Start with your daily energy use in kWh: take the monthly kWh on your electric bill and divide by 30. Then decide how many days the batteries must carry you without sun (consultants call this “days of autonomy”).

Battery size ≈ daily kWh × days ÷ 0.9, because LiFePO4 batteries can safely use about 90% of their rated capacity. Add about 10% if the batteries live somewhere cold. Our Energy Storage Finder does this math for you.

kWh (capacity) is how much energy the battery holds, which sets how long you can run. kW (power) is how much the inverter can deliver at once, which sets what you can run at the same time. A big battery on a small inverter can run small loads for a long time. A big inverter on a small battery can start a well pump but won’t run it for long.

Classic off-grid designs used 3–5 days with lead-acid batteries. With lithium and a backup generator, most homes are designed for 1–3 days. Choose 1 day if a generator can recharge you, 2 for a balanced design, and 3 or more if you have no generator or long cloudy winters.

Motors in well pumps, AC compressors and refrigerators draw 3–6× their running watts for a split second at start-up. If the inverter’s surge rating (and how long it can hold that surge) is too low, the motor won’t start. Check the motor’s nameplate before choosing an inverter.

Match your inverter. As a rule of thumb, 12V suits RVs, vans and small cabins, 24V mid-size cabins, and 48V whole-home systems. Higher voltage means lower current, thinner cable and better efficiency. Every battery, inverter and charge controller in a system must share the same battery voltage.

Yes, with care. Never mix lithium and lead-acid in one bank. When adding lithium, use the same model and generation so the battery management systems can communicate, and keep all modules at the same voltage. If your model is discontinued, it’s often better to start a second, separate bank. Contact us before ordering if you’re unsure.

Yes. There are two paths:

• Replace the grid-tie inverter with a hybrid inverter and move your panels to it.
• AC-couple a battery inverter beside your existing system. Its continuous rating should be at least as large as your PV inverter.

Either way, your solar can then run during outages. See our inverter + battery bundles.

Most US homes are wired split-phase: two 120V legs that combine to 240V. You need a split-phase inverter if you want to back up 240V loads such as a well pump, central AC, electric dryer, range or Level 2 EV charger. You may hear it called “dual-phase.” On spec sheets, look for “split-phase” or “120/240V.”

LiFePO4 batteries can discharge in the cold but must not be charged below about 32°F (0°C). In unheated spaces, choose self-heating batteries, which warm themselves before charging, or use an insulated, heated enclosure.

Plug-in power stations used with extension cords don’t. Anything connected to your home’s wiring does: a transfer switch, a hybrid inverter, or wall and rack batteries. Those must be installed by a licensed electrician, and most areas require a permit. Grid-connected systems also need your utility’s approval.

UL 1973 certifies the battery itself. UL 9540 certifies the complete energy storage system, and many fire codes and permits require it. UL 9540A is a fire-propagation test method, not a certification. Ask your permitting office which listing it requires.