Photovoltaic Design, Installation, Maintenance

Start With the Roof, the Load, and the Limits

Will the proposed photovoltaic (PV) system produce enough energy for the actual loads, fit safely on the available roof, and operate within the electrical limits at the site? If you cannot answer all three before selecting equipment, you are not ready to order modules or an inverter.

Poor input data causes predictable field problems: an array falls short of the customer’s demand, an inverter clips during peak production, a service panel cannot accept the planned connection, or winter shading reduces output far below the sales estimate. The fix starts before hardware selection. Gather measured site data, define the loads by time and energy use, then check the electrical and physical limits.

The Sun-Load-Safety Sizing Loop gives you that order. “Sun” covers the site’s solar resource, roof conditions, orientation, tilt, and shading. “Load” covers energy use, demand, operating hours, and future changes. “Safety” covers voltage, current, temperature, conductor limits, service capacity, clearances, and equipment ratings. After applying the loop, you should have a defensible array size, an inverter range, and a list of conditions that require field verification. The practical takeaway: never size from a roof measurement or an annual utility total alone; size from verified sun, load, and safety inputs.

Build the Sun-Load-Safety Sizing Loop

Start with a site survey that produces numbers another technician can check. Record roof planes, dimensions, pitch, azimuth (the direction the roof faces), obstructions, usable mounting areas, and access paths. Measure or confirm the roof pitch rather than estimating it from a photograph. Note vents, skylights, chimneys, satellite dishes, parapets, and required setbacks. Photograph each roof plane and mark shading sources such as trees, neighboring buildings, and utility poles.

Solar resource data describes the available sunlight. Use a reputable solar resource tool, such as the National Renewable Energy Laboratory’s (NREL) PVWatts calculator where applicable, and select the nearest representative location. Record the monthly peak sun hours (PSH), which express the equivalent hours of full-strength sunlight. A month with 4.5 PSH does not mean the sun shines at full output for 4.5 continuous hours; it provides an energy equivalent used for preliminary production estimates. Use the weaker seasonal value when checking whether winter production meets a stated requirement.

Collect load data from utility bills and the electrical installation. A bill gives energy consumption in kilowatt-hours (kWh), while a load calculation identifies equipment power in kilowatts (kW) and helps reveal when demand occurs. Ask for at least twelve months of bills when available. Record the billing period, kWh, rate structure, demand charges if present, and unusual events such as a vacant house or a temporary electric heater. Then list major loads and their ratings. A 4.5-kilowatt heat pump running for three hours uses about 13.5 kWh before duty-cycle adjustments; a 1.5-kilowatt water heater running for two hours uses about 3 kWh.

Apply the loop in this order:

1. Measure the Sun. Confirm usable roof area, orientation, tilt, shading, and monthly PSH. This prevents an attractive annual estimate from hiding a poor winter or shaded roof plane.

2. Define the Load. Convert appliances and equipment into daily and annual kWh, then identify high-demand periods. This prevents average consumption from masking a short, heavy load that affects inverter selection.

3. Check Safety. Verify maximum voltage, operating current, conductor ampacity (allowable current), overcurrent protection, service capacity, equipment clearances, and temperature effects. This prevents a mathematically correct array from becoming an unsafe installation.

4. Size the Array. Estimate required direct-current (DC) array power from the load, solar resource, and system losses. Keep the loss assumption visible so the estimate can change when site conditions change.

5. Size the Inverter. Match alternating-current (AC) output to the load profile, grid connection, array voltage range, and allowable DC-to-AC ratio. The inverter must start and operate within its voltage window during both cold mornings and hot afternoons.

For a grid-connected preliminary estimate, use:

`Required array power (kW DC) = target daily energy (kWh/day) ÷ [PSH × performance ratio]`

The performance ratio accounts for wiring, temperature, soiling, mismatch, inverter conversion, and other losses. Use a documented design value rather than hiding losses inside an unexplained multiplier. For example, 24 kWh/day with 4.5 PSH and a 0.78 performance ratio requires about 6.84 kW DC. Round the result only after checking available module layouts, roof planes, and electrical limits.

Do not treat this result as a final design. Check monthly production, not just annual energy....