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 ↓
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.
Estimated usable energy: — kWh (after usable % and efficiency)
Runtime (hours)
—
Runtime (hh:mm)
—
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.
Backup Power 101 Check: Solar + Batteries
6 quick questions. Score yourself and get a recommended next step.
Quiz
1) What does kWh mainly tell you?
2) Most grid-tied solar systems without batteries during an outage will:
3) What is a smart first target for most battery backup systems?
4) Why does inverter surge capability matter?
5) If you rely on solar to recharge batteries, you should plan around:
6) Which combo is often the most resilient for multi-day outages?