tools. Make a list of loads that could run together during the business’s busiest period.
Minimum continuous inverter rating = peak simultaneous load ÷ desired loading factor
The loading factor is a design margin selected by the installer; it should not be treated as a universal number. Also check the inverter’s surge rating and duration. A system that runs lights and computers may have a modest peak, while a workshop, restaurant, farm, or cold-storage operation may need careful motor-start analysis.
For a grid-tied system, the inverter may be sized around the array and site electrical limits. For a battery-based or whole-business backup system, the inverter must also support the loads placed on the backed-up panel. Separating essential circuits—such as refrigeration, network equipment, security, and selected lighting—from nonessential circuits can reduce the required inverter and battery size.
4. Estimate battery storage
Battery sizing depends on the loads that must operate during an outage, the desired runtime, and the battery’s usable capacity. Do not size a battery from the business’s total daily consumption unless the goal is to operate the entire business through a defined period without grid power.
Required usable battery capacity in kWh = backed-up load in kW × runtime in hours
To estimate nominal battery capacity, account for the planned depth of discharge and system losses:
Nominal battery capacity = usable capacity ÷ (allowed depth of discharge × battery-system efficiency)
Battery specifications vary, so compare usable kWh, continuous output, surge output, operating temperature range, warranty conditions, and installation requirements—not just the nameplate capacity. LiFePO4 solar batteries are one option often considered for stationary storage; compare their specifications with the site’s safety, space, and operating requirements. See the LiFePO4 versus lead-acid storage guide for a structured comparison.
Inputs and assumptions
Small-business estimates become more reliable when each assumption is explicit. Use these inputs before comparing solar panel kits, commercial solar solutions, or battery backup products.
Energy and operating schedule
Collect at least several billing periods if possible, including the months with the highest and lowest consumption. Note opening hours, seasonal closures, production shifts, weekend use, planned equipment additions, and demand charges if they appear on the bill. A business that consumes most of its power during daylight may need less battery storage than one that operates after sunset.
Site and production conditions
Document roof orientation, tilt, usable area, shading at different times of day, roof age, structural limitations, and available ground space. Treat a roof-area estimate as a screening tool only. The final arrangement must accommodate access paths, equipment clearances, setbacks, wiring routes, and maintenance.
Electrical characteristics
Record service voltage, phase configuration, main-panel rating, existing generator equipment, motor loads, and any sensitive electronics. Confirm whether the proposed solar inverter and battery system are approved for the intended grid connection and backup arrangement. Charge-controller selection also matters in off-grid and some battery-coupled systems; compare MPPT and PWM options using the site’s array voltage and operating conditions.
Financial and utility assumptions
For a commercial project, compare expected solar production with the business’s tariff, export compensation, demand charges, fixed charges, maintenance costs, financing terms, and applicable incentives. Rates, net-metering rules, interconnection requirements, and tax treatment can change, so use current local information when building a purchase decision. Avoid treating a general solar tax credit statement as a guarantee of eligibility.
Worked examples
Example A: daytime retail shop
Assume a shop uses 60 kWh per day, operates mainly during daylight, and the preliminary design uses 4.5 usable peak-sun-hours and a performance factor of 0.80. The estimated array is:
60 ÷ (4.5 × 0.80) = 16.7 kW
Round up to approximately 17 kW before checking the actual panel layout and inverter design. If the shop’s critical outage loads are refrigeration and security equipment totaling 2 kW, and the owner wants four hours of runtime, usable storage begins at approximately 8 kWh. The final nominal battery size must be higher after accounting for depth of discharge, efficiency, temperature, and the battery’s continuous output rating.
Example B: workshop with startup loads
Assume a workshop uses 90 kWh per day but has a 7 kW compressor and several tools that may start together. Daily energy points toward the panel array, while the compressor’s starting behavior affects inverter selection. If the measured simultaneous operating load is 11 kW, an inverter chosen only from the 90 kWh daily figure could be undersized. The design should identify which circuits are backed up, verify motor-start requirements, and consider load sequencing or a soft starter where appropriate.
Example C: small off-grid facility
An off-grid cabin, farm outbuilding, or remote commercial site must be sized for the least favorable production period rather than an annual average alone. Calculate daily loads, add storage for the required autonomy period, account for charging losses, and decide whether a generator is part of the resilience plan. The off-grid solar sizing guide can help organize those additional inputs.
When to recalculate
Revisit the estimate whenever a key input changes. Recalculate after adding HVAC equipment, refrigeration, electric vehicles, pumps, machinery, or extended operating hours. Review it when utility rates, export rules, demand charges, equipment prices, financing terms, or incentives change. Also recalculate after a roof replacement, shading change, service-panel upgrade, business expansion, or a change in backup priorities.
For a practical next step, export the past year of utility bills, list every critical load, and measure uncertain equipment with a plug-in meter or temporary monitor where appropriate. Fill in the worksheet, calculate the array, battery, and inverter ranges, then request proposals that show their assumptions separately. Compare usable battery capacity and backed-up circuits, not just total system size. Finally, have the selected design reviewed for electrical, structural, permitting, and interconnection requirements before ordering equipment. For related residential calculations, see the solar panel system size calculator, while the solar inverter sizing chart provides additional guidance on backup inverter capacity.
