Solar Panels On Beutiful House

How To Properly Size a Backup Solar Generator For Your Home

The average U.S. home consumes 26,000 watts of electrical power every day, or about 1,100 watts per hour.

That’s why most backup solar generators for home standby power deliver 1,000-5,000 watts of power, per hour.

Question is: how long can your backup solar generator keep the power flowing?

This article explains how to properly size a backup solar generator for your home, so you will know exactly what you are buying and you can estimate how long it will keep your appliances on, during a loss of power. 

I will show you how to answer the following questions related to sizing & selecting the right backup solar generator for your needs:

  1. How much energy will I need to power my devices/appliances during a power outage?
  2. How long can I run my appliances before the batteries run out?
  3. How long will it take to recharge my backup solar generator’s batteries, using solar panels?
  4. What additional features should I consider before buying?

STEP 1:  How Much Power Do You Need?

Goal Zero Yeti 1250 Silver With Black Solar GeneratorThe most common expectation for a home backup solar generator is that it will provide enough electricity to power the most critical and most commonly-needed appliances during a power outage, for a set number of days.  

That backup power is measured in watts.

Now, most people don’t really power their whole home after a blackout; they just need the basics.

Your list of “basics” may include a lot of devices, though: a refrigerator, a microwave several times a day, indoor lighting, keeping a laptop or two going, using a CPAP at night, keeping your phones charged, and perhaps running your radio or television a few hours a night.

You may also want to power an air conditioning unit or heater, depending on your local weather conditions and the health of the people in your home.

In order to figure out how much power you need, then, the first question you have to answer is:

What devices and appliances do you need during a blackout?

Make a list of all the devices you will want to power during a blackout. Next, note the Voltage and Power Rating (watts) for each.

home appliance list with power-requirements for solar backup generator

You can find voltage and wattage on the manufacturer’s label of any US electrical device made during the past 75 years.

Older Manufacturer Labels showing Power Rating in watts.

Older Manufacturer Labels showing Power Rating

If you don’t have time to look at every appliance for its voltage and Power Rating, then use the estimated Power Ratings in our Appliance Energy Consumption Chart.  

Next, for each appliance/device you want to keep on, calculate the Average Daily Watts consumed by each.

To do that, for each device:

  1. Look up the Power Rating (watts per hour) from our chart or your appliance’s manufacturer label.
  2. Calculate the Average Daily Watts by multiplying the Power Rating (in Watts) times the number of hours you want to run the device, each day.

Keep in mind that in an emergency scenario, appliance/device usage will differ from your normal on-the-grid life.

For example, your lights will probably only be on a few hours per night. You may use your television much less. And you may not use your air conditioner at all.

You may also be OK not using your washer and dryer for a few days (wait until you see how much power they consume!).

Add up the Average Daily Watts for all of your devices, and you will probably be within +/-30% of total daily power requirement during a blackout or an emergency.

About Those Labels…

Almost all electric-powered devices and appliances list their power consumption, in watts, on their manufacturer’s label.

Wattage is a measure of the power consumed by a device over time.  1000 Watts = 1 kiloWatt, or kW. 

Manufacturer labels also carry the appliance’s Voltage rating:

  • If your appliance uses AC power (alternating current), then your voltage will be 110-120V or 220-240V at 60 Hz in the US; or, 220-240V at 50 Hz internationally.
  • If your device is powered by DC power (direct current), then 12 and 24 VDC are the most common.

If a device’s Wattage is not listed on the manufacturer label, then you can calculate the power consumption in Watts by multiplying its Voltage times its Amperage.  This equation is true whether working with Alternating Current (AC) or Direct Current (DC) voltage.

Amperage is device-specific; it measures the electrical load, or current, a device requires to start up (surge current) and run continuously.

The most important thing you need to understand is that an appliance’s Power Rating (in Watts) does NOT necessarily equal the amount of power it will draw from your generator batteries, every hour, on average, during a blackout.  

The actual energy it draws will depend on the nature of the device.

To learn more, see: “Deciphering Appliance Power Ratings

Adjust Your Estimates for Variable-Power Appliances

Most electric appliances draw power evenly over time; in this case, the Power Rating is a good approximation of the Watts per hour your device will draw from your generator while it is turned on. These are called fixed-rate appliances.

Examples of fixed-rate appliances include devices that use rechargeable batteries, like laptops and cell phones. They also include incandescent & LED light bulbs, radios, TVs and PCs.

However, some devices in your home use variable power. Typically, this means they draw a lot of power up front to get moving, then consume a small amount of power over time to keep them running. These are called variable-power appliances.

Examples of variable-power appliances include a coffee maker; a bread maker; florescent light bulbs; and, most air conditioners.

For a fixed-rate appliance rated at 1000 Watts, its power consumption will reliably be 1000 Watts per hour, while running.

But for a variable-power appliance, the Power Rating is not a good indicator of how much power the device will draw from batteries over time.

Let’s see how this works in the real world:

Let’s say you use an electric coffee maker, a variable power appliance, that’s rated at 1000 Watts. As we explained, this is the coffee maker’s ‘peak power consumption’, or the power it consumes when brewing – for an entire hour.

coffee maker brewing

Mmmm… can’t wait 🙂

In the real world, though, most electric coffee makers only take a few minutes to brew. After brewing, the hot plate will only consume maybe 200 Watts per hour. Then, when the hot plate turns off, the power consumption goes to zero.

Let’s say your coffee maker brews for six minutes and keeps your coffee warm for 54 more minutes, before shutting off.

In this case, you will consume 1000(0.1)+200(0.9) or 280 W in that first hour.

The rest of the day, it’s turned off.

Later, if you want to reheat that cup of coffee in your 1000W microwave for 1.5 minutes on high, you’ll use another 1000(1.5/60) or 25 W.

As you can see, your daily coffee ritual actually consumes only 305 watts per day, or 12.7 watts per hour, of energy on average, each day. Not 1000 Watts per hour, the coffee maker’s Power rating.

Big difference!

Another example of a variable-power appliance is your refrigerator.

refrigerator door open

A 110V refrigerator may be rated at 6 amps for a power rating of 660 watts, but that’s what it draws with its door open, the defrost cycle on and running the ice maker full blast.

A refrigerator’s typical hourly power consumption is far less in the real world, of course, because a fridge runs 23.9 hours/day with the door closed and only needs enough power to keep its well-insulated interior at the same temperature.

In the real world, then, your fridge will probably only consume 100 watts per hour, as the compressor cycles.  

So, your fridge really only consumes about 2,400 watts per day (24 hours x 100W/hour), which is 6 times less than its rating of 660 watts per hour.

A Quicker Way To Adjust for Variable Power Appliances

Don’t worry, you don’t need to do a detailed calculation for every device in your home (unless you want to).

A good rule of thumb for the hourly power requirement of a variable power device is to take 20% of the stated maximum power consumption.

So, to calculate your Average Daily Watts for each variable power device you need to run during a blackout, multiply its Power Rating x 24 hours x 20%.

Want A More Accurate Estimate?

For an even more accurate measurement of actual power usage, you’ll have to use an electricity monitoring device to measure the true energy consumption of your devices in question.

You can measure usage over the course of a day, week, or even month to get a more accurate estimate of average hourly & daily usage.

STEP 2: How Much Battery Capacity Do You Need?

Deka Solar Battery

The next important consideration is how much battery capacity (in watts) you will need to power your devices without access to a live electrical grid.

Sometimes a backup solar generator’s battery capacity is rated in “watt-hours” (same as watts) or in “amp-hours”. If listed in amp-hours, you will need to convert it into watts by multiplying the amp-hours times the average voltage of your appliances, typically 110VAC.

To determine how much Backup Energy (in watt-hours, or watts) that you’ll need stored in your generator batteries to keep your home running during an outage, follow these steps:

  1. Calculate your Total Daily Power Requirement (in watts) by summing the Average Daily Watts for all devices on your list.
  2. Note the number of days you want to ‘keep the lights on’
  3. Calculate your Backup Energy Storage (in watts) you will need to power your home through an emergency, assuming you cannot recharge the batteries while in use. To do this, multiply the number of days times the Total Daily Power Requirement.

Now you know how many watts of battery capacity you need in your backup solar generator.

Well, almost…

The actual battery capacity you need depends on whether your generator can recharge itself while it’s being used, or not.

STEP 3: How Long Does It Take To Recharge The Batteries, Using Solar Panels?

Goal Zero Yeti 400 Solar Generator

Most backup solar generators are designed to stay fully charged until needed.

Then, you run your appliances on the generator’s batteries, until they run out of power.

This is the safest planning assumption to use when specifying a solar generator backup system: assume that your solar panels won’t recharge the system, while it’s being used.

That said, some solar generators offer the option to trickle-charge your batteries using solar power while the batteries are being used to power your home.

In practice, though, most solar generators cannot trickle-charge the batteries nearly fast enough to keep up with demand of your appliances.

They will eventually run out of power, as the following real-world test of a GoalZero 1250 shows:

Example: Energy Usage vs. Battery Capacity (GoalZero Yeti 1250)

Test Results: Energy Usage vs. Battery Capacity for a GoalZero Yeti 1250 charged using one 90W solar panel (click to enlarge)

If your system allows trickle-charging while its batteries are being drained, then knowing how much time it takes to recharge your batteries will tell you how long your system will keep running beyond the capacity of the initial charge.

The time it takes to recharge your batteries is a function of the solar panels you have exposed to a sunny sky and the power rating of the solar panels you’re using (watts per hour).

You also need to consider the maximum speed at which your batteries can recharge, known as the Maximum Trickle Charge Rate (Watts/Hour). This rate is mostly determined by the battery type (lithium ion, gel, lead-acid, etc.).

If your solar panel power output exceeds the rate a which your batteries can be trickle-charged, then you’ve oversized your panels. But that’s OK – on cloudy days, you will be happy that you did.

To calculate your Battery Recharge Time of a particular backup solar generator that allows for trickle-charging while in use:

  1. Calculate the Backup Energy Storage needed to run all of your appliances during an emergency outage, by following Step 2. above (in Watts).
  2. Calculate your Total Solar Panel Power Supply (Watts/hour) by adding up the power ratings for all Solar Panels available to charge your system.
  3. Note the Maximum Trickle Charge Rate (Watts/hour) of your solar generator.
  4. Calculate your Maximum Recharge Rate (watts/hour) by selecting the lesser between the Maximum Trickle Charge Rate and Total Solar Panel Power Supply.
  5. To calculate your Battery Recharge Time, divide your Backup Energy Storage (watts) by your Maximum Recharge Rate (watts per hour) to get the number of hours of daylight required to recharge your batteries.

If your Battery Recharge Time is less than 6-8 hours, then your system should continue to provide power indefinitely, constantly replenishing itself on solar power alone. This is very rare, however.

If your Battery Recharge Time is longer than 6-8 hours, then your generator will run out of battery capacity at some point.

OK, So How Many Days Of Backup Power Do I Really Have?

OK, so you now know a trickle-charging backup solar generator will last longer than its rated battery energy capacity. But how much longer, exactly.

To calculate a more realistic Total Days of Backup Power for a trickle-charging backup solar generator:

  1. Subtract 8 from your Battery Recharge Time. This defines the Hours of Power Lost per Day.
  2. Divide your Backup Energy Storage by the Hours of Power Lost per Day.
  3. The result is the Total Days of Backup Power available from your trickle-charging generator.

As you can see, the ability to trickle-charge your solar generator while it’s being used can lower the total cost of your system significantly, because it requires fewer batteries to provide you with the same number of days of power.

So, make sure you look for this feature.

STEP 4: What Other Features Matter?

Additional backup solar generator features may be important to you, depending upon your lifestyle and emergency requirements.

Proximity to Solar Panels

Roof with Solar Panels

Some generators allow for solar panels to be located far away from the generator itself, while others have panels mounted to or closely linked to the generator unit.

Protection from Damage & Small Hands

Some generators are housed in steel panel boxes or cages, which provide additional protection from shock or tampering in environments in which children might be present.

Power Output Ports

The Best Solar Generators

All home backup solar generators typically provide several 110/220VAC plugs. Some also come with USB chargers and 12V/24VDC outlets.

A generator’s power output is limited by three things: the number of outlets, its inverter surge rating and its continuous power rating.

The number of individual outlets on a generator is therefore directly related to its total power output.

So don’t assume that purchasing a cheaper generator with a few outlets, and then plugging in a power strip to extend it, is a good idea. It’s not. You may overload the generator.

Digital Display

PowerOak 400wh Portable Solar Generator With Ac Power Supply Energy Storage

Most portable solar generators have simple, digital displays that show remaining stored energy, current energy usage, and any problems the unit might be having.

Some provide full LCD screens with programmable features.


Choose the Best Solar Generator

Many home backup solar generators today can be linked together, or chained, with other units to provide more whole-house power. But you need to verify this ability.

Others will allow you to add more batteries and extra solar panels to increase charging speeds.

STEP 5. Choose Your Generator

OK, now that you know how to calculate the basics, you are ready to shop for a backup solar generator.

There are several things you want to check (or calculate) on every generator you look at, to make sure it’s sized correctly for your home:

Energy Capacity

This is the energy storage capacity of the batteries, usually listed in Watts, Watt-Hours or Amp-Hours.

Peak Power Rating (in Watts)

Make sure you select a generator that has a higher Peak Power Rating than your average hourly power requirement.

Divide your Total Daily Power Requirement from Step 2. by 24, to compare.

Power Outlet Ports

GoalZero Yeti 1250: power outlet ports

GoalZero Yeti 1250: power outlet ports

Make sure there are enough ports to supply your appliances (without using extension panels); and, make sure you have the right number of each type to feed your appliances and devices.

Be practical about sharing ports, though – you don’t need 4 USB ports to keep 4 phones charged.

Time To Recharge

How long will it takes to recharge the batteries (hours) with a given set of solar panels?

See Step 3. above to estimate this accurately.

Buy more solar panels, if you want to recharge faster.

Total Days of Backup Power

If your generator can recharge itself while in use, then calculate your realistic backup time using Step 4. above.

If it cannot, then your backup time (in days) = Energy Capacity (watts) / Total Daily Power Requirement (watts/days)

Hidden Limits

Finally, keep in mind that every generator’s electrical circuitry is different.

The amount of power it can deliver continuously and at peak demand is limited by internal components including the inverter, circuit breakers and fuses, and each plug/socket’s amperage limit.

For example, most power outlets in a house are often protected to 15 amps, so most home generators are sized to provide a power per plug up to this rating.

Take care to read the fine print for any special restrictions. These kinds of hidden restrictions are more common with do-it-yourself and cheaper backup solar generator systems.

About the Author David Roberts

I'm a Mechanical Engineer who's obsessed with solar energy and sustainable living.

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