How to Size a Solar, Inverter & Battery System

Decide Why You Want Solar

Before calculating panels or batteries, decide what you actually want the system to achieve.

Your priority may be:

• Backup during power outages
• Lower electricity bills
• Reduced generator fuel consumption
• Solar power during the day
• Backup for critical equipment
• Whole-building backup
• Greater energy independence

A battery backup system, solar-only system and solar + battery system can be very different even when they serve the same building.

Check Whether Your Building Is Single or Three Phase

Your building’s electrical supply matters when selecting the inverter.

A small home may be single phase, while larger homes and many commercial buildings use three-phase power.

For three-phase installations, the system should consider both total demand and how the loads are distributed across the phases.

Large three-phase equipment must also be identified separately.

Calculate Your Maximum Power Demand

The inverter must be large enough to operate the equipment that may run simultaneously.

This is measured in kW or kVA.

Air conditioners, pumps, refrigerators and motors may also require additional power briefly when they start.

Modern storage inverters can often provide substantially more than their continuous rating for a short period, so a momentary motor-starting surge does not always mean the inverter needs to be permanently oversized.

The actual inverter overload specification still needs to be verified before final selection.

Calculate Your Daily Energy Use

Solar and battery sizing also require kWh, not just kW.

For example:

A 1kW appliance running for 5 hours consumes:

1kW × 5 hours = 5kWh

That means two homes with the same maximum load can require completely different solar and battery capacities if one uses its appliances for much longer each day.

This is why a good solar sizing calculator asks both equipment wattage and expected operating hours.

Size the Battery for the Backup Time You Need

Battery size depends on:

average protected load × required backup time.

If your backed-up equipment averages 3kW and you want approximately four hours of backup, the starting energy requirement is:

3kW × 4 hours = 12kWh

The actual battery bank will need to be larger because battery capacity, usable depth of discharge and inverter losses must also be considered.

Lithium and lead-acid batteries should not be treated identically.

Size Solar Panels Using Local Solar Production

Solar panel sizing should not simply assume that every 1kW of panels generates 1kW all day.

Actual solar production changes with:

• Location
• Month
• Weather
• Panel orientation
• Temperature
• Shading
• System losses

For this reason, our Solar Sizing Calculator uses location-based monthly solar-production data.

Rather than sizing only from an annual average, the calculator can look at the lower-producing months and estimate the PV capacity needed to meet the selected energy requirement.

This produces a more useful preliminary solar size for homes and businesses.

Solar, Battery and Inverter Sizes Are Different

Remember:

Inverter kW/kVA determines how much equipment can operate at once.

Battery kWh determines how long your equipment can operate without another power source.

Solar kWp determines how much solar energy can be produced.

A properly designed system balances all three.