Homeowner Edition • Clean Backup Power

Solar + Batteries for Backup Power (What Actually Works)

Solar and batteries can be an excellent backup system — quiet, clean, and indoor-safe — but only if it’s designed correctly. This guide shows what “backup-capable solar” means, what batteries can (and can’t) run, and how to size a setup that survives real outages.

Important: Most standard grid-tied solar systems shut off during an outage. To run during blackouts you need battery storage + an outage-capable inverter + a proper transfer/islanding setup. Take the quiz ↓

kW = how much power at once kWh = how long you can run Critical loads beat “whole-home dreams”

Start sizing your system See the comparison table Avoid common mistakes

What batteries are great at

Batteries shine for essentials + short-to-medium outages: lights, Wi-Fi, refrigeration, medical devices, security, and selected circuits — without noise or exhaust.

Instant switchover (depending on system) for sensitive electronics
Indoor-safe power (no carbon monoxide)
Silent operation and low maintenance
Pairs extremely well with a UPS for computers/medical gear

Best “real-world” target

Design for critical circuits first (fridge, furnace blower/boiler controls, sump, router, lights, outlets), then expand if the math and budget support it.

Where solar changes the game

Batteries alone are limited by stored energy. Solar adds a recharge engine during daylight. With enough sun + correct design, solar can extend your runtime far beyond “battery-only” expectations.

Solar can power loads and recharge batteries during outages (system-dependent)
Clouds/winter reduce production — plan for worst-week, not best-day
Your inverter decides what loads can start and what must be shed
Most homes need a critical-loads panel for clean outage behavior

Reality check

Solar + batteries is not magic. If you try to run large resistive loads (electric heat, big water heaters, multiple HVAC) you’ll drain storage fast unless your system is built (and paid for) at a much larger scale.

Solar & Battery Options Compared

Use this table to match the system type to your outage profile and expectations.

Setup	Works in an outage?	Best for	Common limitations	Typical “win”
Grid-tied solar (no battery)	Usually NO	Lower bills, net metering savings	Shuts down for safety when grid is out	Great ROI, not true backup
Battery backup (no solar)	YES	Silent essentials during short outages	Finite runtime until recharged	Clean backup for critical loads
Solar + battery + outage-capable inverter	YES	Longer outages, self-recharge in daylight	Sun dependent; needs careful load control	Best “generator alternative” for many homes
Solar + battery + generator integration	YES	Multi-day resilience with fuel fallback	Higher complexity + cost	Quiet most of the time; generator only when needed

Quick rule of thumb

If your outages are often multi-day, solar helps — but if you must run big loads (well pump + fridge + furnace + multiple freezers), consider a hybrid approach: batteries for quiet essentials + a generator for heavy lifting.

How to Size Solar + Batteries for Backup

Step 1: Pick your “critical loads”

Refrigerator + freezer
Furnace/boiler controls + blower (or heat pump controls)
Sump pump / sewage pump
Wi-Fi/modem + phone charging
Medical devices (CPAP, concentrator, etc.)
Lights + a few outlets

Why this matters

Backup success is mostly about load control. The smaller and cleaner the outage panel, the easier it is to ride through storms.

Step 2: Understand kW vs kWh

kW (power) = what can run at once
kWh (energy) = how long you can run
Motors have startup surge (pumps, fridges)
Inverters have continuous and surge limits

Common sizing trap

Buying “more kWh” won’t help if your inverter can’t start your pump or compressor. You need both: enough surge kW and enough kWh.

Step 3: Estimate battery capacity (kWh)

Use this simple framework: (Average critical load watts ÷ 1000) × hours of backup = kWh needed. Then add margin for inverter losses and real-world usage.

Practical example

If your critical loads average ~600W and you want 12 hours of runtime: (600 ÷ 1000) × 12 = 7.2 kWh minimum. Add margin and you might target 9–12 kWh of usable storage.

🔋 Battery Runtime Estimator (kWh → hours)

Enter your battery size and your average critical-load watts. We’ll estimate runtime with typical losses. Tip: This is for planning. Real runtime varies with inverter behavior, temperature, and motor starts.

Battery capacity (kWh)

Nameplate capacity. If you have multiple batteries, add them up.

Usable battery (%)

Many systems reserve a buffer. 80–95% is common.

Inverter efficiency (%)

Losses are real. 88–94% is a reasonable planning range.

Average load (watts)

Your “critical loads” average draw during an outage.

Quick presets (adds to average load)

Click common essentials to auto-build an estimate. Edit the watts field anytime.

Estimated usable energy: — kWh (after usable % and efficiency)

Runtime (hours)

—

Runtime (hh:mm)

—

Rule: runtime ≈ (usable kWh ÷ (watts ÷ 1000)). Motor starts can shorten runtime.

Battery “gotcha” (high-draw appliances)

Space heaters, electric cooking, dryers, big water heaters, and large HVAC loads can drain batteries fast. If you want those loads in outages, consider hybrid backup (battery for essentials + generator for heavy lifting).

Step 4: Size solar for “recharge, not bragging rights”

For outage resilience, solar’s job is to refill batteries and cover daytime loads. Production varies by season and weather. A good plan assumes lower winter output and cloudy days.

Design targets that work

Prioritize loads that matter (fridge/heat/water/medical)
Use a critical-load panel so heavy loads don’t sneak in
Plan for “gray week” performance: clouds + short days
Consider a generator port for extreme events

What usually breaks systems

Trying to run electric heat or big water heating on batteries
No plan for motor startup surge
Assuming solar always charges fast (clouds/winter)
No load-shedding strategy

Safety and Installation Notes (Homeowner Edition)

Islanding & transfer equipment

A backup-capable system must isolate your home from the grid during outages. This is typically handled by your inverter system plus transfer hardware and/or a dedicated backup gateway.

Do not “backfeed”

Never energize a panel or utility lines without approved transfer/isolation hardware. This is a safety hazard and can be illegal.

Critical-loads panel strategy

Most successful battery systems power a smaller set of circuits during outages. This keeps your inverter from being overwhelmed and makes runtime predictable.

Best practice

Put “must-run” circuits on the backup panel and keep high-draw loads (electric oven, large water heater, multiple HVAC) off it unless intentionally designed for.

🛠 Key Mistakes to Avoid

Assuming grid-tied solar works during outages (it usually shuts down)
Buying battery kWh without checking inverter continuous + surge limits
Putting random “nice-to-have” circuits on the backup panel
Ignoring winter/cloud production when relying on solar to recharge
No plan for load shedding (pumps + compressors + cooking at once)

The simplest winning rule

Control the loads first. Once you can keep essentials stable, scaling up becomes predictable instead of expensive guessing.

FAQ: Solar + Battery Backup

Can a battery run my whole house?

Sometimes — but it’s usually not the most cost-effective first step. Most homes do best starting with critical circuits and scaling up based on real load math and outage history.

Will solar recharge batteries during a blackout?

Only if your system is designed for it: outage-capable inverter + battery + proper transfer/islanding setup. Standard grid-tied solar typically shuts off in outages.

What about UPS devices?

UPS units are excellent for computers, routers, and medical electronics. They provide instant continuity and protection. Many homeowners pair a UPS with battery backup for the cleanest experience.