Grid Tie Solar Sizing Calculator

Formula used

The calculator sizes a grid-tie PV array using annual energy balance:

• Annual Load (kWh) = Monthly kWh × 12
• Derate = (1 − Losses%) × InverterEfficiency%
• Required kW DC = AnnualLoad × Offset% ÷ (PSH × 365 × Derate)
• Annual Generation = kW DC × PSH × 365 × Derate
• Self kWh = min(AnnualLoad, AnnualGen × SelfConsumption%)
• Export kWh = max(0, AnnualGen − Self kWh)
• Savings = (Self kWh × Tariff) + (Export kWh × ExportRate)

Financial metrics use an upfront cost, yearly O&M, panel degradation, and discounting to compute NPV, IRR (if solvable), discounted payback, and LCOE.

How to use this calculator

1. Enter your monthly kWh from electricity bills.
2. Set peak sun hours and losses for your site.
3. Choose an offset (100% is typical for net metering).
4. Adjust tariff and export credit to match your utility.
5. Enter installed cost, incentives, and O&M for payback and NPV.
6. Press Calculate and download CSV/PDF if needed.

Energy baseline and offset target

Start with monthly kWh from recent bills and convert to annual usage. Choosing a 100% offset targets annual net energy balance, while 70–90% can reduce exports when credit rates are lower. If your load is 600 kWh per month, annual demand is 7,200 kWh. With 90% offset, the target generation becomes 6,480 kWh per year. Seasonal peaks may justify a slightly higher offset.

Solar resource and derate assumptions

Peak sun hours translate solar resource into energy output. A site with 5.5 PSH typically receives about 5.5 equivalent full-sun hours daily. Apply losses for soiling, temperature, wiring, and mismatch, then multiply by inverter efficiency to form a derate factor. For example, 14% losses and 97% inverter efficiency yield Derate = 0.86 × 0.97 = 0.834. Higher derate increases required array size materially.

Sizing outputs and equipment ratios

The sizing engine solves Required kW DC = AnnualLoad × Offset ÷ (PSH × 365 × Derate). It then rounds up to whole modules based on panel wattage, producing an installed DC nameplate. A DC/AC ratio near 1.15–1.30 often balances clipping and inverter cost; the calculator converts array kW DC into a suggested inverter range. Roof checks estimate area using module footprint per panel for feasibility screening.

Retail savings versus export credits

Grid-tie value depends on how much generation is consumed on-site versus exported. Self-consumption uses the full retail tariff, while exported energy earns the export credit. If generation is 7,200 kWh and self-consumption is 70%, then 5,040 kWh offset retail purchases and 2,160 kWh are credited at the export rate. Adjust these inputs to reflect time-of-use or caps.

Investment metrics and sensitivity checks

Upfront cost is estimated from installed cost per kW DC, then reduced by incentives. Annual O&M is modeled as a percentage of gross cost, while production degrades each year. The projection discounts yearly net cashflows to compute NPV and discounted payback; IRR is solved when the cashflow stream crosses zero. LCOE divides discounted lifetime costs by discounted lifetime kWh. Test tariff and installed cost sensitivity.

FAQs

What peak sun hours should I use?

Use the long‑term average PSH for your location, ideally from a solar resource map or your installer. If you only know monthly values, average them across the year to match annual sizing.

Why does the DC/AC ratio change inverter size?

The array is rated in DC watts, while the inverter is AC. A higher DC/AC ratio allows a smaller inverter, reducing cost, but may clip output on very sunny hours. Many designs target 1.15–1.30.

How are losses different from inverter efficiency?

Losses bundle wiring, temperature, soiling, mismatch, and shading impacts. Inverter efficiency represents conversion from DC to AC. The calculator multiplies both into one derate factor to estimate deliverable energy.

What if export credit is lower than the tariff?

When export rates are lower, maximizing self‑consumption usually improves savings. Consider a smaller offset target, shift loads to daytime, or add efficiency upgrades. The self‑consumption input lets you test this quickly.

How is payback shown here?

Simple payback divides net upfront cost by year‑one net cashflow. Discounted payback uses the discounted cashflows across years, reflecting the time value of money. Both assume constant tariff and export rates.

Does this include interconnection limits and shading?

It does not model feeder limits, detailed shading, azimuth, tilt, or hourly net metering rules. Use it for early planning, then confirm with a site survey, utility requirements, and an engineering design.

Example data table

Example assumes self-consumption 70%, tariff 60, export credit 45, and installed cost 180,000 per kW DC.