Electrical Panel Capacity Assessment
Your ADU will need 40-100 amps of electrical capacity, and your main house panel may not have room to spare. Getting this assessment done early prevents costly surprises and helps you plan for separate metering if desired.
Quick Summary
ADU power needs
40-100 amps
Panel upgrade cost
$2,000-8,000
Separate meter cost
$1,500-5,000
Why Electrical Assessment Matters
Electrical work is one of the most expensive components of an ADU build, and inadequate planning can lead to major cost overruns. Many older homes have 100-amp or 150-amp panels that are already near capacity. Adding an ADU without proper assessment can result in:
- Tripped breakers and electrical fires from overloaded panels
- Unexpected $5,000-10,000 panel upgrade costs mid-project
- Delays waiting for utility company to upgrade service
- Regret about not installing separate meters when it was easy
Understanding ADU Electrical Requirements
Typical ADU Amp Requirements
Small ADU (under 500 sq ft)
40-60 amps typical
Electric cooking, mini-split HVAC
Medium ADU (500-800 sq ft)
60-80 amps typical
Full kitchen, larger HVAC
Large ADU (800+ sq ft)
80-100 amps typical
Multiple circuits, EV charging prep
All-Electric ADU
100 amps recommended
Heat pump water heater, induction range
How to Assess Your Electrical Panel
Check Your Current Panel Size
Look at your main breaker (the large breaker at the top of your panel). This tells you your total service capacity:
- • 100 amp: Common in older homes, likely needs upgrade for ADU
- • 150 amp: May work if current usage is low
- • 200 amp: Usually sufficient for most ADUs
- • 400 amp: Plenty of capacity, rarely needs upgrade
Hire a Licensed Electrician for Load Calculation
A professional load calculation determines how much of your current capacity is being used and how much is available for your ADU. Cost: $150-300 for assessment.
- • They'll inventory all current circuits and loads
- • Calculate your existing demand (often 60-80% of panel rating)
- • Determine available capacity for ADU
- • Recommend panel upgrade if needed
Pro tip: Get this assessment before finalizing your ADU design. If you need a panel upgrade, it affects your budget significantly.
Understand Panel Upgrade Options
If your panel doesn't have enough capacity, you have several options:
- • Panel upgrade only ($2,000-4,000): Replace existing panel with larger one
- • Service upgrade ($4,000-8,000): New service from street + new panel
- • Second service ($5,000-15,000): Completely separate electrical service for ADU
Note: Service upgrades require utility company coordination and may take 4-8 weeks. Plan ahead to avoid construction delays.
Decide on Separate Metering
A separate electric meter for your ADU costs $1,500-5,000 but offers significant benefits for rental situations:
- • Tenant pays their own electric bill directly to utility
- • No disputes about utility usage or bill splitting
- • Cleaner tax situation for rental income
- • More attractive to tenants who want control over costs
Separate meters are easiest to install during construction. Adding later is expensive.
Plan the Subpanel Location
Your ADU will need its own subpanel, fed from either your main panel or a separate meter. Consider:
- • Interior location protected from weather (garage, utility closet)
- • Easy access for maintenance and emergencies
- • Distance from main panel affects wire costs (longer runs = more expensive)
- • Code requirements for working clearance around panel
Electrical Cost Breakdown
Common Electrical Pitfalls
Underestimating Power Needs
Don't size your electrical service for today's needs only. Consider future EV charging, heat pump water heaters, and induction cooking. It's cheaper to install extra capacity now.
Forgetting Utility Company Timeline
Service upgrades and new meters require utility company involvement. They may need 4-8 weeks to schedule the work. Start this process early to avoid construction delays.
Skipping Separate Metering for Rentals
If you plan to rent your ADU, strongly consider separate metering. Adding it later can cost $3,000-8,000 and requires reopening walls. During construction, it's much easier and cheaper.
Frequently Asked Questions
Can I use a smaller subpanel to avoid a main panel upgrade?
Sometimes, yes. If your main panel has 40-60 amps of spare capacity and your ADU is small, you may be able to install a smaller subpanel without upgrading. However, this limits your options and may cause problems if you later want to add EV charging or upgrade appliances. Get professional advice on the tradeoffs.
How long does a panel upgrade take?
A simple panel swap (same capacity, new panel) takes 4-8 hours. A service upgrade requiring new wiring from the street may take 1-2 days of work, plus 2-6 weeks of waiting for utility company to disconnect/reconnect service and install a new meter.
What if my panel is a Federal Pacific or Zinsco brand?
These older panel brands are known safety hazards. If you have one, plan for a full panel replacement regardless of ADU needs. Many insurance companies won't cover homes with these panels. Budget $2,500-5,000 for replacement and consider it a necessary safety investment.
Is it worth getting a separate meter for a family member's ADU?
For family use, separate metering is less important. However, it can still help with fair utility splitting, and it preserves the option to rent in the future without expensive retrofitting. If your budget allows, separate meters are worth considering.
Ready for the Next Step?
Once you understand your electrical capacity, the next step is assessing your water and sewer capacity to ensure your ADU can be properly connected to plumbing systems.