How to Use a Kill A Watt to Estimate Your Solar Needs

“How much does it cost to build an off-grid solar system?” 

Well, it depends on how much electricity you use during the day and at night, and what you use the power for.

“Can you just build something without me answering all these questions?”

Sure! But are you ready to pay $30k - 50k for a grossly over-dimensioned system vs. ~$20k for a right-sized whole-house setup simply by understanding your consumption pattern?

Also, knowing how your appliances behave puts you in control of allocating resources — the foundation for achieving energy independence. 

The good news is that collecting data to understand your power consumption pattern isn’t rocket science: Use a Kill A Watt Electricity Usage Monitor, which measures power consumption for each appliance and helps you identify power hogs. (Don’t cut corners with cheap knockoffs. Get the real deal because accuracy is critical.) 

Before starting your data collection effort, let’s discuss what numbers you need to look at and what they mean.

Which numbers should I look at on a Kill A Watt?

You should measure two types of loads to dimension off-grid solar: continuous and intermittent. 

Continuous loads consume a lot of energy over an extended duration (like running a marathon), and intermittent loads require a big burst of power within a short period (like sprinting).

Appliances humming along all day, like a fridge or Starlink, are continuous loads. They don’t consume much energy at any moment, but their 24/7 nature means their electricity consumption adds up (and runs into money).

Things requiring substantial power to jolt into action, like a 2 HP well pump or most power tools, need a system that can support the burst of short but intense power.

So far, so good. We’ll show you how to measure these loads and use the data to dimension a solar system soon.

But first, what data should you collect?

You only need to collect two readings to dimension an off-grid solar system: watts (W) and kilowatt-hour (kWh). Watts measures instantaneous power flow, while kWh measures energy consumed by a 1,000-watt appliance running for one hour — i.e., how much electricity an appliance consumes over a more extensive period. 

How to collect data for solar dimensioning with a Kill A Watt

Collect data by plugging a Kill A Watt into a power source (i.e., outlet, generator, solar, extension cord, etc.) The device should boot up and show numbers with both W and kWh as zero. Then, plug the appliance you want to measure into the Kill A Watt and turn it on. You should see the readings accumulate.

When you unplug the Kill A Watt from the wall, all the numbers will zero out. The device won’t save the data you’ve collected. So, write down all the relevant numbers before unplugging.

So, what do those numbers mean?

For“marathon” appliances, the readings help determine the number of solar panels you need to produce adequate power and how much battery capacity you need for nighttime usage.

For “sprint” appliances, the numbers determine your inverter capacity and, to a lesser degree, battery characteristics. For example, we build custom battery solutions for mobile workshop projects to address the high surge power requirements without adding unnecessary costs or weight.

When we collect data to dimension a portable solution for powering hand tools, we look at starting power, running power, and loaded watts.

After plugging the power tool into the Kill A Watt and starting it, you’ll see a surge in watts. That’s the starting power. Then, the number will settle and show running power. When the power tool performs work (e.g., a saw cutting through wood), the Kill A Watt will show the loaded watts.

Different appliances or tools behave differently. Don’t be surprised that the surge and load ratings are three to seven times higher than the running power.

Take Readings Over Time for Accurate Insights

Some “sprint” appliances draw high but short surge currents upon start, which can be hard for the Kill A Watt to register. You may have to start it a few times, write down all readings, and use the highest number to determine the starting power.

After you have an appliance plugged in (especially the “marathon” ones), go about your regular day and let the Kill A Watt do its thing. After a while (e.g., a full day), and before switching things off or unplugging the device, push the kWh button and write down the number — that’s the total power used by the appliance.

Collect readings for the same appliance over a few days to a week to get an accurate view. You may note down fluctuations and correlate them with changes in behaviors.

Typically, powerful devices/appliances/tools with high surge power don’t use much electricity throughout the day. However, the “silent killers” in the background, e.g., A/C and fridges, often generate shocking numbers. (If you multiply the kWh by the electricity rates, you may be surprised at how much an old, inefficient fridge may cost you!)

Rinse and repeat to get the grand total

To understand your surge requirement and total power consumption, repeat the measuring process for all appliances that draw significant power in your household. 

LED lights, radios, and even TVs aren’t big power hogs. However, anything that moves heat is. These include air conditioners, space heaters, microwave ovens, electric water heaters, kettles, hair dryers, toaster/toaster ovens, etc. You get the idea.

“Wait, we have to repeat that process for every appliance?”

Pretty much. The more details you have, the more accurate the estimation will be.

Don’t want to stare at a Kill A Watt for several weeks? Nor do we. So, we built a thing. We can gather data on your whole-house usage with our proprietary device to identify surge and continuous loads in as little as 24 hours and gather the information we need (and plot fancy charts) to dimension a solar system appropriately.  

Then, we'll recommend a strategy or power source to get the biggest bang for your buck using the data and insights (from our device or your Kill A Watt measurements).

“What does strategy mean? Aren’t you selling solar?”

Well, solar is often the answer, but not always. Our goal is to help our clients achieve energy independence and resilience cost-effectively. Sometimes, it means recommending a solution that’s not part of our solar offerings to help them do more with less. 

For example, someone with infrequent and minimal backup needs might be better off buying a commercially available portable power station (although they aren’t suitable for backup against prolonged outages). Meanwhile, a client who wants to run power tools occasionally during an outage may use a generator for that purpose while putting the rest of the house on solar to save thousands on inverter and battery capacities.

We may also advise some clients to improve their household’s energy efficiency to lower the costs of their solar solution. Some simple tactics include improving insulation, upgrading to energy-efficient appliances (e.g., fridge and A/C), and identifying mystery loads.

So, yeah, we can be annoying, asking all those questions and measuring usage. But that’s how we help clients lower the cost of off-grid solar while enhancing energy independence without sacrificing their lifestyle preferences.