How much power does your house actually use?

Four numbers describe a load, and each one rules out a different mistake. People usually buy on the wrong one.

The fourth number is not a property of your house at all, it is a decision. Storage is daily energy multiplied by the number of days you want to ride out, divided by how deeply you are willing to discharge the battery. The calculator uses two days at 85 % depth of discharge, a common planning value for lithium iron phosphate packs. Change either assumption and the number moves; the page prints the formula so you can.

Note what surge does not do: it does not scale with how long an appliance runs. A compressor surges once per start cycle whether it runs for ten minutes or eight hours. If you cycle the fridge (two hours on, two hours off), surge applies every time it restarts, so a plan built on cycling does not reduce the surge requirement at all.

One more distinction worth holding onto: watts and Wh are not interchangeable. A 1,500 W space heater running for one hour uses 1,500 Wh; a 150 W refrigerator running for ten hours uses the same energy at one tenth the instantaneous draw. Two devices can cost the same in energy and demand wildly different equipment.

Every row below is a typical US 120 V appliance, not one specific model. A 10,000 BTU window unit is not a 3-ton central system, and a modern inverter fridge draws less than the 1990s unit in your garage. The ranges come from appliance energy labels and manufacturer specification sheets; when the label on your own appliance disagrees with this table, the label is right and you should type its number into the calculator.

You do not have to trust this table. Every appliance sold in the United States carries a label with the numbers you need, and reading it takes about twenty seconds per appliance.

Look for a plate or sticker, usually inside the door or on the back. It will list voltage (120 V in the US), amperes, and sometimes watts directly. If only amps are printed, multiply by 120: a refrigerator rated 1.25 A draws about 150 W. If only a model number is printed, put that number into the manufacturer's specification page, that is where the surge figure usually lives, because the label rarely shows surge current.

Two traps. First, nameplate watts on a heater or microwave can be double the running draw: a microwave labelled "1,000 W cooking power" pulls roughly 1,500 W from the wall, because the magnetron is not perfectly efficient. Second, an inverter compressor fridge may list a very low average draw, that is a daily average, not a peak, and it still surges on start. When a label gives an annual kWh figure (many ENERGY STAR labels do), divide by 365 to get daily energy, then divide by your estimated run hours to sanity-check the running watts.

A practical shortcut: if the label says 120 V and a single amp figure, the running watts are amps × 120. A 5 A label is 600 W; a 0.75 A label is 90 W. If the appliance has a motor, compressor, pump, fan, blower, assume the worst case surge is three to five times that figure unless the manufacturer publishes a surge number. If it has a heating element, the surge equals the running draw, because resistive loads do not surge. That one rule covers most of what a homeowner needs to type into a load calculator, and it is why the table above shows surge equal to running draw for the heater and the microwave.

Storage is the number people get wrong most often, because they buy the capacity printed on the box instead of the usable capacity they will actually get.

Two days is a planning choice, not a law. It covers the common case where an outage lasts through the night and the array cannot refill until the next afternoon. If your outages are measured in hours, one day is enough; if you live where winter storms take the grid down for three days, plan for three.

Depth of discharge is the second half. A lithium iron phosphate pack is commonly planned at 80–90 % usable; lead-acid is planned at 50 % to protect cycle life. The calculator uses 85 %, which sits inside the lithium range. This is why a "5 kWh" battery does not give you 5 kWh of outage: at 85 % it gives about 4.25 kWh, and a two-day plan on 2,170 Wh a day needs 5,106 Wh of nameplate to supply 4,340 Wh of usable energy.

There is also an efficiency loss you should not ignore: every battery loses a few percent between what goes in and what comes out, and the inverter that turns DC into household AC loses a few more. The calculator does not add a separate inverter allowance, which means its storage figure is slightly optimistic by design, treat 5,106 Wh as a floor for a two-day plan, not a target to hit exactly.

These solve different problems, and the calculator prints both numbers so you can see which one your case needs.

A bigger battery buys time. It lets you ride through a longer outage with no charging at all. Panels buy autonomy: they let you stay up indefinitely as long as the sun returns, because they refill what you use each day. A house that uses 2,170 Wh a day and has 5,106 Wh of storage can run two days with no sun. The same house with 723 W of panels refilling at four peak sun hours a day can run forever, but it will be dark on the second cloudy day.

Most practical setups are a small battery plus a modest array: enough storage for one night, enough panels to refill by the next afternoon. The array figure assumes 75 % system efficiency, which accounts for charge controller losses, wiring, and the fact that panels rarely sit at their rated output. On a balcony or a wall mount, expect worse than a roof: partial shading, a non-ideal tilt, and no active cooling in summer all cost you a few percent.

If your daily energy is under about 2,500 Wh, panels are optional; the battery alone will outlast most outages. Above 5,000 Wh, panels stop being optional, because the battery you would need to cover three days gets expensive faster than the array does.

The order of purchase matters here. Buy the battery for the night first, then the array for the days. Doing it the other way round leaves you with generation you cannot store, which is the most common way people waste money on this category.

One more check before you buy an array: confirm what the station or wall accepts as input. A 400 W panel set feeding a station with a 200 W solar input limit will simply clip, and you will pay for panels you cannot use. The input limit is printed on the specification sheet, and it is the number people forget to read. If you plan to expand later, buy a unit whose solar input can grow with the array rather than one that caps it on day one.

These are the three cases this calculator sees most often, worked through with the same arithmetic it uses. Every number below is reproducible: set the same appliances and hours on the right and you will get the same totals.

Look at the gap between peak and surge in the second case. The pump runs at 800 W but starts at 2,400 W, so the surge figure is three times the running figure. This is the single most common reason a station that "should work" shuts off the moment the pump starts. Always compare your surge figure to the station's surge rating, not to its running rating.

The third case is the one that changes a purchase decision. Adding an air conditioner roughly triples daily energy but only adds about 3,000 W of surge, because a single compressor surging is smaller than the sum of every inductive load you might run. If you plan to run the AC only during the day, you can cut its hours and watch the storage number fall, the calculator updates as you type.

One: sizing on running watts only. A 1,000 W station will run a 150 W fridge and will not survive the 600 W start. Buy on surge.

Two: treating nameplate capacity as usable capacity. A 5 kWh battery at 85 % depth of discharge gives 4.25 kWh. If you plan a two-day backup on 2,170 Wh a day, 5 kWh nameplate is the floor, not a generous margin.

Three: sizing for the worst month only, or for the best month only. Four peak sun hours is a year-round average for much of the US. Sizing in June leaves you short in December; sizing in December leaves money on the roof in June. The sun hours page covers the monthly spread.

Two more belong to the same family even though this page handles them elsewhere. Counting every appliance in the house inflates the number you are trying to size, dryers, ranges and central air are 240 V and will not connect to a 120 V station at all. And forgetting that storage and generation are separate purchases leads to buying one of each badly: a battery with no array is a countdown timer, an array with no battery is a daytime-only supply, and the calculator prints both figures precisely so the two do not get confused.

The common thread is that every one of these mistakes is cheap to make and expensive to discover. A wrong surge figure shows up as a device that shuts off; a wrong storage figure shows up as a dead battery on day two. Both are arithmetic problems, and both are fixable here before you spend anything.

This is a 120 V load calculator. It does not size a 240 V circuit, a transfer switch, a subpanel, or a generator inlet, and it is not a substitute for an electrician's load calculation. Anything that involves opening a panel, adding a breaker, or connecting a battery to house wiring needs a licensed electrician and, in most jurisdictions, a permit.

It also does not model duty cycles in detail. A refrigerator does not run continuously for eight hours; it cycles on and off, and the compressor's duty cycle depends on room temperature and how often the door opens. Using eight hours is a deliberately conservative stand-in for a warm kitchen. If you want a tighter number, set the fridge hours to six and compare; many people find the difference changes nothing about which battery class they buy.

What it also cannot tell you is whether your panel can accept the load at all. A 15 A kitchen circuit and a 20 A garage circuit impose different limits, and neither of them appears in a Wh total. That question belongs to an electrician with a meter in hand.

The full library behind these defaults, with 150 appliances and the provenance of every row, is in the appliance wattage table.