How to Size Solar Power for Backup: Loads, Season, and Storage

A useful solar estimate begins with measured loads and location-specific solar data. It does not begin with house size or a generic package label. The result is a planning range that should be checked against the manuals, site conditions, local requirements, and a qualified designer for permanent home wiring.

Use the Solar Power Sizing Calculator to organize the inputs, then follow the steps below to understand and challenge the result.

Step 1: Define the backup loads

List only the devices that need solar-backed power during the planned event. Write down:

• Measured running watts or watt-hours
• Hours of use per day
• Starting demand for motors and compressors
• Whether loads may run at the same time
• Which loads can be deferred when energy is low

For plug-in equipment, a suitable energy meter can capture watt-hours over a normal day. For pumps, hardwired equipment, and large motors, use manufacturer data or a qualified electrician. Do not guess at medical-device requirements; confirm the backup plan with the equipment provider and clinician.

Calculate daily energy for each load:

watts x hours used per day = daily watt-hours

If a refrigerator cycles, measure it over a representative period rather than multiplying its maximum label wattage by 24 hours. Repeat measurements in the season that matters, because ambient temperature and household use can change the result.

Step 2: Separate energy from power

Watt-hours describe energy over time. They help size batteries and daily solar production.

Watts describe power at one moment. They help size inverter output and wiring.

Add the loads likely to run together, then compare that total with the exact inverter’s continuous rating. Check starting requirements separately. A refrigerator, well pump, or sump pump may need more power to start than to run. The appliance and inverter manufacturers should confirm compatibility when it is not explicit.

Avoid universal multipliers such as “three times every motor.” Starting behavior varies by equipment, controls, voltage, and inverter design.

Step 3: Use location and season data

NLR’s PVWatts estimates photovoltaic production from location, array size, orientation, system losses, and weather data. Use the actual proposed location and examine monthly output, not only an annual average.

For outage planning, the relevant month is often the one with low production and high critical-load demand. Shade, snow, smoke, panel temperature, orientation, and temporary placement can reduce output. A portable panel moved throughout the day does not have the same production profile as a fixed roof array.

PVWatts models grid-connected photovoltaic production. It is a strong starting point for solar resource and array output, but it does not prove that a battery, portable station, or off-grid controller will accept or store that energy.

Step 4: Estimate panel energy

Use daily load energy and modeled daily array production in the same units. Rather than dividing by a generic peak-sun-hours number and adding a fixed margin, compare at least three scenarios:

1. A representative day in the planning month
2. A low-production day when loads must be reduced
3. A recovery day when the array must power loads and recharge storage

The exact charge controller or power station has input-voltage, current, power, and connector limits. A larger panel array is not useful if it exceeds those limits or if the system cannot use the available energy.

Step 5: Size usable battery energy

Start with daily watt-hours and the number of days or hours the battery must support before recharge. Then use the exact product’s allowed usable capacity, temperature limits, standby use, and conversion efficiency.

daily load watt-hours x planned days = load energy before losses

Do not assume every battery can use the same percentage of its nameplate energy. The battery-management settings, chemistry, temperature, age, discharge rate, and warranty conditions matter. Use the complete system manual.

Autonomy is also a policy decision. More stored energy may increase cost, space, weight, fire-code requirements, and recharge time. A written load-shedding plan can be safer and more affordable than trying to preserve every normal load.

Step 6: Check the whole power path

Trace the energy from panel to load:

1. Panel or array
2. Connector and conductors
3. Charge controller or solar input
4. Battery
5. Inverter or DC output
6. Appliance

Each stage has ratings and losses. Use the manufacturer’s efficiency information where available and keep a conservative range when it is not. Do not combine components merely because their headline wattage looks similar.

For stationary systems, a qualified designer should address protection, disconnects, grounding, transfer, permits, and utility interconnection. The DIY Battery Planning Guide explains why individual component ratings are not a residential system approval.

Worked planning example

Suppose measured critical loads total 1,200 Wh per day. The loads that can overlap draw 350 W, and one motor has a manufacturer-documented starting requirement of 700 W.

The next steps are not to buy a 1,200 W panel or a 1,200 Wh battery. Instead:

• Model monthly solar production for candidate array sizes in PVWatts
• Choose a low-production scenario and decide which loads can be shed
• Compare the complete battery system’s usable energy at the planning temperature
• Confirm continuous and starting compatibility with the inverter manufacturer
• Confirm the array is within the solar input limits
• Test the finished setup with the real loads

This approach produces a range and a documented fallback. It is more useful than a single exact-looking number based on generic assumptions.

Validate before relying on it

Run a controlled outage drill. Start with a fully prepared system, disconnect the normal source as the manufacturer directs, and operate only the planned loads. Record energy used, solar input, faults, temperature, and remaining capacity.

Repeat in the planning season if possible. A summer test does not establish winter solar performance. Stop if equipment overheats, reports a fault, or does not behave as the manual describes.

Adjust the method for the setting

A seasonal building needs a difficult-month autonomy and unattended-load plan; use Off-Grid Cabin Solar Sizing for that case. A portable trip has deployment-time, shade, packing, and connector limits; use Camping Solar Panel Sizing instead.

For every setting, the next step is the same: replace assumptions with measured loads and location data, then record which load will be shed on a low-production day.

Primary sources

• NLR: PVWatts Calculator
• NLR: PVWatts API and Model Overview
• Department of Energy: Homeowner’s Guide to Solar
• Department of Energy: Solar and Resilience Basics