Solar battery sizing goes wrong in the same way most often: people buy the biggest battery they can afford instead of the one that actually matches their goals. Sizing correctly means defining what you want the battery to do — critical-loads backup, whole-home backup, or maximizing self-consumption — and working the math backward from there.
What “Sizing” Actually Means
Battery sizing has two separate dimensions that both matter:
Capacity (kWh) — how much total energy the battery can store and discharge before it’s empty. This determines how long it can run your loads.
Power (kW) — how much energy the battery can deliver at one instant. This determines how many appliances (especially high-surge ones like AC compressors or well pumps) can run at the same time.
A battery can have plenty of capacity but not enough power rating to start your air conditioner, or plenty of power but not enough capacity to last through the night. Both numbers need to be sized correctly.
Step 1: Define Your Goal
| Goal | Typical Capacity Needed |
|---|---|
| Critical loads only (fridge, wifi, some lights/outlets) | 3–5 kWh |
| Extended backup (adds well pump, medical equipment, partial HVAC) | 10–15 kWh |
| Whole-home backup (including AC/heat) | 20–30+ kWh, often multiple units |
| Self-consumption / TOU rate arbitrage (no backup priority) | 10–15 kWh, sized to daily solar excess |
Step 2: Calculate Your Load
List out what you actually want powered and for how long, then add up the watt-hours. A typical critical-loads list looks like this:
| Appliance | Watts | Hours/day | Daily kWh |
|---|---|---|---|
| Refrigerator | 150W | 24 (cycles) | ~1.2 kWh |
| Wifi router + modem | 20W | 24 | ~0.5 kWh |
| LED lighting | 200W | 5 | ~1.0 kWh |
| Well pump | 1,000W | 1 | ~1.0 kWh |
| Phone/laptop charging | 60W | 4 | ~0.24 kWh |
| TV | 100W | 4 | ~0.4 kWh |
| Total (example) | ~4.3 kWh/day |
Step 3: Apply Depth of Discharge and Round-Trip Efficiency
Modern lithium iron phosphate (LFP) batteries — the standard chemistry in home systems like the Tesla Powerwall 3, Franklin aGate, and Enphase IQ Battery — offer 95–100% usable depth of discharge (DoD), meaning nearly the full nameplate capacity is usable. Round-trip efficiency (energy lost converting AC to DC and back) typically runs 90–95%.
In practice, plan for roughly 90% of nameplate capacity being usable. So if your calculated daily need is 6 kWh, look for a battery rated around 6.5–7 kWh to comfortably cover it.
Step 4: Decide How Many Days of Backup You Want
Multiply your daily usable need by how many outage-days you want to cover without sun. Most homeowners size for 1 day (a typical outage), while those in storm-prone regions (hurricane, wildfire, or ice-storm areas) often size for 2–3 days, or plan on solar recharging the battery during multi-day outages if panels remain undamaged and the sky isn’t fully overcast.
Step 5: Match the Power (kW) Rating to Your Simultaneous Loads
Add up the running watts of everything you might need on at once, plus the highest single starting-surge load (AC compressors and well pumps can briefly draw 2–3x their running wattage on startup). The battery’s continuous and peak power ratings need to clear that number.
| System | Continuous Power | Usable Capacity |
|---|---|---|
| Tesla Powerwall 3 | 11.5 kW | 13.5 kWh |
| Franklin aGate | 10 kW | 15 kWh |
| Enphase IQ Battery 5P | 3.84 kW (stackable) | 5 kWh per unit |
| SolarEdge Home Battery | 5 kW | 9.7 kWh |
Stackable systems like the Enphase IQ line let you add units later as your needs grow, which is often a smarter starting point than buying one oversized battery up front.
Not sure what size battery fits your home? Get a free, no-obligation sizing quote from a vetted local installer: (855) 427-0058
Cost and the Federal Tax Credit
Installed home battery systems typically run $8,000–$16,000 in 2026 depending on capacity and brand. As of 2026, the Section 25D residential tax credit has expired (it ended December 31, 2025), so homeowners who buy and own their battery outright no longer receive the 30% federal credit. The Section 48E commercial credit remains active through 2027, but it applies to installers offering leases and PPAs, where the installer — not the homeowner — claims the credit.
Don’t Oversize — It’s the Most Common Mistake
A bigger battery than your actual critical-load list requires doesn’t add real backup value; it just adds cost. Size to your real needs first using the steps above, then choose a system that can be expanded with additional modules later if your needs grow — most current-generation systems are designed to be added to rather than replaced.
Frequently Asked Questions
How do I calculate what size solar battery I need?
List your critical appliances with their wattage and daily runtime, total the watt-hours, divide by usable capacity (roughly 90% of nameplate for modern LFP batteries), then multiply by how many backup days you want to cover.
What’s the difference between kWh and kW when sizing a battery?
kWh (kilowatt-hours) is total stored energy — how long the battery lasts. kW (kilowatts) is power output — how much it can deliver at one instant, which determines how many appliances can run simultaneously, including high-surge loads like AC compressors.
How many batteries do I need for whole-home backup?
Most whole-home backup setups need 20-30+ kWh of capacity, which usually means two or more stacked battery units (for example, two Powerwall 3 units or four to six Enphase IQ5P units), sized around your home’s actual peak simultaneous load.
Can I add more battery capacity later?
With most current-generation systems, yes — stackable systems like the Enphase IQ line and modular systems like the Powerwall 3 and Franklin aGate are designed to accept additional units as your needs grow.
Does the 30% federal tax credit still apply to solar batteries in 2026?
Not for homeowners who buy and own their battery outright – the Section 25D residential credit expired December 31, 2025. The Section 48E commercial credit is still active through 2027 for installers offering battery leases or PPAs.
How long will a battery power my house during an outage?
It depends entirely on sizing: a 5 kWh battery might cover a few critical circuits for 12-24 hours, while a 20-30 kWh whole-home system can run a full house for a day or more, longer if solar panels are recharging it during daylight.
Is it better to oversize or undersize a solar battery?
Neither – right-sizing to your actual calculated needs is best. Oversizing wastes money on unused capacity, while undersizing leaves you short during an outage. Choosing an expandable system lets you start right-sized and add capacity later if needed.
Summing Up
Sizing a solar battery correctly means starting with your actual goal and load list, not the biggest number in a brochure. Calculate your daily watt-hour need, apply a realistic usable-capacity factor, decide how many backup days you want, and make sure the power (kW) rating covers your simultaneous and surge loads. To get a free sizing quote based on your home’s real electricity use, call (855) 427-0058 or visit us.solarpanelsnetwork.com.
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