Backup power questions, answered plainly

Add up the running watts of everything you will run at once, add the biggest motor’s startup surge, then divide by the power factor (about 0.8) to get the VA rating, and add roughly 20 percent headroom. Our inverter size calculator does all of this for you.

For anything with a motor or sensitive electronics (a fridge, AC, pump, or computer), yes. Modified sine runs those hot and shortens their life. It is only a safe saving on simple resistive loads like plain lights and basic fans.

Motors surge to three to five times their running watts for a second or two at startup. If the inverter cannot supply that surge, it trips into overload. Size for the surge, not just the running load, and for a non-inverter AC use a low-frequency (transformer-based) inverter that holds voltage through the start.

VA is apparent power; watts is real power. They are linked by the power factor, about 0.8 in a typical home, so a 1,000 VA inverter delivers roughly 800 usable watts. Divide your watts by 0.8 to find the VA rating to shop for.

Not all of it. Flooded and tubular lead-acid, and AGM or gel, should be treated as about 50 percent usable for safe sizing. Lithium (LiFePO4) safely gives up around 80 to 90 percent. That usable share, not the label rating, sets your real backup time.

Usually, over its life. Lithium lets you use most of its capacity, lasts three to five times longer, and barely loses energy under heavy loads. It costs more up front but often works out cheaper per usable kWh over the years.

It depends on the battery capacity, the load, and the chemistry. As a rough guide a 150 Ah 12V tubular battery runs a 300 watt load for around 2.5 to 3 hours. Use the battery backup time calculator for your exact numbers.

Yes, and it is not optional. They release hydrogen while charging, which is explosive at just 4 percent in air. Install them in a ventilated space with airflow at the top of the enclosure, away from any spark or switch. Sealed AGM, gel, and lithium do not have this problem.

Start from your real daily energy use in kilowatt-hours, divide by your area’s peak sun hours, then add 25 to 30 percent for losses. That gives the panel size. The battery is sized separately, from how much of that energy you need after dark.

The number of hours a day your panels effectively make full rated power. It is not the same as daylight hours. A sunny area averages around five, less in winter or under cloud, and it is the single biggest variable in solar sizing.

Your bill shows your real consumption in units (kWh) and reveals which appliances dominate it. Sizing solar or backup from guesses wastes money; sizing from your actual usage is the difference between a system that fits and one that does not.

Yes. Every calculator and guide is free, with no sign-up and no personal details required. The site is supported by advertising.

They are solid planning estimates built on standard engineering math and typical appliance values. Always confirm against the rating plate on your own equipment, and use a licensed electrician for installation.

No. Never tie an inverter into house circuits without a listed transfer switch or interlock. Done wrong, it can push power back onto the grid and electrocute a lineman, and destroy the inverter when the grid returns. Have a licensed electrician install proper transfer equipment.

The content and calculators are authored and reviewed by Engr. Syed Farrukh Anwar, a Registered Professional Electronics Engineer. His credentials are listed on the author page with links to the independent registers where you can verify them.