What Size Off-Grid Solar System Do I Need?

I'm Doug Mycko, owner of SolarMan Consulting. One of the most common questions I get is, “What size solar system do I need?”

There isn't one number. We need at least three different answers:

Step 1: Make a Real Load List

Write down what the house will actually use. Don't stop with lights and refrigerator.

Step 2: Calculate Daily Energy in kWh

For example, a 100-watt load operating for 10 hours uses about 1 kWh.

Do this across the meaningful loads and add them together. For variable-speed equipment such as mini-splits, measured consumption is better than assuming the nameplate maximum runs continuously.

Step 3: Separate Daytime and Nighttime Loads

This is extremely important off grid.

A load running during strong sunshine may be powered largely from the PV array. The same load running after sunset has to come from stored battery energy.

Two homes using the same total daily kWh can therefore need different battery capacity.

Step 4: Size the Battery Bank

Battery capacity is normally expressed in kilowatt-hours. Start with the energy the home needs when solar production is unavailable, then add appropriate reserve and cloudy-weather strategy.

But kWh isn't the entire battery specification. The battery bank also has to deliver enough current for the inverter and large loads.

Step 5: Size the Inverter for Power, Not kWh

The inverter has to carry the loads that can operate simultaneously.

If the refrigerator uses 150 watts and the house uses 15 kWh per day, neither number tells us whether a 4 kW, 6 kW or 12 kW inverter is appropriate. We need to know the largest realistic simultaneous load and any starting surge.

A Water Pump Can Change the Inverter Size

A 1 HP pump may not consume much daily energy if it runs only briefly. But starting the motor can demand several times its running current.

That short event may determine whether the inverter and battery bank are large enough even though the pump contributes relatively little to daily kWh.

Air Conditioning Changes the Battery Calculation

A modern mini-split may have a relatively gentle start, but running for eight to twelve hours overnight can add many kWh to the battery requirement.

If overnight A/C is important, tell me before the system is designed. Adding it later can change both battery storage and PV recovery requirements.

Step 6: Size the Solar Array to Refill the Battery

The array has to cover daytime consumption and replace energy used overnight.

Then we have to consider the actual solar resource where the system will operate.

A Big Island design should reflect the local microclimate. East Hawaiʻi and Kona do not have identical weather patterns, and even nearby communities can experience different cloud cover.

How Many Solar Panels Do I Need?

Once we know the required PV-array wattage, panel count becomes simple arithmetic.

For example, an 8,000-watt target using 400-watt modules would require about 20 modules before any project-specific adjustment.

Panel voltage, inverter/charge-controller MPPT limits, string configuration, roof or ground-mount space and electrical code requirements still have to be checked.

More Panels Are Not a Substitute for Enough Battery

A huge solar array can recharge quickly in good weather, but it cannot power the house at midnight without storage or another source.

Likewise, a huge battery bank with too little PV can slowly fall behind because the array cannot replace what the house consumes.

Step 7: Decide How the Generator Fits In

I consider backup generation part of the design conversation for a fully off-grid home.

The question isn't simply whether you own a generator. We need to know how much charging power the inverter can use, what house loads may run simultaneously and how much generator runtime you're willing to accept after poor solar weather.

How Much Reserve Should I Have?

There is no universal number because customers use off-grid homes differently.

Some customers are comfortable starting a generator after an unusually cloudy day. Others want substantially more battery and PV so generator operation is rare. That preference changes the economics of the system.

Should I Size From My Utility Bill?

If you're converting an existing grid-connected home, utility history can be useful because it shows real energy consumption. But it does not tell the whole story.

A monthly kWh number does not reveal instantaneous peak loads, pump starting surge, when energy is consumed or how much overnight storage is needed.

What If This Is a New House?

For a new off-grid house without historical consumption, we build the load estimate appliance by appliance.

I would rather spend time getting the load assumptions right than make the system expensive by guessing high—or unreliable by guessing low.

Use the SolarMan Whole-System Sizing Calculator

I've built a calculator specifically to help organize the sizing problem. Use it as a starting point, then we can review the loads that have the biggest effect on the final system.

What Size System Can I Get for My Budget?

Budget matters, but I reverse the question: first determine what equipment the loads require, then decide where the design can be adjusted without creating a system that disappoints you.

SolarMan offers equipment packages at several price levels, from smaller basic systems to larger whole-home packages.

A Simple Sizing Example

Imagine a house estimated at 18 kWh per day. Suppose 8 kWh occurs during solar-producing hours and 10 kWh occurs overnight.

That immediately tells us more than “18 kWh per day.” We know the battery must support roughly the overnight energy plus reserve, while the next day's PV must support daytime loads and replace the overnight energy.

Then we separately inspect the largest loads—perhaps a water pump, microwave and mini-split—to determine inverter and battery power requirements.

That is whole-system sizing.

Big Island Loads I Ask About Every Time

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Frequently Asked Questions

SolarMan Consulting provides off-grid solar design, equipment supply, troubleshooting and commissioning support. Electrical installation requiring contractor licensing should be performed by appropriately licensed personnel.