Frequently asked questions
Every answer is shown in full below, followed by 27 power terms in plain English.
Sizing
How big a battery do I need?
Add up daily watt-hours, divide by usable depth of discharge and inverter efficiency, then multiply by days of autonomy. See how to size a battery bank.
How many panels do I need?
Divide daily watt-hours by peak sun hours and a derate of 0.77 (NREL PVWatts manual). See solar panel sizing.
Can a battery run my refrigerator?
Usually yes, if the inverter covers the compressor's start-up surge and the battery covers its daily watt-hours. Check the appliance's energy label for annual kWh.
Batteries
Is lithium better than lead-acid?
For regular cycling, LiFePO4 usually offers more usable capacity and cycles. See LiFePO4 vs lead-acid.
Can I charge a battery in freezing weather?
Many lithium batteries block charging below about 32 °F (0 °C) unless they have built-in heating. Check the manufacturer's charge temperature range.
Why does my battery show less than its rating?
Inverter losses, BMS reserves, temperature and age all reduce usable energy.
Safety and installation
Can I plug a power station into a wall outlet to power my house?
No. That backfeeds the wiring and can endanger utility workers. Use a transfer switch or interlock installed by an electrician.
Do I need a permit?
Systems connected to home wiring or the grid usually do. Check your local building department and utility.
Are there tax credits?
Federal and state incentives change; check IRS.gov and your state energy office for current rules before relying on one.
Power terms
- AGM
- Absorbent glass mat sealed lead-acid battery.
- Amp-hour (Ah)
- Battery charge capacity at a given voltage.
- Backfeed
- Power flowing into utility lines from a home source; dangerous.
- BMS
- Battery management system that protects cells.
- Charge controller
- Regulates charging from panels to battery.
- Critical loads panel
- Subpanel holding the circuits a backup system powers.
- Cycle life
- Number of charge-discharge cycles a battery is rated for at a stated DoD.
- Days of autonomy
- Days a battery can run loads without recharging.
- Depth of discharge (DoD)
- Share of a battery's capacity that is used.
- Derate
- Factor for real-world losses in solar output.
- Inverter
- Converts DC battery power to AC household power.
- Kilowatt-hour (kWh)
- 1,000 watt-hours.
- LiFePO4 (LFP)
- Lithium iron phosphate battery chemistry.
- MPPT
- Maximum power point tracking controller.
- Net metering
- Utility billing credit for solar exported to the grid; rules vary.
- NMC
- Nickel manganese cobalt lithium chemistry.
- Open-circuit voltage (Voc)
- Panel voltage with no load; rises in cold.
- Pass-through charging
- Using a power station while it charges.
- Peak sun hours
- Hours of equivalent 1,000 W/m² sunlight per day.
- Pure sine wave
- Inverter output that closely matches grid power.
- PWM
- Pulse-width modulation controller.
- State of charge (SoC)
- How full a battery is, as a percentage.
- Surge rating
- Short-term peak power an inverter can supply.
- Temperature coefficient
- How much panel output changes per degree.
- Transfer switch
- Device that isolates home circuits from the grid during backup.
- Watt (W)
- Rate of power use or production.
- Watt-hour (Wh)
- Energy: one watt for one hour.