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Solar PV System Sizing Reference Cheat Sheet
A printable reference covering energy use, PV array size, battery capacity, inverter rating, charge controller sizing, and safety factors for grades 9-12.
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Solar PV system sizing estimates how many solar panels, batteries, and power electronics are needed to meet an electrical load. This cheat sheet helps students organize the main calculations used for off-grid and hybrid solar designs. It is useful because undersized systems lose power reliability, while oversized systems cost more than necessary. The goal is to connect energy needs, sunlight availability, and equipment ratings in a clear design process. The core idea is to start with daily energy use in watt-hours, then account for system losses and available peak sun hours. PV array power is found from adjusted daily energy divided by peak sun hours. Battery sizing depends on days of autonomy, battery voltage, allowable depth of discharge, and efficiency. Inverter and charge controller ratings are selected from maximum load power, surge power, system voltage, and PV current.
Key Facts
- Daily energy use is E_daily = sum of power x time = sum of P x t, where power is in watts and time is in hours.
- Adjusted energy is E_adjusted = E_daily / system efficiency, where system efficiency is often estimated from 0.70 to 0.85.
- Required PV array power is P_array = E_adjusted / peak sun hours.
- Number of panels is N_panels = P_array / panel wattage, rounded up to the next whole panel.
- Battery energy needed is E_battery = E_daily x days of autonomy / depth of discharge.
- Battery capacity in amp-hours is Ah_battery = E_battery / battery voltage.
- Inverter continuous rating should be at least P_inverter = total simultaneous AC load x safety factor, often using 1.25 or higher.
- Charge controller current is I_controller = P_array / system voltage x safety factor, commonly using a factor of 1.25.
Vocabulary
- Photovoltaic array
- A group of solar panels wired together to convert sunlight into electrical power.
- Peak sun hours
- The number of equivalent hours per day when sunlight averages 1000 watts per square meter.
- Depth of discharge
- The fraction of a battery's stored energy that can be used before recharging is needed.
- Inverter
- A device that converts DC electricity from panels or batteries into AC electricity for standard loads.
- Charge controller
- A device that regulates power from the PV array to safely charge batteries and protect the system.
- Days of autonomy
- The number of days a battery bank should power loads without enough solar charging.
Common Mistakes to Avoid
- Using watts instead of watt-hours is wrong because watts measure power rate, while watt-hours measure energy used over time.
- Forgetting system losses is wrong because real PV systems lose energy in wiring, batteries, inverters, temperature effects, and charging.
- Rounding the number of panels down is wrong because a partial panel cannot be installed and the system would not meet the required array power.
- Ignoring battery depth of discharge is wrong because using the full rated battery capacity can damage batteries and shorten their life.
- Sizing the inverter only from average load is wrong because the inverter must handle the largest simultaneous load and startup surge from motors or appliances.
Practice Questions
- 1 A cabin uses four 10 W lights for 5 hours and one 60 W fan for 3 hours each day. What is the daily energy use in watt-hours?
- 2 A system needs 1800 Wh per day, has an estimated efficiency of 0.75, and receives 4 peak sun hours per day. What PV array power is required?
- 3 A 12 V battery bank must supply 1200 Wh per day for 2 days with a maximum depth of discharge of 50 percent. What battery capacity in amp-hours is needed?
- 4 Explain why two homes with the same daily energy use may need different PV array sizes if they are located in different climates.