Planning backup power, batteries and solar

Backup power is a list of what you want to run and for how long. Everything else (the generator or the battery, the size of the inverter, the number of solar panels) follows from that list. Starting a motor takes several times its running power, which is why a generator that looks big enough on paper can still trip when the fridge or the pump starts.

This planner starts with the list of loads and ends with a solar plan. Connecting a generator to a house is regulated work and must be done with a transfer switch by a licensed electrician.

Step 1: List the loads and the hours

Write down the lights, the fridge and freezer, the furnace fan, the well pump, the internet and the phones, with their running watts, starting watts and the hours you will use them. Start with the essentials and add comforts later.

Step 2: Size a generator

A generator must supply the running watts of everything on at once, plus the largest starting surge. Add a margin of 20% or so. Fuel use depends on the load, and the generator must be run outdoors, away from windows, to avoid carbon monoxide.

Step 3: Or size a portable power station

A power station is a battery with an inverter, rated in watt-hours of storage and watts of output. The runtime is the usable watt-hours divided by the load, less inverter losses. It is quiet and can be used indoors, but its energy is limited.

Step 4: Add solar panels

The number of panels depends on the energy you use each day, the sun hours at your location and the losses in the system. Solar can recharge a power station or a battery bank, or offset your bill with a grid-tied system.

Step 5: Wire it safely

The wire and the breakers between panels, batteries and inverters have to be sized for the current, and low-voltage runs lose a lot of voltage over distance. Use the right fuses and follow the equipment maker's instructions.

Before you start

• The list of essential loads with running and starting watts.
• A safe place to run a generator outdoors, with a carbon monoxide alarm indoors.
• A transfer switch or interlock installed by a licensed electrician.
• Fuel storage that is safe and legal, and rotated.
• The permit and the utility's rules for grid-tied solar.

Mistakes that cost people money

• Sizing a generator for the running watts only, then tripping on motor starts.
• Running a generator in a garage or near a window.
• Back-feeding the house through an outlet instead of a proper transfer switch.
• Expecting a small power station to run a heater or a space-hungry load.
• Under-sizing the wire on a low-voltage solar run.

Frequently asked questions