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.
Understanding Solar PV System Sizing Reference
A useful design starts with a load schedule, not a list of appliances. Record each device, its rated power, how long it runs, and when it runs. Timing matters because several appliances may operate together.
A kettle, pump, refrigerator, and power tool can create a much larger short-term demand than their daily energy total suggests. Some labels show maximum power rather than normal operating power, so measurements from a plug-in energy meter can improve the estimate.
Motors and compressors need extra power for a few seconds when starting. This starting surge is one reason a system that looks adequate on paper can shut down in real use.
Peak sun hours are not simply the number of daylight hours. They represent an amount of solar energy equivalent to a certain number of hours at full standard sunlight. A long bright day can still have fewer peak sun hours if the sun is low in the sky.
Weather, season, latitude, dust, shading, and panel angle all change output. Even a narrow shadow across part of one panel can greatly reduce production, especially when panels are connected in series.
Students should use conservative solar data for the worst expected month when reliability matters. A summer-only system can use a different assumption from a year-round cabin or emergency power system.
Panel wiring changes voltage and current. In a series connection, panel voltages add while current stays limited by the lowest-current panel. In a parallel connection, currents add while voltage stays about the same.
The chosen arrangement must stay inside the charge controller input limits under cold, sunny conditions. Solar panel voltage rises as temperature falls, so a controller can be damaged if the maximum input voltage is ignored. Wire size matters too.
Long, thin cables waste energy as heat and cause voltage drop. Higher system voltages can reduce current for the same power, which can allow smaller cables, but they require equipment rated for that voltage.
Battery capacity is not the same as usable stored energy. Lead-acid batteries last longer when they are not deeply discharged, and their available capacity falls at high discharge rates and in cold weather. Lithium batteries usually allow deeper discharge and have higher efficiency, but their battery management system may limit charging or discharging in extreme temperatures.
A battery bank must provide enough current as well as enough energy. A bank with sufficient amp-hours may still struggle with a large inverter load if its current rating is too low.
Fuses or circuit breakers belong close to batteries because battery cables can supply very high fault current. Correct grounding, disconnects, weatherproof enclosures, and manufacturer limits are essential parts of a safe design.