A business using 20,000 kWh per month would need about 128 kW DC of solar to generate electricity equal to 80% of its annual use, assuming a yield of 1,500 kWh per installed kW each year. That works out to roughly 233 panels at 550 W each. Daytime use, roof space, and export limits can still change the final size.
The examples below are for grid-connected systems. Yield, electricity prices, consumption profiles, and equipment dimensions are calculation assumptions, not site forecasts or industry averages.
Estimate Solar Capacity From Your Electricity Use
Required capacity (kW DC) = annual electricity use × target generation share ÷ annual solar yield
Annual solar yield means the usable AC electricity generated in a year for each installed kilowatt of panels, after modeled system losses.
For a business averaging 20,000 kWh per month:
- Annual consumption: 20,000 × 12 = 240,000 kWh.
- Target solar generation: 240,000 × 80% = 192,000 kWh.
- Required capacity: 192,000 ÷ 1,500 = 128 kW DC.
The table below gives a quick comparison. Every row assumes annual generation of 1,500 kWh per installed kW DC.
| Average monthly use | Annual use | 50% generation | 80% generation | 100% generation |
|---|---|---|---|---|
| 5,000 kWh | 60,000 kWh | 20 kW DC | 32 kW DC | 40 kW DC |
| 10,000 kWh | 120,000 kWh | 40 kW DC | 64 kW DC | 80 kW DC |
| 20,000 kWh | 240,000 kWh | 80 kW DC | 128 kW DC | 160 kW DC |
| 50,000 kWh | 600,000 kWh | 200 kW DC | 320 kW DC | 400 kW DC |
| 100,000 kWh | 1,200,000 kWh | 400 kW DC | 640 kW DC | 800 kW DC |
A system that generates the same number of kWh as the business uses in a year can still import power at night and export power during the day. So “100% annual generation” does not mean a zero electricity bill.
Collect 12 Months of Bills and Interval Data
Start with the actual kWh from 12 consecutive bills. Suppose a business uses 30,000 kWh a month for four months and 15,000 kWh a month for the other eight:
(30,000 × 4) + (15,000 × 8) = 240,000 kWh annually
If you sized the system from the busiest month alone, annual consumption would look like 360,000 kWh. That is 50% above the real figure.
Interval meter readings show something monthly bills cannot: when the electricity is actually being used. They reveal daytime demand, weekend shutdowns, and seasonal changes.[1]
| Information to collect | What it determines |
|---|---|
| Monthly consumption in kWh | Initial annual generation target |
| Interval meter readings | Solar electricity the business can use directly |
| Import and export tariff | Value of avoided purchases and surplus production |
| Demand-charge rules | How peak demand affects the bill |
| Operating calendar | Weekend, holiday, and seasonal surplus |
| Equipment changes | Future consumption and planned savings |
Use kWh, not the dollar total on the bill, when estimating solar capacity. Prices can change while the underlying electricity use stays the same.
Adjust Capacity for the Site’s Solar Output
Two businesses can use the same amount of electricity and still need different solar capacities. Roof direction, tilt, shading, and location all affect production.[2]
For the same 192,000 kWh annual target, capacity changes like this:
| Assumed annual yield | Required capacity |
|---|---|
| 1,200 kWh per kW DC | 160 kW DC |
| 1,400 kWh per kW DC | Approximately 137 kW DC |
| 1,500 kWh per kW DC | 128 kW DC |
| 1,600 kWh per kW DC | 120 kW DC |
| 1,800 kWh per kW DC | Approximately 107 kW DC |
These are sensitivity examples, not forecasts for particular sites. PVWatts is one established model used to estimate photovoltaic output.[3]
One small trap here is double counting losses. If the forecast already includes temperature, wiring, inverter, and other modeled losses, do not subtract them again.
Confirm the modules behind your capacity estimate
Send Tongwei your project location, target DC capacity, roof type, and delivery schedule. Ask which module models and power classes are available for your project.
Calculate How Much Solar You Will Use Directly
Imagine one 30-minute interval with 40 kWh of building consumption and 55 kWh of solar generation. The business can use 40 kWh on-site. The other 15 kWh is surplus.
That surplus has to go somewhere. Depending on the project, it may be exported, stored, or curtailed.
A hypothetical 128 kW system could look like this over a full year:
| Electricity flow | Annual amount |
|---|---|
| Business consumption | 240,000 kWh |
| Solar generation | 192,000 kWh |
| Solar used directly | 153,600 kWh |
| Solar exported | 38,400 kWh |
| Electricity still imported | 86,400 kWh |
The solar system generates an amount equal to 80% of annual consumption, but only 64% of total consumption is supplied directly by solar: 153,600 ÷ 240,000.
Its self-consumption rate is different: 80% of the solar generation is used on-site: 153,600 ÷ 192,000.
Weekends can make a noticeable difference. An office that closes two days a week is closed for roughly 104 days each year, when equipment and lighting demand may be much lower.
Compare the Savings From Smaller and Larger Systems
Now compare three system sizes. This example uses 240,000 kWh of annual consumption, a yield of 1,500 kWh per kW DC, avoided imports worth $0.15/kWh, and exports paid at $0.05/kWh.
The direct-use figures are illustrative. There is no battery or export curtailment in this example.
| Capacity | Annual generation | Used directly | Exported | Annual energy value |
|---|---|---|---|---|
| 80 kW DC | 120,000 kWh | 114,000 kWh | 6,000 kWh | $17,400 |
| 128 kW DC | 192,000 kWh | 153,600 kWh | 38,400 kWh | $24,960 |
| 160 kW DC | 240,000 kWh | 168,000 kWh | 72,000 kWh | $28,800 |
For the 128 kW option, the annual energy value is:
(153,600 × $0.15) + (38,400 × $0.05) = $24,960
Going from 128 kW to 160 kW adds 48,000 kWh of annual generation, but the extra annual value is only $3,840 under these assumptions. The reason is simple: much of the extra electricity is exported at the lower rate.
If that extra capacity costs $30,000, the simple payback before additional operating costs is $30,000 ÷ $3,840 = approximately 7.8 years.
This example leaves out financing, taxes, incentives, degradation, and changes in demand charges. If electricity prices change by time of day, each interval needs to be valued at the rate that actually applies.
Work Out Panel Count and Roof Area
Panel count = target capacity in watts ÷ individual panel wattage
For a 128 kW target using 550 W panels, 128,000 ÷ 550 = 232.7. Round that up and you get 233 panels, with a combined capacity of 128.15 kW DC.
The area examples below assume each panel occupies 2.6 m². These are generic example modules, not specifications for a particular Tongwei model.
| Target capacity | 550 W panel count, rounded up | Panel surface area |
|---|---|---|
| 50 kW | 91 | Approximately 237 m² |
| 100 kW | 182 | Approximately 473 m² |
| 128 kW | 233 | Approximately 606 m² |
| 200 kW | 364 | Approximately 946 m² |
| 500 kW | 910 | 2,366 m² |
Panel surface area is not the same as usable roof area. Access routes, obstacles, setbacks, and row spacing still take space. Commercial PV design also has to deal with site measurements and relevant building and safety requirements.[4]
If the approved layout fits only 200 of the 550 W panels, the available capacity falls to 200 × 550 ÷ 1,000 = 110 kW DC. At the assumed yield, that produces 165,000 kWh a year, or about 69% of the example business’s annual consumption.
Higher-wattage modules change the panel count. Tongwei lists the TWMNH-66HD module with a maximum power of 655 W. At that power class, 196 modules total 128.38 kW DC.
Fewer panels still do not guarantee less roof area. A more powerful panel may simply be a larger panel.
Check module dimensions before fixing the roof layout
Share your proposed panel count, roof drawing, and mounting requirements. Request the module dimensions, weight, and installation documents your designer needs to compare the options.
Check Inverter Capacity and Export Restrictions
Do not lump every capacity figure together. A proposal should separate these three numbers:
| Specification | Example |
|---|---|
| Panel capacity | 128 kW DC |
| Inverter output capacity | 100 kW AC |
| Permitted grid export | 30 kW |
The DC-to-AC ratio in this example is 128 ÷ 100 = 1.28. An inverter rating below the panel rating is not automatically a design problem. The important part is whether the production model accounts for any output clipped by the inverter limit.[5]
Export restrictions are another limit. If the available AC solar output is 100 kW, the site is using 50 kW, and grid export is capped at 30 kW, there is still 20 kW left. That power needs another approved load, battery charging, or curtailment.
If the same condition lasts for two hours, the affected energy is 20 × 2 = 40 kWh.
Calculate Demand-Charge Savings Separately
A 100 kW solar array does not automatically cut billed demand by 100 kW. Demand-charge savings depend on the tariff and on whether solar is producing when the billing peak occurs.[6]
At an illustrative demand charge of $20 per kW per month:
| Reduction in monthly billing demand | Monthly saving |
|---|---|
| 5 kW | $100 |
| 10 kW | $200 |
| 20 kW | $400 |
A 100 kW array therefore does not automatically save $2,000 a month in demand charges. If the billing peak happens after sunset, solar alone may do very little to reduce it.
Include Confirmed Changes in Electricity Use
Future loads can change the answer quite a bit. A machine averaging 20 kW for six hours a day over 250 working days adds 20 × 6 × 250 = 30,000 kWh annually.
At a yield of 1,500 kWh per kW DC, producing the same annual amount would need another 20 kW DC of solar. Use the machine’s expected average operating power here, not automatically its maximum nameplate rating.
Savings work the other way. If a lighting upgrade cuts annual use by 15,000 kWh, the equivalent solar requirement falls by 10 kW DC at the same yield.
Timing still matters. A new machine running on the night shift increases annual consumption, but it may add almost nothing to direct daytime solar use.
Size Batteries for the Load and Duration
If a business needs 25 kW of essential power for four hours, the required delivered energy is 25 × 4 = 100 kWh.
Now assume 90% usable battery capacity and 95% discharge-path efficiency. The starting nameplate calculation becomes 100 ÷ (0.90 × 0.95) = approximately 117 kWh.
That still excludes extra reserve and future capacity decline. Storage also has conversion losses, which have to be included when estimating the energy that actually reaches the load.[7]
Check These Figures Before Approving the System
- Exact module model, power class, panel count, and total DC capacity.
- Inverter AC capacity and permitted grid export.
- Monthly generation, direct consumption, exports, and curtailed production.
- Scaled layout, structural assessment, and required electrical upgrades.
- Separate energy-charge and demand-charge savings.
- Installed cost and annual savings for smaller and larger alternatives.
- Operating costs and a lower-production scenario.
Even small equipment changes can ripple through the design. A different panel wattage changes the count, while a different physical size can change what actually fits on the roof.
Request a quote for your selected module quantity
Send the module model or required power class, quantity, destination, and target delivery date. Ask Tongwei to confirm availability, supply terms, and the documents included with the quotation.
Finally
For a business using 20,000 kWh each month, the example gives 80 kW, 128 kW, and 160 kW DC for 50%, 80%, and 100% annual generation at a 1,500 kWh yield. The best size is not simply the one that produces the most electricity. What the business can use during the day, export limits, roof space, and the cost of adding more capacity all matter.