When sizing a solar system, whether it’s a grid-tied with net metering or an off-grid setup with batteries, there are two crucial questions to address:
- How much energy (kWh) are you looking to offset?
- How much sunlight will be available for the solar panels to convert into energy?
Once these two variables are taken into account, sizing the solar system becomes a matter of straightforward calculations.
In this article, I’ll explore the electricity consumption of heat pumps, offering estimates and providing accurate methods for estimating the energy usage of your specific heat pump.
I’ll also explore the amount of sunlight that will be accessible to your solar panels for conversion into electricity, explaining how it’s measured and how you can determine it for your particular location.
I will then illustrate how to combine these two pieces of information to, at the very least, obtain a good estimate of the solar power you require, whether you are considering grid-tied or off-grid scenarios.
This article will serve as a comprehensive guide, providing you with most, if not all, of the necessary information. Let’s get started right away.
I get commissions for purchases made through links in this post.
How much electricity does your heat pump use?
There are various units of measurement in electricity, including Amps, Volts, and Watts, which respectively measure Electrical Current, Electrical Potential (Voltage), and Electrical Power.
However, what matters most in our context, and what utility providers bill you for, is “Electrical Energy.”
Electrical Energy represents the actual amount of electricity that your appliances consume over a specific period, conventionally measured in kiloWatt-hours, or kWh for short.
For example, you might say that a refrigerator consumes 2 kWh of energy per day, or that the average U.S. household uses 1000 kWh of energy per month.
So, how much electrical energy does a heat pump consume?
Well, it varies depending on several factors:
- The heat pump’s heating/cooling capacity (tons/BTUs).
- Its energy efficiency.
- Operating conditions, such as:
- Hours of use
- Outdoor temperatures
- Indoor temperature settings
- Whether the heat pump is cooling or heating
- The quality of insulation
But if we set aside these operating conditions for a moment, we can estimate a heat pump’s energy usage based on its cooling and heating capacity and energy efficiency.
Let’s begin by comparing a heat pump’s energy use during the cooling and heating seasons.
Heat Pump’s Energy Consumption: Cooling vs. Heating
In the cooling season, heat pumps typically consume between 0.6 and 0.85 kWh of energy per hour for every ton (12,000 BTUs) of cooling capacity. On the other hand, during the heating season, they use between 1 and 1.6 kWh of energy per hour for every ton of heating capacity.
For instance, a 3-ton (36,000 BTU) heat pump will typically use between 1.8 and 2.6 kWh of energy per hour of use when cooling, and between 3 and 4.8 kWh of energy per hour when heating.
Here’s a table to help you visualize this information better, categorizing heat pumps by cooling capacity and estimating their hourly energy consumption during the cooling season based on efficiency:
Capacity Rating (Tons/BTUs) | Hourly Energy Use (kWh/hour) for cooling | ||
Most Efficient | Least Efficient | ||
1 Ton (12,000 BTUs) | 0.43 kWh/hour | 0.85 kWh/hour | |
1.5 tons (18,000 BTUs) | 0.64 kWh/hour | 1.3 kWh/hour | |
2 Tons (24,000 BTUs) | 0.86 kWh/hour | 1.7 kWh/hour | |
2.5 Tons (30,000 BTUs) | 1.07 kWh/hour | 2.1 kWh/hour | |
3 Tons (36,000 BTUs) | 1.3 kWh/hour | 2.6 kWh/hour | |
3.5 tons (42,000 BTUs) | 1.5 kWh/hour | 3 kWh/hour | |
4 Tons (48,000 BTUs) | 1.7 kWh/hour | 3.4 kWh/hour | |
5 Tons (60,000 BTUs) | 2.2 kWh/hour | 4.25 kWh/hour |
And here’s a table that categorizes heat pumps by their heating capacity and estimates their hourly energy consumption during the heating season based on efficiency:
Capacity Rating (Tons/BTUs) | Hourly Energy Use (kWh/hour) for heating | ||
Most Efficient | Least Efficient | ||
1 Ton (12,000 BTUs) | 1.05 kWh/hour | 1.6 kWh/hour | |
1.5 tons (18,000 BTUs) | 1.6 kWh/hour | 2.4 kWh/hour | |
2 Tons (24,000 BTUs) | 2.1 kWh/hour | 3.2 kWh/hour | |
2.5 Tons (30,000 BTUs) | 2.6 kWh/hour | 4 kWh/hour | |
3 Tons (36,000 BTUs) | 3.1 kWh/hour | 4.8 kWh/hour | |
3.5 tons (42,000 BTUs) | 3.7 kWh/hour | 5.6 kWh/hour | |
4 Tons (48,000 BTUs) | 4.2 kWh/hour | 6.4 kWh/hour | |
5 Tons (60,000 BTUs) | 5.25 kWh/hour | 8 kWh/hour |
While these estimates serve as an initial guideline and should give you a good idea of the energy usage of your heat pump, the specific efficiency ratings (SEER for cooling and HSPF for heating) of your heat pump can provide a more accurate estimate of its energy usage during different seasons.
Here are the formulas for calculating hourly energy consumption using these ratings:
For Cooling Season:
Hourly Energy Consumption for Cooling (kWh/hour) = (BTU rating ÷ SEER rating) ÷ 1000
For Heating Season:
Hourly Energy Consumption for Heating (kWh/hour) = (BTU rating ÷ HSPF rating) ÷ 1000
Find a better explanation with examples on this page: How much electricity does a heat pump use?
You can then use these hourly energy consumption values along with your daily usage hours to estimate the daily energy consumption:
Daily Energy Consumption (kWh/day) = Hourly Energy Consumption (kWh/hour) x Daily Usage (hours/day)
As I’ll explain later in this article, the daily energy use of your heat pump will be crucial when sizing the solar panels for those planning an off-grid system.
For those of you who are planning a grid-tied system, you’ll be more interested in the annual energy consumption of the heat pump.
Heat Pump’s Annual Energy Consumption
In general, a heat pump typically uses between 2000 and 3500 kWh of energy annually for every ton (12,000 BTUs) of its heating/cooling capacity.
For instance, a 3-ton (36,000 BTU) heat pump can have an annual energy consumption ranging from 6,000 to 10,500 kWh, covering both heating and cooling needs. Conversely, a 5-ton (60,000 BTU) heat pump may consume between 10,000 and 17,000 kWh of energy annually.
However, it’s essential to note that the exact yearly energy usage of a heat pump isn’t solely determined by its heating/cooling capacity. It can significantly vary due to factors like its efficiency, seasonal use patterns for both heating and cooling and external influences like climate conditions.
To provide you with a preliminary reference, here’s a table comparing heat pump annual energy usage based on their cooling/heating capacity (tonnage/BTUs) and energy efficiency:
Capacity Rating (Tons/BTUs) | Annual Energy Use (kWh/year) (Heating and Cooling) | ||
Most Efficient | Least Efficient | ||
1 Ton (12,000 BTUs) | 2,100 kWh/year | 3,400 kWh/year | |
1.5 tons (18,000 BTUs) | 3,200 kWh/year | 5,100 kWh/year | |
2 Tons (24,000 BTUs) | 4,200 kWh/year | 6,800 kWh/year | |
2.5 Tons (30,000 BTUs) | 5,300 kWh/year | 8,500 kWh/year | |
3 Tons (36,000 BTUs) | 6,300 kWh/year | 10,200 kWh/year | |
3.5 tons (42,000 BTUs) | 7,400 kWh/year | 11,900 kWh/year | |
4 Tons (48,000 BTUs) | 8,400 kWh/year | 13,600 kWh/year | |
5 Tons (60,000 BTUs) | 10,500 kWh/year | 17,000 kWh/year |
These estimates were calculated using the U.S. Department of Energy’s (DOE) testing protocols for determining the energy consumption of central air conditioners and heat pumps.
You can gauge your heat pump’s yearly energy consumption by multiplying its hourly energy usage during the cooling and heating seasons by their respective annual usage cycles (hours/year), and then summing these values:
Annual Energy Use (kWh/year) = (Hourly Energy Consumption (cooling) x Average Cooling Usage Cycle (hours/year)) + (Hourly Energy Consumption (heating) x Average Heating Usage Cycle (hours/year))
According to the U.S. DOE’s criteria for measuring the annual energy use of central air conditioners and heat pumps:
- For cooling-only air conditioners or heat pumps that provide both cooling and heating, the average annual usage cycle for cooling is 1,000 hours per year.
- For heating-only heat pumps or heat pumps that provide both cooling and heating, the average annual usage cycle for heating is approximately 1,600 hours per year.
Applying these guidelines, the formula to calculate the annual energy consumption of a heat pump providing both cooling and heating becomes:
Annual Energy Use (kWh/year) = (Hourly Energy Consumption (cooling) x 1,000 hours/year) + (Hourly Energy Consumption (heating) x 1,600 hours/year)
You can find more detailed information on this topic here.
Now, moving on to another crucial variable to consider: sunlight, how much you get of it, and its impact on your system’s size.
How much sunlight will your solar panels receive?
The amount of energy a solar panel generates over a specific period is directly related to the amount of sunlight it receives during that same period.
In solar applications, the sunlight a given area receives over a certain duration is measured in kWh/m2 (kilowatt-hours per square meter) and referred to as Peak Sun Hours (1 Peak Sun Hour = 1 kWh/m2).
For example, if a solar panel receives an average of 5 kWh/m^{2} per day, it could be said that the panel receives 5 Peak Sun Hours per day.
Historical data on the average Peak Sun Hours in a specific location can be used in conjunction with the power rating of a solar system (in Watts or kiloWatts) to predict the system’s energy production (in Watt-hours or kiloWatt-hours) in that location:
Energy Production (Watt-hours or kiloWatt-hours) = Solar Power Rating (Watts or kiloWatts) x Peak Sun Hours
For instance, if you install a 1 kiloWatt (1,000 Watt) solar system in a location that typically receives 5 Peak Sun Hours a day, the system is expected to generate approximately 5 kWh (5,000 Wh) of energy daily in that area.
Conversely, if you know the amount of energy you require a solar system to produce and have historical data on Peak Sun Hours for that location, you can calculate the necessary system size as follows:
Solar Power Rating (Watts or kiloWatts) = Energy Production (Watt-hours or kiloWatt-hours) ÷ Peak Sun Hours
However, to account for inherent system losses and typical energy efficiency of about 80%, it is advisable to apply a multiplier of 1.25 when determining the size of your solar array:
Solar Power Rating (Watts or kiloWatts) = (Energy Production (Watt-hours or kiloWatt-hours) ÷ Peak Sun Hours) x 1.25
Okay, now you know the math, but how do you determine the Peak Sun Hours in your location?
You can estimate the average daily Peak Sun Hours for your area using historical data with the PVWatts Calculator, a free tool provided by the National Renewable Energy Laboratory (NREL).
To begin, visit the tool and enter your address:
As an example, when I enter an address in Albuquerque, New Mexico into the PVWatts, it generates the following results in the “Results” section of the tool:
Once you’ve submitted your address, the tool will provide a table displaying monthly averages for each month, along with an annual average for the entire year.
For instance, in a location like Albuquerque, the annual average is approximately 6.31 Peak Sun Hours per day. However, during specific months, such as May, it can receive as much as 7.78 Peak Sun Hours per day, while in December, it may receive as little as 4.45 PSH/day.
As I’ll clarify below, the annual average PSH value is generally used when sizing grid-tied solar systems, while the monthly average PSH values are more relevant for sizing off-grid solar systems.
Additionally, it’s important to note that the PVWatts Calculator defaults to assuming that your solar panels will be positioned at a 20-degree angle, equivalent to a 4-5/12 roof pitch, and facing due South, which is equivalent to an Azimuth angle of 180 degrees.
However, the actual orientation of your solar panels will significantly impact the amount of sunlight they receive (Peak Sun Hours) and, consequently, the energy they produce.
Therefore, it’s essential to visit the “System Info” section of the tool and adjust the default inputs to match the orientation of your planned setup:
For example, if it’s going to be a roof-mounted system, and you’re unsure of the exact Azimuth angle of your roof, you can use apps like Commander Compass Go for iOS or Azimuth Compass for Android to determine the Azimuth angle accurately.
Similarly, if you have doubts about the Tilt Angle of your roof, apps such as Measure for iOS or Bubble Level for Android can be used to measure the tilt angle of the roof section precisely.
Once you’ve provided the accurate details that describe your future solar panel setup to the calculator, you’ll be able to generate more precise estimates for monthly and yearly Peak Sun Hour averages.
Now, let’s delve into the actual size of the solar installation you’ll require, starting with grid-tied systems.
How many grid-tied solar panels do you need to run a heat pump?
In grid-tied solar systems, also known as on-grid or grid-connected systems, specifically those with net metering, the grid essentially functions as a massive battery.
In this setup, if your energy consumption exceeds what your solar panels generate, you pay for the additional energy used. Conversely, if your solar panels produce more energy than you consume, your electricity provider compensates you for the surplus energy.
Because solar panels generate more power in the summer and less in the winter, forming an annual cycle, this arrangement enables you to use the excess energy your solar system generates during the summer to cover your energy needs in the winter.
In other words, when determining the size of a solar system that can run your heat pump without incurring additional utility costs, the annual energy production of the system should align with the annual energy consumption of the heat pump.
As a general rule of thumb for grid-tied solar systems, you would typically require approximately 1.4 to 2.3 kW (1,400 to 2,300 Watts) of solar panel capacity for every ton (12,000 BTUs) of heating/cooling.
To provide a clearer picture, here’s a table estimating the solar panel system size needed to offset 100% of the energy usage of heat pumps that provide both cooling and heating, based on their Capacity (Tons/BTUs):
Capacity Rating (Tons/BTUs) | Annual Energy Use—Heating and Cooling (kWh/year) based on Heat Pumps’s Energy Efficiency (SEER2/HSPF2) |
1 Ton (12,000 BTUs) | 1.4 – 2.3 kW |
1.5 tons (18,000 BTUs) | 2.1 – 3.4 kW |
2 Tons (24,000 BTUs) | 2.8 – 4.6 kW |
2.5 Tons (30,000 BTUs) | 3.5 – 5.7 kW |
3 Tons (36,000 BTUs) | 4.2 – 6.9 kW |
3.5 tons (42,000 BTUs) | 4.9 – 8 kW |
4 Tons (48,000 BTUs) | 5.6 – 9.2 kW |
5 Tons (60,000 BTUs) | 7 – 11.5 kW |
These figures should provide a reasonably close estimate of the solar power you’ll need. However, please keep in mind that these calculations are based on the U.S. national annual average of approximately 5 Peak Sun Hours per day.
To determine the size of the system you require for your specific location, you can use the annual energy usage of your heat pump, combined with the Annual Peak Sun Hours in your area:
Required On-Grid Solar Power (kW) = (Annual Energy Consumption (kWh) ÷ Annual Peak Sun Hours) x 1.25
For example, let’s say you have a 3-ton heat pump that uses 8,000 kWh of energy per year for both cooling and heating and let’s assume that your solar panels would, on an annual average, receive 4 Peak Sun Hours per day.
Since it’s an annual average, we can multiply those daily Peak Sun Hours by 365 (the number of days in a year) to determine the annual PSH that your panels would have access to:
Annual Peak Sun Hours = Daily Peak Sun Hours x 365
Annual Peak Sun Hours = 4 x 365
Annual Peak Sun Hours = 1,460 PSH
Now, let’s determine the required system size:
Required On-Grid Solar Power (kW) = (Annual Energy Consumption (kWh) ÷ Annual Peak Sun Hours) x 1.25
Required On-Grid Solar Power (kW) = (8,000 kWh ÷ 1,460 Peak Sun Hours) x 1.25
Required On-Grid Solar Power (kW) = (5.48 kW) x 1.25
Required On-Grid Solar Power (kW) = 6.85 kW
Assuming you’re using residential solar panels rated at 350 Watts (0.35 kW) each, you would need:
Number of solar panels = Required Solar Power (kW) ÷ Individual Solar Panel Rating (kW)
Number of solar panels = 6.85 kW ÷ 0.35 kW
Number of solar panels = 19.56 panels
How many off-grid solar panels do you need to run a heat pump?
In off-grid solar systems, also known as standalone or independent systems, the absence of a grid connection requires a different approach to energy generation and storage.
In an off-grid setup, your solar panels generate electricity that is immediately used to power your home via a battery bank. Any excess energy produced during sunny periods is stored in that battery bank for use when the sun isn’t shining, such as at night or on cloudy days.
This setup means you only have access to energy as long as your batteries remain charged.
Therefore, it’s essential to size your solar panels based on a daily cycle. In other words, the daily energy production of your solar panels should match the daily energy consumption of your heat pump, plus any additional appliances you plan to run.
While your heat pump’s energy usage varies daily depending on climate, daily usage, and whether it’s cooling or heating, it’s prudent to size your off-grid system based on a worst-case scenario. A good approach is to size your system based on:
- The daily energy consumption of the heat pump in the heating season (winter) which, as seen in the table above, will represent the peak energy usage of your heat pump.
- And on the Peak Sun Hours in the same season.
Required Off-Grid Solar Power (kW) = (Daily Energy Consumption (kWh) ÷ Daily Peak Sun Hours) x 1.25
This ensures that your solar panels are sufficiently large to produce enough energy even during peak energy consumption periods when sunlight may be suboptimal.
Let’s illustrate this with an example:
Suppose you have a 3-ton (36,000 BTU) heat pump with a daily energy consumption peaking at 35 kWh per day in the winter.
Additionally, you want your solar panels to power other appliances, such as a refrigerator, freezer, TV, laptop chargers, lights, etc., which collectively use up to 10 kWh of energy per day.
Your total daily energy consumption is:
Daily Energy Consumption (kWh/day) = Heat pumps’s energy consumption (kWh/day) + Other appliances energy consumption (kWh/day)
Daily Energy Consumption (kWh/day) = 35 kWh/day (heat pump) + 10 kWh (other appliances)
Daily Energy Consumption (kWh/day) = 45 kWh/day
Now, let’s assume that in the winter, specifically in December, you only receive an average of 4.5 peak sun hours a day.
With these figures, we can calculate the size of the solar array as follows:
Required Off-Grid Solar Power (kW) = (Daily Energy Consumption (kWh) ÷ Daily Peak Sun Hours) x 1.25
Required Off-Grid Solar Power (kW) = (45 kWh ÷ 4.5 Peak Sun Hours) x 1.25
Required Off-Grid Solar Power (kW) = (10 kW) x 1.25
Required Off-Grid Solar Power (kW) = 12.5 kilowatts
So, to ensure that the solar panels produce enough energy to run the heat pump and additional appliances during the winter, the system must be rated at 12.5 kilowatts (12,500 Watts) or higher.
If we use solar panels rated at 350 Watts (0.35 kW) each, we would require :
Number of solar panels = Required Solar Power (kW) ÷ Individual Solar Panel Rating (kW)
Number of solar panels = 12.5 kW ÷ 0.35 kW
Number of solar panels = 35.7
For a system of this size, we would need 36 panels rated at 350 Watts each.
Now, if you’re about to go off-grid, and you have a ducted central heat pump, you might want to consider switching to a ductless heat pump for a simple reason: it’s way more energy-efficient.
According to Energy Star, ductless mini-split heat pumps use 30% less energy compared to old-school ducted central heat pumps. That’s a big deal, especially when you’re off the grid and every bit of energy counts.
See, central heat pumps rely on a network of ducts to distribute warm or cool air, and these ducts can leak heat, which isn’t great for efficiency.
In contrast, ductless heat pumps eliminate this heat loss by directly delivering conditioned air to the target space. Plus, you get to choose which rooms get heated or cooled, which alone will decrease energy consumption significantly.
They are also typically designed with energy-saving features like variable-speed (inverter) compressors, which adapt to the specific heating or cooling needs of a room, further reducing energy consumption.
So, if you’re all about maximizing your off-grid setup’s efficiency, and you want to shrink the size of your solar system while still staying comfortable, transitioning to a ductless heat pump could be a wise choice.
In any case, for an off-grid solar system to be complete, you’ll need more than solar panels. A complete off-grid solar system consists of the following components:
- Solar panels
- A battery bank
- A solar charge controller
- An inverter
- And of course, wires and overcurrent protection devices, such as fuses and circuit breakers.
Let me give you an overview.
The battery bank
In an off-grid solar system, the battery bank plays a crucial role by storing the energy produced by the solar panels during the day for use at night or when sunlight isn’t available in general.
Typically, the battery bank consists of multiple batteries wired together, and its total energy storage capacity (kWh) should match your daily energy consumption. However, it’s advisable to account for scenarios where sunlight is limited for consecutive days.
This can be addressed by having a battery bank that has an energy capacity of 2 to 5 times your daily energy consumption, or by incorporating a backup generator.
Related: What size generator to run a heat pump?
For instance, if your peak daily energy consumption reaches 30 kWh, you would require a battery bank with a “usable” energy capacity of at least 30 kWh.
The key term here is “usable” because the battery bank may have a total energy capacity of 30 kWh, but due to factors such as battery chemistry, you may only be able to use a portion of that capacity.
For more detailed information on solar batteries and how to size them, I recommend referring to my comprehensive guide on sizing solar battery banks. In this guide, you’ll find a thorough explanation of solar batteries and the sizing process.
The solar charge controller
Another critical component of a solar setup is the solar charge controller, which serves multiple functions.
It connects the solar panels to the battery bank, preventing the battery bank from discharging into the solar panels at night, and it safeguards the battery bank from overcharging.
In addition to that, a solar charge controller, particularly an MPPT solar charge controller, enhances the efficiency of the solar panels through the process of Maximum Power Point Tracking (MPPT). This ensures that the solar panels produce their maximum output.
The process of sizing an MPPT charge controller takes into consideration factors such as the Voltages of the solar panels and battery bank, temperature, Amperage, etc.
To spare you the hassle of doing all the math on your own, I’ve made available an MPPT Charge Controller Calculator which will use the details that describe your system to determine the specifications of the MPPT that you need.
The inverter
Solar panels generate Direct Current (DC) electricity, which is also the type of electricity stored in a battery bank. DC electricity typically has low voltage (Volts) but high current (Amps).
On the other hand, most household appliances operate on Alternating Current (AC) electricity, which has higher voltage, typically 120/240 Volts, and lower current. AC power is what you would find in a standard residential electrical outlet.
An inverter plays a crucial role in an off-grid solar system by converting the low-voltage, high-current DC energy from the battery bank into high-voltage, lower-current AC energy that your appliances require for operation.
When sizing an inverter for your off-grid solar system, the primary specification to consider is the inverter’s continuous power capacity in watts. This capacity should be equal to or greater than the combined power consumption of your appliances:
Inverter’s Continuous Power Capacity (Watts) > Continuous Power Usage of Your Appliances
Additionally, It’s also important to consider power surges that certain appliances, such as your heat pump, may experience during operation when selecting an inverter.
For certain appliances, these surges in power usage can occur during start-up. For example, when the heat pump’s compressor kicks in, its power usage can surge up to six times the normal power usage (wattage).
To account for these surges, you can use a straightforward formula:
Inverter’s Surge Power Capacity (Watts) > Highest Starting Wattage (Watts) + Combined Running Wattage of the Other Appliances
If you have a central heat pump, it’s likely to have the highest surge wattage among all your appliances. Therefore, the formula becomes:
Inverter’s Surge Power Capacity (Watts) > Heat Pump’s Starting Wattage (Watts) + Combined Running Wattage of the Other Appliances
To determine the starting watts of your heat pump, you can refer to this article for guidance.
The wires and fuses/circuit breakers
Once you’ve determined the sizes of all the components we’ve discussed, the final step is to size the wires and fuses/circuit breakers that will connect each two of these components.
This process can be a little complex, but to assist you, I’ve created several guides and calculators that should simplify it:
- What size wire from the solar panels to the solar charge controller?
- What size fuse or circuit breaker from solar panels to charge controller?
- What size wire from the solar charge controller to the battery?
- What size fuse or circuit breaker from the solar charge controller to the battery?
- Battery to inverter wire size calculator
- What size fuse or circuit breaker between the battery and the inverter?
These resources will help ensure that your off-grid solar system functions efficiently and safely.